GN-z11 is a galaxy that's 32 billion light years away. Here's the note from wikipedia:
At first glance, the distance of 32 billion light-years (9.8 billion parsecs) might seem impossibly far away in a Universe that is only 13.8 billion (short scale) years old, where a light-year is the distance light travels in a year, and where nothing can travel faster than the speed of light. However, because of the expansion of the universe, the distance of 2.66 billion light-years between GN-z11 and the Milky Way at the time when the light was emitted increased by a factor of (z+1)=12.1 to a distance of 32.2 billion light-years during the 13.4 billion years it has taken the light to reach us.
Also, despite the age of the universe (13.8B years), the diameter of the observable universe is 93 billion light years. That's because the universe expanded (and probably still is expanding) faster than light.
Can you ELI5 - how can we observe something 93 billion light years away, if the light has only been traveling for 13.8B years? Shouldnt the "observable" distance be the the age of the universe?
Your intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" .
I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot.
In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
As light is travelling within the expanding universe does the wavelength of distant light change over long time periods or does it appear the same because our observations are expanding too(my head hurts)>
Yes, the wavelength gets longer. That is, in fact, how we measure the distance: for a known frequency (say, the electrons jumping up and down in a hydrogen atom), we observe that this color is "redder" the further away an object is. The number "z" mentioned above is called the redshift because of that, and it's how we know the distances of the furthest objects.
Except for fact than in balloon analogy, ant is moving in a direction by balloon surface, while in our case our "balloon" is perfectly still in all directions.
Imagine that balloon is not inflated at all, but ant magically moves 70mm instead of 10mm. How this is possible???
I can accept, that our part of Laniakea is expanding, but not whole Universe.
It kind of makes sense to me with some further caveats to the analogy.
If you think of the ant as a 2 dimensional creature living on a two dimensional surface of the balloon, the expansion is occurring in 3 dimensional space. The two-dimensional ant has no way to observe the space north or south (above and below, to us) its plane of existence.
Now, imagine that the balloon is in a vacuum. It had a burst of air (the big bang) for a moment to get it started but it's not the further addition of air that's driving the expansion, it's the desire to equalize pressure between the exterior vacuum and the interior matter.
With that in mind, push everything up a dimension. We're 3 dimensional creatures living in a 3 dimensional universe. That 3 dimensional universe exists as a plane in 4 dimensional space.
The 4 dimensional balloon (that our universe is surrounding) is continuing to expand into the "nothingness" on the ana plane of the 4th dimensional universe because of the "pressure" imbalance on the kata plane of the 4th dimensional universe. (In 4th dimensional spatial terms, ana and kata indicates directions on the w axis, similar to up/down on the x axis, left/right on the y axis, and north/south on the z axis)
At least, that's how I see it in my head, but I'm an odd ball who has never had a problem intuitively visualizing 4th spatial dimension objects and how they might interact with 3rd dimensional space.
As for the increased speed of the expansion... actually, I hadn't thought about that. Does it indicate that equilibrium will never be found? The balloon analogy might be a useful tool, but our universe isn't made of rubber so it might have the capability of being able to expand infinitely without breaking and without its "nature" constraining the expansion speed.
Anyway, if this is a reasonable analogy, the "nothingness" in the ana direction isn't as interesting to me as what might comprise the "pressure" that's in the kata direction that's causing the expansion. It's nothing we'd ever be able to observe or sample since it's outside of our plane of existence, but it's awfully fun to speculate about the composition and nature of it.
> In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
Thus the expansion isn't caused by the movement of the photon but from something else? Then what caused that movement in the first place? And what cause the expansion?
You're right; the expansion of the universe is not related to photons. It's caused by the pressure of dark energy. There are some hypotheses but no one knows for sure what dark energy is.
If I understand correctly, the theory is that spacetime itself was created by the big bang. As-in, space, and time itself didn't exist prior to the big bang.
That is we see red-shifts when we observe distant objects. And comparing that to other distance estimates, we see that far away things are more red-shifted. As if they are moving away from us.
When we build cosmological models to explain this, we could choose reference coordinates for time and space in different ways. But if we choose to define time and distance in the usual way then we get a picture in which the distances between all galaxies is increasing with time.
If space works as a balloon then the growth happens in all the volume of the universe (not just on the “boundary”). Is that how it works? Doesn’t that imply that we are getting “bigger”?
Think of the expansion of the universe as a superlatively weak repulsive gravitational force that scales with the distance to an object. It is so weak at short scales that it is more weak than attractive gravity by a greater factor than attractive gravity is weaker than electromagnetism. It is so small at short scales that even the normal attraction of gravity is so much stronger as to make it undetectable.
However, at scales so large that gravitational attraction has attenuated to nothing, this repulsive component of gravity is still getting stronger. At the scale of galaxy clusters, it is pushing everything in the universe apart in all directions: faster the farther apart they are. At a certain distance, it becomes so strong that objects will appear to be moving away from you faster than the speed of light.
The distance at which that happens is called your Cosmological Horizon, and it has similarities to the event horizon of a black hole turned inside out. Unlike a black hole, however, there isn't a single unique event horizon. Every distinct gravitationally-bound object (galaxy cluster) in the universe has its own cosmological horizon; every other object is being pushed out towards it, and vice versa.
(I am not a physicist) As far as I understand the current theory (i.e. our best guess), the 4 fundamental forces are somehow countering the expansion so that, even as everything in the universe expands, including e.g. every single atom, the forces are even stronger and are pulling stuff together.
I think this just opens more questions than it resolves, and would really like to hear a better explanation.
But just thinking of the electromagnetic forces in terms of what is required from them to keep matter together in the context of that same matter effectively expanding faster than the speed of light is... at best weird.
The speed of expansion depends on the size of the object. Human-scale things aren't expanding at the speed of light; you need to get to larger-than-galaxy-cluster scales for that to happen.
(In fact space inside galaxies isn't expanding at all, only space between galaxies. But I digress.)
Where is the waves? 13.8B years is too short to make Universe smooth again after such epic event. It's like to see green grass again just 1 second after nuke blast: mathematical formulas will need lot of magic constants for this to explain.
The universe isn’t smooth. The CMB is not uniform, and at a very large scale the universe is composed of attractors and voids, which stem from the anisotropy of the early universe.
For me, CMB is just light of distant stars with z=1100.
Universe is composed of attractors and voids at any scale.
Anyway, I see no shockwave of any kind in CMB. Where it is? Such massive explosion, with energy of 1E53 atomic bombs and no shockwave at all just miliseconds (in scale of Universe) after the Bing Bang. How this is possible?
What exactly are you looking for when you say shockwave? As far as we understand it, the big bang didn't explode into something, so there isn't an outer medium to "hold" a shockwave in the obvious sense of a radially expanding region of turbulence.
Also the frequently depicted diagram of expanding from a point doesn't really intend to depict the shape of it. The universe might still be infinitely large and the big bang occurred "everywhere" within it. Where is the shockwave in something that has no edge?
The shockwaves are everywhere - you, right now, are in a density node.
Inflation wasn’t something that happened from a single point - it happened as a result of vacuum collapse, spontaneously, all over the place. Some of these inflationary zones went much faster than others, and where they intersected, regions of increased density, and therefore matter, occurred.
The filamentary nature of the very macroscopic universe reflects these shock intersection zones - they’re full of galaxies. We think that the voids are likely centred around points where inflation occurred.
So yeah. You’re the shockwave. Sorry man.
Here’s a decent vid from PBS Space Time where they explain some of this nice and clearly - I recommend their channel generally if these topics interest you. https://youtu.be/72cM_E6bsOs
Look into the inflationary portion of the BB timeline, and also add in the fact that those shockwaves have had the same 13.8B years to expand and dissipate. The CMB variations _are_ the remnants of that initial period, attenuated after all this time.
I'm bit confused. Regardless of expansion, these objects are right now > 13.8B ly away. So the light from them still needs that much time to get here. If light from them has arrived sooner, we would see less redshift and perceive them as a closer. No?
The light we see is from when the galaxy was closer. As the light left the galaxy and travelled towards us, the universe expanded "between" the photons and the galaxy.
If someone in a train traveling at 108km/h throws a ball at you while passing and you catch the ball two seconds later, you also received a ball from someone “60 meters away” even though the ball only traveled a few meters. The point is: when the ball was thrown the train was much closer. Likewise, the other object was much closer when it sent out the light we see today.
Well, it's an interesting question. Imagine a star that is 13B years old. Basically it means, the light from the star has been traveling for 13B years to reach us. You might say, then the star must be 13B light years away but that's not correct.
The thing is we cannot see such a star in visible light spectrum anymore. The light of the star has shifted to infrared or microwave. Why? Because while the light was traveling in spacetime, the spacetime itself expanded and stretched the light wavelength. When you take a redshift into account, you can correct the distance and figure out that the star must be, say, 40B light years away.
By the way, note that this is all about the observable universe. There might be stars beyond our observation horizon that we might never see due to rapid expansion of the universe.
Two objects can move away from eachother faster than the speed of light as seen by a third observer.
The space between objects can also expand faster than light, the object itself can still not move faster than light. If the space between two objects is expanding faster than light then for all intents and purposes, the other objects no longer exists to another. There is no way to contact or observe that object other than the light that was send your way before space went FTL.
If you have a laser pointer and flick your wrist fast enough, the spot on the ground that your cat chases can move faster than light without violating any laws of physics.
That's perhaps easier to see, if you think of a giant laserpointer aimed at the moon.
Similarly, a shadow on the wall can move faster than the speed of light.
An outside observer that just sees the spot on the moon (but has no clue that it's produced by you flicking your wrists), just sees a spot that moves insanely fast.
No information, energy nor matter travels faster than light here, of course.
Gotcha, so it appears to be moving faster, but relative to its point of reference it is not. I am wondering tho, what laws of physics state that there just is _no way_ of moving faster than light? Apologies for the lack of knowledge, I genuinely don't understand / know where this limitation is coming from.
I'm not sure if there is any single law that states nothing moves faster than the speed of light.
Quantum mechanics limit the spread of information to the speed of light, most wave functions including the fundamental forces propagate at the speed of light and acceleration requires increasing amounts of energy the close your come to the speed of light.
I guess one way to look at where this comes from is to look at Conway's Game of Life. A cell in this game spawns if enough neighbors are present and with some trickery you can make things that move. But due to the rules of the game, nothing moves faster than 1 square per round.
There is no explicit rule that the speed of light in that game is 1 square per round, it's just that the way the rules work, the fastest thing could only possibly be that fast. Everything is limited to this speed that doesn't exist in the game.
If you changed the rules to allow cells to die or spawn depending on cells up to 1 square inbetween in distance, the speed of light would be 2 squares and none of the rules will explicitly state this limit.
The comparison that helped me understand it was seeing the Universe as a balloon.
My friend and I both live at different points on the surface of a balloon with distance A. I throw a ball at light speed while the balloon is being inflated, and the distance between us is B when the ball reaches him after T time.
To my friend it seems that the ball was thrown at B / T speed, which is faster than the speed of light.
You cannot travel faster than light in spacetime but spacetime metric can expand faster than light. There is no speed limit on the evolution of spacetime itself as far as we know.
No, it's observed as 93 billion light years. That's why we call it the "observable" universe.
There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion.
But of course the meaning of "observed" is kinda strange here, since we're not directly measuring the distance but estimating it based on from other observations.
Where I supposed that "observed as <distance>" just means how much red-shifted its light is- we don't really have any other way to measure such distances. This in turn only measures how much the space has expanded between us and the original starting point, much closer than 13 billion light years away. So we can say that we're seeing a star that is "now" 46 billion ly from us, but must have been only a few billion ly away when it emitted the light we're receiving. Correct?
You're right. The star could not have been more than a few billion light years away when it emitted the photons that we're seeing. Otherwise, not only would the photons have not reached us yet, they will probably never reach us because the universe is expanding faster than the speed of light.
I feel so stupid when talking and hearing about physics... but we are not in any meaningful way "at the center" of the universe are we? Or is every point in some sense at the center (a point of reference thing)? I'm asking because why would it be 46.5 "each way"?
Imagine we exist in a 2D universe, but one that happens to be the surface of a sphere. Any point you pick on that surface is "at the center of the universe."
Cosmologists talk about "horizons" a lot, and the analogy of standing on a sphere actually works quite well. Remember that horizons only make sense on curved surfaces. You can't see beyond the point where certain features of spacetime (black holes, expansion, or sheer distance) prevent signals from reaching you, just as you can't see beyond a mountain range or the curvature of Earth itself. Of course you'll need to extrapolate the analogy to three, four, or more dimensions, but the basic idea is the same.
Basically, ignoring wormholes and black holes and assuming that spacetime is locally flat everywhere and it's mathematically a manifold, my question is: what's the shape of the (global) universe?
Global as opposed to observable. So we might have a hard time answering that question. How would you be able to distinguish between the (n-dimensional equivalent of) a torus vs a flat infinite space, if you can't see the repetition?
You'd even have a hard time distinguishing a hypersphere from a flat infinite space, if the hypesphere was big enough so that we can't tell it's curvature apart from no curvature.
Or the universe might be weirdly shaped, and we just happen to live in the flat part.
So I guess the question comes down to:
* assuming no edges
* assume Copernicus at least for space (we might have a special position in time)
* What's the simplest theory about the shape of the global universe that satisfies our observations?
I suspect general relativity toys around with such questions, because I know that they sometimes look at cosmological (toy) models for the whole universe, and not just what's in the light cone of one particular observer.
Aah of course... somehow we should believe that at some stage something accelerated to an impossible - according to all current observation - speed and then decelerated, somehow dissipating this energy somewhere.
Just to fit a bunch of observations that wouldn't otherwise make sense for the current model.
I remember the story of a chap from Pisa having a hard time trying to budge a bunch of clerics to consider his observations proof that their model was broken. ;)
Seriously... what's the difference between 'universe expansion' and 'travel'? A particle can't travel faster than c but it can effectively if the universe expands in addition to its travel speed? Are there any established sci-fi concepts that replace the concepts of wormholes with 'universe compression'?
Yes, it's called Alcubierre Drive, where spacetime is compressed in front and expanded behind a ship to drive it through the spacetime like a gravitational wave.
Just because the universe is expanding at c doesn't mean everything inside it is getting bigger at c. Due to the way mass curves spacetime, mass stays more or less the same size (in comparison to the rest of the empty universe, anyway) - it is mostly only the EMPTY parts of space that get stretched out at c. This means that distances between things are getting bigger all the time... BUT things are still able to travel through space, so they get further away even faster.
Imagine two people (which are galaxies) walking in the same direction on a travelator (which is spacetime). Ordinarily one person would reach the end, and then in a predictable amount of time so would the other.
But if the travelator is expanding evenly in both directions, then the distance between the two people will increase, even though they're walking at the same speed.
One of the interesting things is that if everyone of us did manage to travel almost at the speed of light then time stops and you can be in universe wherever you want in matter of your seconds. In other words, whole universe just becomes giant wormhole and you teleport from wherever to wherever in matter of seconds at whim. If you reach to the edge of time dimension, the space dimension basically seize the exist.
I'm not sure that's right since things are light-years away it means you also need to travel just as long (or is this just as far as outside obeservers are concerned?). But even so, the universe around you would still get older while you travel like that so in a few trips you will reach the heat death of the universe.
That is just for outside observers. With time and space dilation, things do become instantaneous at light speed. It is actually kind of fun to think about that in the frame of reference of a photon which ‘is’ constantly coming in and out of existence as it interacts with charged mass.
> what's the difference between 'universe expansion' and 'travel'?
Travel happens inside the universe and expansion doesn't? If a rock is sliding along a stretchy sheet, stretching the sheet will make other things on the sheet move farther away from the rock, potentially faster than the fastest that the rock can slide, even though the rock doesn't move any faster.
Don't think there is a way to tell the difference without stepping outside the universe to look back on the system relative to another frame.
But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.
Not that i'm truly able to wrap my head around it, but at the moment of the big bang, time and space themselves exploded into existence.
The expanding balloon analogy is often used with the wrong audience and badly misleading there: the surface of the balloon is a 2d analogy for the universe and it's expansion, rather than the interior of the balloon being a 3d analogy.
But that would be a wrong analogy. Cool thing about a balloon surface is that it expands distances between fixed points on it (ants if you want). But that doesn’t happen exactly the same way to a space inside of a balloon, it just grows at its border without dragging any reference frames.
A way of thinking about it is that space isn't "something". Space is just the gaps between "something".
It seems like a lot of people think of the universe as a sort of container/volume that things exist inside, and then what is "outside the box" is a natural question, because there is a boundary to pass.
If you instead think of it as simply a description of the extent of a set of objects, it's easier. Picture a simulated universe with no fixed boundary, just a set of objects with coordinates.
"The universe" is just the set of objects, and it's volume is just the volume encompassing all of those objects at any time. The simulation doesn't contain any "space". Space in the simulation is just an absence of objects, and there's nothing special about "outside" or "inside" in that situation. If the objects move further away, the universe gets bigger. If you "travel" to the edge of the universe, and travel further, the universe gets bigger. If you try to traverse the current "edge of the universe", the edge moves with you.
There are theories where there might be something "outside" our universe in one or more dimensions, but there's no reason why something absolutely needs to have an "outside".
Does that include photons? Photons that have travelled the furthest are, by the above definition, at the boundary of the universe - an ever expanding boundary - which they will never exceed. If this is so and you were 'outside' the universe, would it look like a black hole? Does a universe have an event horizon?
Don't you find the fact there is something to be equally weird? I mean, first there was nothing and everything was as it should be. Then: something! And time!
This is why I think the notion of free will is wrong. The initial nothingness wasn't nothing. It was mass, but without time. The way the particles were arranged in this nothingness determines a future that has already been carved in mass.
> But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.
Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. (But even if that's the case, that still has nothing to do with the expansion we've been talking about.)
Space does get bigger, but that's separate. What eternal inflation is claiming is that some fourteen billion years ago there was a false vacuum collapse, and we're living inside the bubble that resulted, but said bubble is itself still expanding.
It won't ever get anywhere, since space outside the bubble is inflating much, much faster than inside. It means there's a border to our universe, which is expending at lightspeed, but it's far outside the Hubble horizon and thus entirely unreachable.
There is no such thing as “read-only” in physics. If we can observe the other galaxy, that means it is affecting the particles in this galaxy. Even if it’s just a single molecule in a telescope responding to a photon that hit it from the other universe.
I guess that makes sense, thanks. I was wondering if it might have been something like the theories of what might happen much later in the universe's life, where things eventually get so far apart while still accelerating that light can't travel between them. But it couldn't be something like that if we can see the other galaxy, right?
We can see light that was emitted from it billions of years ago, before we split apart. Similarly, they can see light that ours emitted billions of years ago.
They can never be affected by anything we do now, or vice versa. This doesn't make it "read-only", though -- we can't read it either. All we're seeing is their distant past, back when we were mutually able to reach each other.
True, but we can see galaxies that are already over the "light horizon." In other words, we can see light emitted from those galaxies when they were within the light horizon (the point where they were receding from us at less than c), and that light has a causal effect on us. Light emitted from the galaxy now, however, will never reach us because it is already receding from us faster than the speed of light.
This still doesn’t add up. For one, now it’s saying that when the universe was .4 billion years old we were already 2.6 billion light years apart, so now you have to explain that first.
I thought the answer was more like "all our figures are quite rough currently and a lot of work needs to be done".
They mention that the current universe age fits "within the bars", for example but also mention arguments for universe being younger as well, which would again put the measurements at odds.
Basically, both of these values involve a string of assumptions and while getting every single one of them to high accuracy is a remarkable achievement, the chance of some glitch in either measurement is significant.
And it's only older than the universe if you assume the Big Bang was the beginning of the universe, which there's really no evidence to suggest. Pretty much everything else in the universe all seems to suggest that it all originated from a similar point in time and space and that it rapidly expanded outward. Okay. But what was going on in this particular space back then is completely unknown, so... could there have been some stars forming already from matter that wasn't part of the Big Bang? Why not?
Quark Gluon Plasma (observed at the RHIC), at really early in the universe there was too much temperature and pressure to have stars or even atoms. At least that's my understanding.
Yes but Quark Gluon Plasma existed immediately following the Big Bang. We arrive at the Big Bang by reverse engineering lots of evidence we see in the universe. What evidence is there that nothing else currently inside the bubble of the observable universe could possibly have been there, very far away from the Big Bang, at the time of the Big Bang?
I don't see why I'm being down-voted because as of yet no one has answered that question. I'm not saying the Big Bang didn't happen. I'm saying we have no observations that give us any confidence we'll never encounter something that wasn't part of the Big Bang. This star actually pre-dating the Big Bang is extremely unlikely, I agree. The simpler explanation is the uncertainty in it's age estimation. But equating "the observable universe" with "stuff that originated in the Big Bang" is an assumption we simply don't have much reasoning for, other than we haven't seen much other stuff. But then... we also don't see many stars this old.
Mostly because space and time are most likely a result of the Big Bang. For all we can tell, "the beginning" was an infinitely dense singularity of space time - there is no "very far from the Big Bang", because, there's no space.
Yes, you can somewhat debate that - we're not entirely clear what happened in the first 10^-33 seconds - but we don't know anything about that.
So you might as well say "what evidence is there that the universe wasn't the spawn of two extremely short-lived green bunnies", and the answer is "none", too.
I thought that theory had issues with quantum gravity?
But either way, inflation would happen in that 10^-33 span I mentioned above. We simply don't know.
And even if we assume it holds, inflation is an extremely rapid expansion of space-time, so the idea that there were things "very far from each other" still does not really hold.
And IIRC, Penrose and Hawkings disproved the idea that inflationary models can avoid an initial singularity[1]. I'd be somewhat surprised to see them disproven.
All the evidence suggests that Big Bang did not happen at one identifiable point. Instead it happened everywhere simultaneously. So there is no place in our universe "very far away from the Big Bang".
Not the above poster, but I want to understand. I have half-understood thoughts that conflict with each other. Any help showing where I misunderstand is appreciated, but this is probably a failure of my brain to be able to cope.
* All space was contained in the big bang, so "everywhere simultaneously" makes sense from that perspective.
* ...but the idea that space is expanding implies an outer "edge" (, which doesn't make sense if space isn't infinite. (or is it infinite, and just getting "more" infinite, in the way that the space on the number line between 0 and 1 is infinite, but "less" infinite than between 0 and 2?
> ...but the idea that space is expanding implies an outer "edge"
No, it doesn't.
For example: consider an infinite number of marbles in an infinitely long line, all of them are touching each other.
Now, get an infinite number of helpers stationed along the infinitely long line, and have them all move the marbles in front of them to the right and stagger them out in the process, so that there's now an inch between each marble.
You now have an infinite number of marbles an infinitely long line, but it's a longer line than it started out, because there's now an inch between all the marbles.
A less brain melting way to imagine the "expansion without edge" thing is the classic balloon system. Imagine your entire universe exists across the plain of a semi-deflated balloon and then someone slowly begins to blow it up. The "space" in your 2D world is expanding, but there are no edges.
Which leads to the follow up question: if you travel far enough in one direction do you end up where you started? (well, no, not if space expands faster than c).
If it wasn't for the expansion, and if the universe had positive curvature, then yes.
As it stands, there's not only the expansion, but the universe also seems to be flat -- meaning it's spatially infinite, not just borderless. That observation is also compatible with a very, very small curvature, but in that case the size would still be so large that it wouldn't be practically possible to circumnavigate even if it wasn't expanding.
> * ...but the idea that space is expanding implies an outer "edge"
Space expension just means that the metric used to measure distance increases with time. It doesn't imply a border.
Consider the function equivalent to the Euclidian distance on a line :
d(x1,x2) = sqrt((x2-x1)^2)
Now, you can consider the function :
d2(x1,x2,y) = y * d(x1,x2)
If you view x1 and x2 as positions on one axis and y as a position on the other one, you can think of d2 as the distance between x1 and x2 expanding as you go higher in the plane.
Space expansion works in the same way but y is actually time.
Others have mentioned the "expanding ballon" analogy. But personally I find this more intuitive:
Imagine you are baking sweet rolls with raisins in them. When you are proofing your dough, the yeast makes the dough expand in each point, so all the raisins become further apart.
Now imagine starting with an infinite dough. As it is proofing, it will still be infinite, but there will be a measurable increase in the distance between raisins, and the speed of how quickly a specific raisin moves away from another one increases linearly with how far they are apart when you start measuring.
That's all "expansion of space" really is. We can see other stars moving away from our own, and the stars far away are moving away faster than those close to us. And since looking outwards is looking backwards in time, we can only see as far away as corresponds to the first stars that were formed.
And maybe you've heard that we can observe "expansion is slowing down". That is actually the fact that we measure a deviation from linearity in speed vs. distance.
Layman here. To your second bullet, yes, the universe can be infinite but also be expanding. A rough analogy I've heard is that there are infinite points on the surface of a balloon, but if you blow it up more, the distance between any two given points will be greater than it was, so it has expanded. I don't think we know if the universe is infinitely large or not though.
You are probably being downvoted because the big bang, our ability to estimate the age of the universe, etc is believed to be a fact by many including astronomers and astrophysicists. And yet there are so many unanswered questions that it is reasonable to question those things. In this case, it calls into question our age estimation for both planets and the universe.
That's great, but The Big Bang is more or less chasing a status as the canonical answer to an intensely desirable question.
It wants to be the answer, yet the facts don't really align with the premise of the question. We just shoe horn a litter of observations into some post-hoc rationale for how the universe "began" and then we retcon anything that doesn't make sense, when we think more deeply about it.
It's fine to have cosmic background radiation, and elemental distributions and ratios, and sure that all kind of lines up, pointing to some large, highly influential event in the past.
But just because we can chart certain facts doesn't mean the story stops where our charts end.
I'll concede that something really important happened 13-ish billion years ago, and that it represents a significant temporal horizon, beyond which we cannot draw conclusions, yet because we cannot see beyond the peaks of some mountain range does not mean the edge of the world resides on the other side.
All the mental backflips required to rationalize The Big Bang, and then from the other side of your mouth, murmur that it actually wasn't a "bang" but instead an "expansion" really just result in a colossal defrocking of the theory.
Hey look guys, here comes
emperor Big Bang in his
super comfy new clothes!
Sorry, but if it's not actually a bang, don't call it a bang.
And if it seems that we don't really know what was going on, as we try to recreate a march back inward to the hypothetical origin singularity, and every time we try to walk events backward, they stop making sense, we just need to say we don't really know.
The Big Bang is a smelly theory, and it smells wrong. There's a lot of evidence of a really powerful event some 13 billion years ago, and it may have been transformative, but there's no basis to the claim that it was truly event zero.
Just as possible is that we're the fallout of some localized very large bang, that cleared out a radius of space time that is more distant than we can observe. This idea is not interchangeable with the big bang origin story of the universe. It's not an Occam's razor victory in favor of the big bang.
If we are stuck within a spatial temporal crater as the result of a non-unique very large bang, it would mean that we can look differently at how we model the bang, and come up with saner conclusions than "oh, it just popped into existence, and then went kablooey, and that's okay because the very idea of time is bound to that ever even happening at all."
Like, sorry, that hand wavey whitewash doesn't adequately paper over the possibility of retconning some nonsense into the narrative, because it would be a threat to academic authority to say "we don't know" regarding the nature of a hypothetical origin singularity.
I get that there's no evidence beyond the reaches of our deepest observations, but it's lazy to put forward that because we can't grasp at evidence beyond certain terrestrial limits, then we get to say that we know conclusively that the observable limits are known to be the actual limits and thus the assured origin of all space time.
What we should say is that there is a depth beyond which we cannot summon further evidence, and while there seems to be some tumultuous event back there, we don't know what happens prior to that or where that limit comes from.
So your chief complaint is that people call it a bang instead of an expansion? Obviously from my comments you'll see I agree we're a little over-confident in all the specifics, but the question isn't "was it event 0", which I don't think anyone has really claimed, but could events, specifically this star, that occurred outside of the horizon of that expansion now have moved into our visible universe.
If that's not what you're saying I really cannot see what point you're making.
But why would that matter? If, like you said "...all originated from a similar point in time and space and that it rapidly expanded outward", any star forming next to it would be pushed out of the observable edge.
Hence, no one would ever know if a star was there in the first place.
The Big Bang was not (or not only) an expansion of matter, it's an expansion of space itself; the known universe is an enormous place that expanded out of a microscopic one. If there was space before the Big Bang, presumably it's been displaced by the sudden appearance of the entire universe as we know it. Not just the things in it but all the empty parts too, especially the empty parts.
Even ignoring complicated questions about gravity and space metrics and their evolution over time, if a big bang occurred in an occupied region of space, we'd see the results of these two things crashing together. We see similar, but much smaller, such things all over the place in space. We do not see any collisions best explained this way.
As I've said in another comment, in most of the time since the Big Bang space was been extremely sparse. If there's 1 example of a star that could maybe be older than the rest of it, what makes you think we'd be seeing collisions more frequently if some stuff existed at the time of the Big Bang. I'm not even suggesting there was a second Big Bang that immediately preceded the one we already talk about. But other than theory, we know nothing about pre-Big Bang conditions, so I don't see how we can reason with confidence about how nothing within the current bubble of the observable universe could possibly have existed outside of that bubble at any point. Maybe with the star's current direction it's consistent with the same Big Bang, but we seem awfully confident about conditions we've never observed.
Because as "extremely sparse" as it may be, colliding gas clouds still light up. To try to express in English what properly ought to be math, space is really, really, really empty... but it isn't really, really, really, really empty. Everywhere there's a bit of at least hydrogen gas, and we'd probably see it if even two of the emptiest regions of space were colliding.
I'm presuming this is happening at some non-trivial relative velocity on the grounds that it's hardly a "big bang" if it's actually in the same velocity vector as the location in which it occurs, and doesn't have any new material coming out.
You're kind of overextrapolating the "we know nothing about pre-Big Bang conditions" based on the English phrasing of a more mathematical idea. We can eliminate some things, like as we are discussing, the possibility that the Big Bang occurred in an already-existing universe in some way that it could create a visibly older star showing up a mere 100-200 light years from us. I can't eliminate the possibility that there's some "collision" happening out of our past light cone, or that a Big Bang occurred and that taking into account metrics of space this time, that it simply "blew" all the old universe out of our past light cone as our universe inflated. But I can eliminate the idea of some older universe being something we could co-exist in local space with. It would produce visibly obvious results that we do not see, a fairly violent cataclysm that would have results that we could see. As we parse the Cosmic Microwave Background, we're parsing twitches and fluctuations far, far smaller than such an event would have created. This would be writ large on the sky, not something that could sneakily inject a star into our galaxy that we only notice after hundreds of years of astronomical observations.
We can't make very many positive assertions about pre-Big-Bang conditions, but we can make a few negative ones like this.
I'd also point out it's not an intrinsically crazy idea. Some variants of String Theory hypothesize situations in which it would be possible for a "Big Bang" to occur in an existing universe like ours, due to membrane collisions. It could well have happened elsewhere. It just doesn't seem to have happened in our past lightcone.
Colliding bits of material produce light. Even colliding hydrogen clouds produce detectable signatures. If, for the sake of argument, two big bangs occurred and we could see where they met, there would be a large visible region in the universe putting out some sort of photons. (Not necessarily visible ones.)
More subtle things would be visible too, like, yes, interacting gravity fields, but it wouldn't take such clever analysis; the impact points would literally light up in the sky.
What's the evidence of that? I'm not arguing, but between cosmic background radiation, doppler shift of stars and the relationship between speed / distance, all the evidence of the big bang I've heard doesn't suggest anything about space itself.
The theory of cosmic expansion states that space is being created between objects in the universe (rather than them simply moving away from each other). Therefore, all space in the universe, or at least the ~90 billion light year observable universe, was created by the big bang itself.
Okay - that makes sense. Upon reviewing the Wikipedia page, that sounds to be a much more accepted consensus than I thought it was, with more reasons to accept it than I thought.
Correct me if any of the following is wrong, though: when we say the universe is expanding, we mean that matter is spreading out. The further apart the matter is, the faster it's spreading out. If we back-trace the position of stars, they all coincide at roughly the same point in time and space. This led us to believe that at some point in time all of this matter was together, incredibly dense, and in a very "hot", high-energy state. This led to the prediction that has since been confirmed, that from the furthest reaches of the observable universe, we would see cosmic background radiation. And as time goes, that cosmic background radiation is getting further and further away, so the observable bubble is expanding.
Maybe I have some of that wrong, but that's my understanding. None of that means that the fabric of space (as in space-time) is actually expanding. And even if it did, none of that means that nothing could have existed outside of the observable bubble billions of years ago, and could have since been passed by the observable edge and now be inside that observable bubble. Unless maybe the source of the background radiation was still dense / high-energy enough to absolutely obliterate everything in it's path. And maybe it was? But I just don't see how we can reason about what was outside of the observable bubble.
Perhaps a big bang that's 'just' an explosion in spacetime, rather than of spacetime, would not work with known physics because that concentration of that much matter would be a black hole? Until now, I never thought to ask anyone.
That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region.
Furthermore, our telescopes have only observed from the vicinity of the sun - we have not actually seen what the universe looks like from another star. Of course, we can figure out what it would look like, but if that is your argument, then it is circular.
> That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region.
The image isn't enough to demonstrate the difference. But the speed of light is, along with the way acceleration and redshift work.
> Of course, we can figure out what it would look like, but if that is your argument, then it is circular.
A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular.
You might have some specific circular argument in mind, but there are non-circular arguments for what GP is saying.
> The image isn't enough to demonstrate the difference...
Precisely.
> ...But the speed of light is, along with the way acceleration and redshift work.
Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.)
> A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular.
It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)
Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all.
> What set of measurements and calculation do you have in mind that avoid this circularity?
Any measurements that would actually work, I think.
You're the one asserting circularity, so I think it's up to you to explain what specific argument would be circular. Especially when you're apparently talking about theoretical arguments that swebs might have.
At best, you are claiming that swebs' argument was correct because there is a different argument for the point he claimed has been demonstrated - but that is, as you say, "a very different thing." My original objection to that argument stands, and you have agreed that it does!
> Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all.
Who is making any claim about demonstrating the opposite? You can see from my original post that I agree that spacetime is expanding; I merely disagree with swebs' argument for claiming to know that it is.
>> What set of measurements and calculation do you have in mind that avoid this circularity?
> Any measurements that would actually work, I think.
In that case, you will have no difficulty in stating, or providing citations for, the actual measurements and calculation that get the job done - if any such set of measurements exist, that is. Otherwise, your reply is equivalent to "I don't know."
My point is that I do not think that there is any set of deduced remote measurements that a) show, as a certainty, that space itself is expanding, and b) can be deduced without implicitly or explicitly assuming a position on the issue. Current cosmology expects that the large-scale measurements will show the same results as from Earth (e.g. the same Hubble constant, if and only if you derive it using the same assumption about space itself expanding), but the absence of a difference between there and Earth would fail to satisfy a).
Furthermore, I notice that you have still not made clear which redshift measurements support your claim that the issue has been resolved experimentally. A citation would be sufficient.
I probably could have been more explicit. To be clear, the opposites here are 'the big bang is an expansion of spacetime' and 'the big bang is an expansion in spacetime.' Swebs claimed the diagram showed the former, but it acually literally shows an expansion in space (which, it so happens, also works for an expansion of space, and so does not, as it stands, show a way to choose between the two.)
Let's go over the circularity issue in more detail. It starts with Swebs' statement that "[The big bang as a uniform expansion in spacetime] wouldn't work out because it wouldn't match what our telescopes are observing." Here we have a claim that the 'uniform expansion in spacetime' hypothesis (UEIS for short), which we all agree is wrong, has already been ruled out by astronomical observation.
Next, Swebs offers a specific argument for that claim, in the form of a diagram which shows that, in a uniformly expanding universe, each point appears to be that from which the expansion is ocurrring. One problem with that argument, which we both apparently agree on, is that this would be true for UEIS (for one thing, the image can be taken as a diagram of a demonstration using marbles on a table, which would literally be a UEIS.)
There is a second problem with it, however: none of our telescopes have observed the universe from the perspective of a distant galaxy, so we cannot say that our telescopes have shown that everywhere seems to be at the center of the expansion. We can deduce what it looks like from distant galaxies, if and only if we make some assumptions about the dynamics of the universe (there may be other observations from Earth that have already ruled out UEIS empirically, in which case the rest of this comment is moot. You have claimed that there are, but so far, you have have not presented any details.)
There are two possibilities: either it looks the same as from Earth (putting aside details local to the specific point of observation) or there is some difference. In the former case, which would hold if the expansion we observe is a uniform expansion of spacetime (UEOS), and which therefore is what we all, and cosmology in general, assumes to be so, I do not think these observations would rule out the possibility of UEIS. Therefore, for observations made in a distant galaxy to empirically settle the UEIS/UEOS issue, they must differ from those from Earth, in a way that is diagnostic of, and therefore causally dependent on, what form the expansion takes. Furthermore, because the local and remote results would differ in this case, UEOS would be ruled out.
So, when we try to deduce what can be observed from a distant galaxy, we must either assume that it is the same as from Earth (subject to local corrections), which will fail to resolve the issue, or we must deduce that there is some difference - but what difference? As shown in the previous paragraph, the relevant differences must be causally dependent on what form the expansion takes, so you have to make assumptions about the latter in order to deduce the former. There's the circularity, and it is general - i.e. not dependent on what specific measurements are being considered.
> You have claimed that there are, but so far, you have have not presented any details.
You can measure stars and watch the distances accelerate, or look at how the redshift of a star changes over time instead of staying static. These might take a while to do, but this is about what we can measure, not what we already have.
> As shown in the previous paragraph, the relevant differences must be causally dependent on what form the expansion takes, so you have to make assumptions about the latter in order to deduce the former.
This is that part where I see a breakdown in your logic. It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy. In that case, there would be no circularity.
Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
> This is about what we can measure, not what we already have.
That is not so, this discussion started with a claim that we already have observational proof that distinguishes between the two scenarios. I have already suggested that the Hubble constant could, in principle, be measured without cosmological assumptions, but we do not, AFAIK, have good enough data, and you have not offered anything to alter that opinion.
>... look at how the redshift of a star changes over time instead of staying static.
Up until now, you have been very vague about what sort of measurements you have had in mind. This is getting closer, but under which scenario does the redshift stay static?
> This is that part where I see a breakdown in your logic...
All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
> It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy.
There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
> Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
Note that there are some conditionals here that you have to be careful of. What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)
There's definitely some deep miscommunication going on. I'm just going to try to clarify a couple parts...
> All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
It's circular if we assume the form of the expansion. If the real form influences our measurements, and we use that to figure out what telescopes would show, which lets us calculate the real form, then nothing is circular.
> Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
No, those are very different statements. "We can tell from here what telescopes would show" could be part of a valid argument. "We need to go there to know" is not necessary.
> There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
I'm talking about the "none of our telescopes have observed the universe from the perspective of a distant galaxy" stuff.
Okay, look, this is clearly not working. If you want to format your argument as a numbered chain of logical statements, I can probably give you a response you'll understand. Otherwise I'm giving up. Big blobs of paragraphs are not conducive to debating whether there "must" be certain assumptions.
> In what way is that not exactly the sort of circularity that we just agreed about above, given that the real form is the issue to be decided?
Because measurements are not assumptions.
If we use assumptions about UEOS/UEIS to figure out what telescopes show, and use that as evidence for UEOS/UEIS, that's a circular argument.
If we use measurements to figure out what telescopes show, without any assumptions about UEOS/UEIS, and then use that as evidence for UEOS/UEIS, that's not a circular argument.
(And obviously there are always some assumptions when being sufficiently pedantic. It's an assumption that the sun still exists, etc. That's why I'm specifically saying "assumptions about UEOS/UEIS".)
> If we use assumptions about UEOS/UEIS to figure out what telescopes show, and use that as evidence for UEOS/UEIS, that's a circular argument.
Exactly, and that is what my comment to Swebs was intended to forestall (I have already made this point, several posts back.) That comment, being a reply to Swebs, must be read in that context (obviously, as you had not even joined the thread yet), and in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation. In other words, the resolution of UEOS/UEIS was, in fact, the conclusion that Swebs was claiming to have observational proof of. Using a pre-existing resolution of UEOS/UEIS to deduce what observations might be made elsewhere simply didn't enter into the discussion, as in that case, the issue being debated would have already been resolved.
> Because measurements are not assumptions.
I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
> in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation.
At most that was just being wrong, not having a circular argument.
> In other words, the resolution of UEOS/UEIS was, in fact, the conclusion that Swebs was claiming to have observational proof of.
Yes, Swebs was making a conclusion about UEOS/UEIS. But both the circular and non-circular arguments do that. Swebs was not using assumptions about UEOS/UEIS to reach that conclusion.
> I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
That doesn't matter.
If I say "A implies B", and I have no evidence for A, then I have not proven B. But it is not a circular argument.
You can have an argument that is both valid and unproven.
The argument "Measurements will/might/do tell us UEOS/UEIS" is not circular.
>> in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation.
> At most that was just being wrong, not having a circular argument.
I did not call Swebs' argument circular. I will repeat, for at least the third time now, that my comment about circularity was intended to forestall the use of a circular argument, of the sort that you have acknowledged is possible, in attempts to correct it.
>> I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
> That doesn't matter...
It is not intended to be an argument for the circularity of anything, it is simply a reply to your comment about measurements, pointing out the lack of them in this discussion.
I know. Swebs has not proposed a full argument. We're talking about hypothetical arguments that swebs, or someone else, could propose. Right?
> I will repeat, for at least the third time now, that my comment about circularity was intended to forestall the use of a circular argument, of the sort that you have acknowledged is possible, in attempts to correct it.
Here are the things you said:
"Of course, we can figure out what it would look like, but if that is your argument, then it is circular."
"you have to make assumptions about the latter in order to deduce the former"
"I do not think that there is any set of deduced remote measurements that a) show, as a certainty, that space itself is expanding, and b) can be deduced without implicitly or explicitly assuming a position on the issue."
"What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)"
So, correct me if this summary is wrong, "Any argument based on figuring out what it looks like has to be a circular argument."
Do you stand by that statement, or am I misinterpreting something?
Because that's what I disagree with. I think it's possible to make an argument, based on figuring out what it looks like, that is not circular. This argument would use measurements as the basis for its figuring/deducing.
> circular argument, of the sort that you have acknowledged is possible
It's always possible to make a circular argument. What matters is whether it's possible to make a non-circular argument.
I am assuming that, in your statement of my position, where you wrote "figuring out what it looks like" you meant "figuring out what it looks like elsewhere." I also want to make it clear that I take "based on" to mean that the deduced remote measurements are a necessary part of the argument. Nevertheless, it is still not quite there: as it stands, even after these adjustments, it leads to the conclusion that Swebs' argument is circular, yet we are both agreed that it is not.
This goes back to the part of Swebs' argument that says, in effect, "if we were to observe the expansion of the universe from a distant galaxy, it would look like it does from here" (specifically, that we are at the center in both cases.) My position is that I do not think one could make any successful argument of that form - i.e. which depends on comparing what one would measure at a remote location with what one observes here - without it being circular. My reason for thinking that is so is that I suspect that, if the argument is valid (Swebs' was not), you cannot deduce what those remote measurements are without begging the question.
Now we get to the critical part: my reason for thinking it would be begging the question is that, if the measurements are capable of resolving the UEIS/UEOS issue, then I think they must have a causal dependency on which way UEIS/UEOS goes, and thus, on account of that dependency, one could not deduce what would be measured remotely without first picking one or the other.
Update: Let's look at it from the other direction: if your deduced remote measurements do not differ depending on whether you assume UEIS or UEOS, then how are they going to resolve the issue? This is what makes Swebs' argument an invalid one.
Note that this does not rule out UEIS/UEOS being settled by local measurements, and in fact, early in this thread, I mentioned what I thought might be a possible candidate: directly determine the Hubble constant without reference to redshift, through measurements over time of objects whose distance can be measured by other means. Such methods would not depend on comparing local and deduced remote measurements, and would render moot the whole issue of whether we can deduce relevant remote measurements, as well as all arguments that rely on doing so.
So my position is that I do not think there are any arguments that are both valid and non-circular, that depend on comparing deduced remote measurements to local ones, that show we already have observations that resolve the issue, and I will stand by it until I am persuaded otherwise. One valid counter-example would be sufficient.
Okay, I understand everything you're saying, so let me put this as simply as possible.
Let's posit that we're taking local measurements and using them to determine UEIS/UEOS.
We know it's possible to do a whole bunch of abstract math and get an answer, without involving remote galaxies.
But maybe that's hard math to do.
What if it's easier to extrapolate our measurements to a remote galaxy?
So we calculate what telescopes would show in a remote galaxy as an intermediate step.
And from that we actually determine UEIS/UEOS.
That would be a non-circular argument, in which the deduced remote measurements are a necessary step.
It's not the only argument we could have made, but it's a useful and valid one.
Is that hypothetical argument detailed enough to satisfy you? It's something that should apply to just about any measurements. There's an unlimited number of ways to prove anything. There's no reason "deduced remote telescope measurements" can't be an intermediate step for all sorts of proofs that look at stars for data.
> Update: Let's look at it from the other direction: if your deduced remote measurements do not differ depending on whether you assume UEIS or UEOS, then how are they going to resolve the issue? This is what makes Swebs' argument an invalid one.
I know that my local measurements will differ based on UEIS/UEOS. But I don't know specifically how. Just looking at my local measurements by themselves, I have no idea which one they support. And in the process of deducing the remote measurements, I don't assume either one. I just plug in the numbers.
If you want a math analogy, then uhhh think of the local measurements as a composite number, and factoring it to deduce the remote measurements? And the question we ultimately want to answer can easily be solved with the prime factors.
It would be possible to calculate the answer without factoring, but much harder.
And factoring does not depend on assuming the answer. It's just factoring. It depends only on the composite number.
I am not sure that I have made every implication of my position clear to you, as the arguments in the paragraph beginning "now we get to the critical part", and even more so in the update following it, imply that if there is some calculation where one can just plug in the numbers without explicitly or implicitly taking a position on UEIS/UEOS, it will not yield a result that resolves the issue (unless it merely duplicates a set of measurements that could just as well have been made locally, in which case the excercise would have been pointless.)
I think the best thing we can do at this point is to agree to disagree, especially as we are disagreeing over a hypothetical set of measurements and a calculation that neither of us can specify.
> I think the best thing we can do at this point is to agree to disagree
I would say yes to this except for your parenthetical there. Reading that, I think we're finally on the same page.
> unless it merely duplicates a set of measurements that could just as well have been made locally, in which case the excercise would have been pointless
Yes, it does duplicate the measurements that could have been made locally. But to whoever is writing the proof it's easier to use them in the 'remote galaxy' form. So it's not pointless. It's a critical part of that particular proof.
So with that noted down, I think everything is resolved?
> I do not see how differing telescopes have anything to do with the issue.
How our telescope's observations would differ if they were placed in a different galaxy.
> it would have to be done without any tacit assumption about whether UEIS or UEOS applies
Yes! Now we're getting somewhere.
When you say "the argument is circular" that depends on all possible arguments using assumptions about UEIS/UEOS. I'm saying that you can make arguments that don't make assumptions about UEIS/UEOS. It's possible to make a circular or non-circular argument.
> In what way is that not exactly the sort of circularity that we just agreed about above, given that the real form is the issue to be decided?
Because I'm talking about measurements! You know, the same thing telescopes do. They measure. If you make measurements instead of assuming, then you don't have a circular argument.
> Mainly because you haven't said what measurements you think will substitute for Swebs' "the expansion appears to originate from wherever we are."
I don't have any in mind. I'm not saying there are any. I'm just saying that if the argument exists, it can be expressed in a non-circular form.
> point out the first statement in there that you disagree with
"We can deduce what it looks like from distant galaxies, if and only if we make some assumptions about the dynamics of the universe"
If "some assumptions about the dynamics" is supposed to include UEIS/UEOS assumptions, then this is where I disagree. We do not necessarily have to make those assumptions to make those deductions. You have not given any proof that all possible ways to deduce what it looks like will require those assumptions. We may be able to remove all of those assumptions with the right local measurements. Then we would have a non-circular argument.
> when we say the universe is expanding, we mean that matter is spreading out
It's been a while, but I recall reading in a book that's actually the space between everything that's expanding, much like you state in your second paragraph. For example, if you blow up a balloon, two points on it will appear to be moving apart from each other even though they're stationary.
But that's also somewhat misleading as it gives the impression that it's just interstellar space (reinforcing the non-scientific connotation of space) that inflates. At some point even atoms may not be able to hold together. See https://en.wikipedia.org/wiki/Big_Rip Whether the Big Rip is true or not, it's useful as a supplemental illustration.
Space itself is actually growing as well as celestial bodies having some relative motion that on average moves away from a common point. The growth in space itself is called inflation. Last time I studied cosmology (university) this was an accepted fact although the precise mechanism that explains this inflation was not agreed upon (scalar field, dark energy etc). I think there is some debate about whether inflation is speeding up or slowing down or how long it has occurred for but AFAIK the concensus is that it is a real thing.
To make an analogy, it is as if you had an elastic sheet with objects moving on its surface. The elastic sheet can be expanded which causes the distance between any two fixed points to grow.
(it's been many years since I studied physics so any corrections to my comments are appreciated)
We've been here before, with stars older than the age of the universe. Last time, we thought that there was only one kind of Cepheid variable. This lead to underestimating the age of the universe. Later, astronomers realized the nearby Cepheid variables (used for calibrating the period/luminosity relationship) differed systematically from the bright Cepheid variables that we could see in distance galaxies.
Understanding the difference between Type I and Type II Cepheid variables lead to a greater estimate for the age of the universe, resolving the difficulty. So I expect some interesting astronomy to come out of tracking down the cause of this discrepancy.
Hypothesis 3: Due to inherently incomplete information, our understanding of the age of the universe is not yet perfectly correct, and the universe may be older than current models and theory.
Strap yourselves in folks, maybe The Big Bang is but one event woven amid a deeper tapestry of events, significant only in its capacity to occlude deeper periods of time.
It should surprise no one that we might be wrong about some centralized, focal aspect of the universe, given how wrong we've been before, about things we assumed to be the center or origin, previously.
This remind me of a similar story with the age of the earth and the solar system. Scientists using new methods were starting to discover that Earth was far, far older than originally predicted...so old, that it seemed to predate our sun. It turns out it our estimates of the solar system's age were wrong, but it took some time to figure out why.
I imagine something similar is going on here. The article mentions that once they got an age older than the universe, they started looking at how to make the star younger...finally by acknowledging the margin of error in estimates. That sounds like bad science to me.
> The article mentions that once they got an age older than the universe, they started looking at how to make the star younger...finally by acknowledging the margin of error in estimates. That sounds like bad science to me.
Later (in section "Taking a closer look at the age of the universe"), they tell that your theory is also being looked into.
I think, by Occam's Razor, the most likely explanation is that there's a skybox surrounding the solar system that Voyager 1 may run into any day now, like Truman Burbank at the end of The Truman Show. It seems to be a more parsimonious explanation of the available facts that we're looking at the result of a super duper planatarium projector, as opposed to an actual vast and frequently inexplicable universe.
If you're going in that direction, you may as well say that the universe is a simulation. Planck limits on measuring time and space as a continuum would pretty useful for putting a hard cap on required rendering resources per volume of space-time...
Simulation doesn’t have to obey the rules. For non-scientists it is enough to “tell” them how it works without actually rendering it, cause they’ll never check. For scientists, you have to make their brain believe that all is consistent with complex computations all the way down.
Imagine that you know physics and math very good. But then simulation suddenly fails due to segfault and you realize that all your knowledge was just a gibberish nonsense and your work sessions and discussions were dream-like experience. Otherwise it was just a pretty dumb 3d simulation slightly better than a modern AAA game.
Edit: I mean, a simulation argument opens a huge can of worms, if you consider the perspective of a lazy simulation developer them-self.
While a simulation doesn't have to obey the rules, any given simulation is likely to resemble a system that does adhere to rules, since a simulation is indeed simulating something else.
Even if some external observer can pause, rewind, intervene and violate rules, the reason a simulation exists is to model something else.
This "something else" will have some kind of rules, and the goal of the simulation will be similarity to the actual realm external to the simulation.
Meanwhile, a lazy replication of a model by an unmotivated author would likely impose finite quantities upon scales of interaction, since a simulation won't be able to recreate a real time version of something larger than the external reality itself.
We wouldn't notice a lagged simulation that tries to consume its footprint, but it's probable that an artificial creation would impose caps on aspects of a system to prevent runaway reactions that produce useless simulations. To us, those sorts of limitations would resemble extra physical laws, and while incontrovertible to the simulated entity, such limits might confront intuition in strange ways.
Without giving too much away, it's not from this universe.
You really should read it, it's one of the best books in the culture series and Banks was a phenomenal writer.
It also contains the Affront, a species that are almost comically awful and coined the term "outside context problem":
"The usual example given to illustrate an Outside Context Problem was imagining you were a tribe on a largish, fertile island; you'd tamed the land, invented the wheel or writing or whatever, the neighbors were cooperative or enslaved but at any rate peaceful and you were busy raising temples to yourself with all the excess productive capacity you had, you were in a position of near-absolute power and control which your hallowed ancestors could hardly have dreamed of and the whole situation was just running along nicely like a canoe on wet grass... when suddenly this bristling lump of iron appears sailless and trailing steam in the bay and these guys carrying long funny-looking sticks come ashore and announce you've just been discovered, you're all subjects of the Emperor now, he's keen on presents called tax and these bright-eyed holy men would like a word with your priests."
"An Outside Context Problem was the sort of thing most civilisations encountered just one, and which they tended to encounter rather in the same way a sentence encountered a full stop."
Why can't the star be older because of relativity? I imagine that, particularly in the early universe while everything was nearer to everything else, relative background gravity was higher (and thus relative time) is different than now. Would the star not age at a different rate if it's a remnant from violent beginnings of the universe?
I think, if I understand your question correctly, that a star couldn't age more, only less, from our perspective.
EDIT: I could be wrong though, and additionally I believe the star being referenced by the article is actually in a similar reference frame so relativity likely isn't the answer anyways.
No, but we (our planet, stuff in our vicinity) could age less.
What we'd need is most stuff (that gave rise to most stars and most of our measures of the age of the universe) to be in a higher-gravity region compared to the star in question. It seems unlikely, but I suppose it's possible...
[Edit: I thought you were talking about GR time dilation due to gravity. A re-read shows that I may have been in error.
Also, for a star 190 ly away, any relativistic explanation, either special or general, is probably wrong.]
I don't think there's such a concept as "higher-gravity region" in modern physics, unless you're thinking about "in the vicinity of a giant star, black hole, or something equally massive."
However, if most of the known universe is sitting next to a gigantic mega-blackhole (or enough of them to cover the whole sky), then surely we would have noticed by now...
> I don't think there's such a concept as "higher-gravity region" in modern physics, unless you're thinking about "in the vicinity of a giant star, black hole, or something equally massive."
I think at the beginning of the universe, there might be a lot of regional discrepancies. They might even just be "temporary" regional discrepancies from gravitational waves due to the inherent nature of the universe growing and everything still crashing together.
The cosmic microwave background tells us there were practically no discrepancies at all, actually. Not too surprising, since at the densities back then even a small local fluctuation would easily have become a black hole.
The very early universe wasn't perfectly uniform, but it was really, really close.
It reminds me of the same question asked in this book, "The Birth of Time: How Astronomers Measured the Age of the Universe" by John Gribbin. The bottom line was that the calculations always have some approximations, which can throws off a number by a large factor.
Start over space theories all over again. Lets not build our assumptions on previous generation findings.
Only if we take path of how the previous geniuses went through we will have a flawless exploration. Instead of doing that, if we work on someone else's work, its like eating someone else's recipie and trying to remake the tase of it without understanding the ingredients in it.
Given the norm for question titles, the answer must be "It can't." Also simply based on what a universe is.
What we have are just different estimates for the age of our universe. And none of them involve simple measurement, obviously. So the challenge is finding the artifact(s) that generate the disagreement.
Does the microwave background come from the centre of the universe where the big bang was; and the light from stars on the far side, flying in the opposite direction to us, take longer to reach us than the microwave background does?
The Big Bang happened everywhere in the Universe, at once. Our CMBR is coming in from all directions, at all times, as more distant locations(eg further away from us at the start) finally reach us.
I would REALLY love to read this, but space.com is one of those asshole sites that wants to take a gigabyte of memory (I'm not joking) and wants a full 30 to 45 seconds of CPU just to load the page.
It loads almost instantly without javascript. I can understand not using adblocking but why are you complaining about loading times when not using umatrix or noscript?
wow what an interesting article. Finding not one, but multiple stars that are much older than the predicted age of the universe means something is likely very wrong with one of our theories: dark energy, star aging, or using the cosmic background as a measure of the universe's age.
Okay... This implies the light was stretched along with the space, right? Because otherwise, the photons would just... Never reach here if space between us and the light expanded faster than c.
A question - how common are such old stars? Because it is right in our backyard. 190 light years away are nothing. And if those stars are rare - that makes our neighborhood unusual - we have life and some of the oldest object in the universe in almost the same spot.
Almost everything of interest essentially happens in our backyard because that's the place we can see with the most detail. It's a little like looking out at the Universe and coming to the conclusion that we're in the middle of it because the "edges" are all around us.
Even if this was rare and there happened to be tonnes of very old stars only close to us, this could again be coincidental, especially if intelligent life is more common than we currently know.
This is precisely true, only very small stars last very long, and small stars are very dim. All the old stars that we see must be very close. The bright stars that we see that are far away only live for a few million years.
190 light years is in our backyard compared to the size of the universe, but I was curious about how many stars are that close to us. It turns out that we have around 100k stars within 190 light years.
0.120 stars/parsec^3 * 4/3 * pi (190 ly * parsec/3.26ly)^3
Here we go again... "scientists would look at the ripples in the fabric of space and time". Am I the only one who finds this expression very confusing? What fabric? There is no fabric. Space and time are coordinates. They do not exist physically at all. There cannot be a fabric of space and time.
It's a metaphor to help humans grasp it better. Space and time does exists physically.
Does a shortest path between you and your workplace exists that you can traverse? Is it a straight line? No, it's not, but there's one path that you as a being can walk along physically. The buildings, roads, obstacles force you to diverge from the ideal straight line. That's another analogy of what space & time is. Fabric is just shorter.
GN-z11 is a galaxy that's 32 billion light years away. Here's the note from wikipedia:
At first glance, the distance of 32 billion light-years (9.8 billion parsecs) might seem impossibly far away in a Universe that is only 13.8 billion (short scale) years old, where a light-year is the distance light travels in a year, and where nothing can travel faster than the speed of light. However, because of the expansion of the universe, the distance of 2.66 billion light-years between GN-z11 and the Milky Way at the time when the light was emitted increased by a factor of (z+1)=12.1 to a distance of 32.2 billion light-years during the 13.4 billion years it has taken the light to reach us.
[1] https://en.wikipedia.org/wiki/GN-z11#Notes
Also, despite the age of the universe (13.8B years), the diameter of the observable universe is 93 billion light years. That's because the universe expanded (and probably still is expanding) faster than light.
Can you ELI5 - how can we observe something 93 billion light years away, if the light has only been traveling for 13.8B years? Shouldnt the "observable" distance be the the age of the universe?
Your intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" .
I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot.
In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
As the ant, so the photon.
The ant and balloon analogy is really good from an intuition stand point. Great explanation! Well done :)
As light is travelling within the expanding universe does the wavelength of distant light change over long time periods or does it appear the same because our observations are expanding too(my head hurts)>
Yes, the wavelength gets longer. That is, in fact, how we measure the distance: for a known frequency (say, the electrons jumping up and down in a hydrogen atom), we observe that this color is "redder" the further away an object is. The number "z" mentioned above is called the redshift because of that, and it's how we know the distances of the furthest objects.
Except for fact than in balloon analogy, ant is moving in a direction by balloon surface, while in our case our "balloon" is perfectly still in all directions.
Imagine that balloon is not inflated at all, but ant magically moves 70mm instead of 10mm. How this is possible???
I can accept, that our part of Laniakea is expanding, but not whole Universe.
Sometimes I say as joke that in reality the universe are not expanding but the things inside that are shrinking.
What'll really blow your mind is that it's expanding faster, not slower.
It kind of makes sense to me with some further caveats to the analogy.
If you think of the ant as a 2 dimensional creature living on a two dimensional surface of the balloon, the expansion is occurring in 3 dimensional space. The two-dimensional ant has no way to observe the space north or south (above and below, to us) its plane of existence.
Now, imagine that the balloon is in a vacuum. It had a burst of air (the big bang) for a moment to get it started but it's not the further addition of air that's driving the expansion, it's the desire to equalize pressure between the exterior vacuum and the interior matter.
With that in mind, push everything up a dimension. We're 3 dimensional creatures living in a 3 dimensional universe. That 3 dimensional universe exists as a plane in 4 dimensional space.
The 4 dimensional balloon (that our universe is surrounding) is continuing to expand into the "nothingness" on the ana plane of the 4th dimensional universe because of the "pressure" imbalance on the kata plane of the 4th dimensional universe. (In 4th dimensional spatial terms, ana and kata indicates directions on the w axis, similar to up/down on the x axis, left/right on the y axis, and north/south on the z axis)
At least, that's how I see it in my head, but I'm an odd ball who has never had a problem intuitively visualizing 4th spatial dimension objects and how they might interact with 3rd dimensional space.
As for the increased speed of the expansion... actually, I hadn't thought about that. Does it indicate that equilibrium will never be found? The balloon analogy might be a useful tool, but our universe isn't made of rubber so it might have the capability of being able to expand infinitely without breaking and without its "nature" constraining the expansion speed.
Anyway, if this is a reasonable analogy, the "nothingness" in the ana direction isn't as interesting to me as what might comprise the "pressure" that's in the kata direction that's causing the expansion. It's nothing we'd ever be able to observe or sample since it's outside of our plane of existence, but it's awfully fun to speculate about the composition and nature of it.
> In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
Thus the expansion isn't caused by the movement of the photon but from something else? Then what caused that movement in the first place? And what cause the expansion?
You're right; the expansion of the universe is not related to photons. It's caused by the pressure of dark energy. There are some hypotheses but no one knows for sure what dark energy is.
If I understand correctly, the theory is that spacetime itself was created by the big bang. As-in, space, and time itself didn't exist prior to the big bang.
And and expansion is a lengthening of something within a frame of reference. What is the frame of reference?
The visible galaxies.
That is we see red-shifts when we observe distant objects. And comparing that to other distance estimates, we see that far away things are more red-shifted. As if they are moving away from us.
When we build cosmological models to explain this, we could choose reference coordinates for time and space in different ways. But if we choose to define time and distance in the usual way then we get a picture in which the distances between all galaxies is increasing with time.
If space works as a balloon then the growth happens in all the volume of the universe (not just on the “boundary”). Is that how it works? Doesn’t that imply that we are getting “bigger”?
Think of the expansion of the universe as a superlatively weak repulsive gravitational force that scales with the distance to an object. It is so weak at short scales that it is more weak than attractive gravity by a greater factor than attractive gravity is weaker than electromagnetism. It is so small at short scales that even the normal attraction of gravity is so much stronger as to make it undetectable.
However, at scales so large that gravitational attraction has attenuated to nothing, this repulsive component of gravity is still getting stronger. At the scale of galaxy clusters, it is pushing everything in the universe apart in all directions: faster the farther apart they are. At a certain distance, it becomes so strong that objects will appear to be moving away from you faster than the speed of light.
The distance at which that happens is called your Cosmological Horizon, and it has similarities to the event horizon of a black hole turned inside out. Unlike a black hole, however, there isn't a single unique event horizon. Every distinct gravitationally-bound object (galaxy cluster) in the universe has its own cosmological horizon; every other object is being pushed out towards it, and vice versa.
(I am not a physicist) As far as I understand the current theory (i.e. our best guess), the 4 fundamental forces are somehow countering the expansion so that, even as everything in the universe expands, including e.g. every single atom, the forces are even stronger and are pulling stuff together.
I think this just opens more questions than it resolves, and would really like to hear a better explanation.
No. Just because space is expanding doesn't change the electromagnetic forces holding you together.
But just thinking of the electromagnetic forces in terms of what is required from them to keep matter together in the context of that same matter effectively expanding faster than the speed of light is... at best weird.
The speed of expansion depends on the size of the object. Human-scale things aren't expanding at the speed of light; you need to get to larger-than-galaxy-cluster scales for that to happen.
(In fact space inside galaxies isn't expanding at all, only space between galaxies. But I digress.)
It's not faster than light at the scale where there's matter in the universe. Across a planet or solar system it's not measurable.
It's when you measure between one side of the observable universe to the oposite side that the expansion speeds get crazy.
> Doesn’t that imply that we are getting “bigger”?
Yes, if by "we" you means "large scale cosmological structures".
But galaxies stay roughly constant size because gravity holds them together. Smaller things are also held together by various forces.
Where is the waves? 13.8B years is too short to make Universe smooth again after such epic event. It's like to see green grass again just 1 second after nuke blast: mathematical formulas will need lot of magic constants for this to explain.
The universe isn’t smooth. The CMB is not uniform, and at a very large scale the universe is composed of attractors and voids, which stem from the anisotropy of the early universe.
For me, CMB is just light of distant stars with z=1100. Universe is composed of attractors and voids at any scale.
Anyway, I see no shockwave of any kind in CMB. Where it is? Such massive explosion, with energy of 1E53 atomic bombs and no shockwave at all just miliseconds (in scale of Universe) after the Bing Bang. How this is possible?
What exactly are you looking for when you say shockwave? As far as we understand it, the big bang didn't explode into something, so there isn't an outer medium to "hold" a shockwave in the obvious sense of a radially expanding region of turbulence.
Also the frequently depicted diagram of expanding from a point doesn't really intend to depict the shape of it. The universe might still be infinitely large and the big bang occurred "everywhere" within it. Where is the shockwave in something that has no edge?
The shockwaves are everywhere - you, right now, are in a density node.
Inflation wasn’t something that happened from a single point - it happened as a result of vacuum collapse, spontaneously, all over the place. Some of these inflationary zones went much faster than others, and where they intersected, regions of increased density, and therefore matter, occurred.
The filamentary nature of the very macroscopic universe reflects these shock intersection zones - they’re full of galaxies. We think that the voids are likely centred around points where inflation occurred.
So yeah. You’re the shockwave. Sorry man.
Here’s a decent vid from PBS Space Time where they explain some of this nice and clearly - I recommend their channel generally if these topics interest you. https://youtu.be/72cM_E6bsOs
Look into the inflationary portion of the BB timeline, and also add in the fact that those shockwaves have had the same 13.8B years to expand and dissipate. The CMB variations _are_ the remnants of that initial period, attenuated after all this time.
What do you expect that shockwave to look like? Circles expanding from the center of the explosion?
Yep. Something like that: https://www.youtube.com/watch?v=EmdBFoQtVvU .
I'm bit confused. Regardless of expansion, these objects are right now > 13.8B ly away. So the light from them still needs that much time to get here. If light from them has arrived sooner, we would see less redshift and perceive them as a closer. No?
The light we see is from when the galaxy was closer. As the light left the galaxy and travelled towards us, the universe expanded "between" the photons and the galaxy.
And how do we know that now the galaxy is much further away? Just by the red shift of the light emitted over time?
As far as I understand, yes. The red shift of the light can be measured, which tells us for how long the light has traveled.
Yep - I kind of understand that. Thanks.
If someone in a train traveling at 108km/h throws a ball at you while passing and you catch the ball two seconds later, you also received a ball from someone “60 meters away” even though the ball only traveled a few meters. The point is: when the ball was thrown the train was much closer. Likewise, the other object was much closer when it sent out the light we see today.
Well, it's an interesting question. Imagine a star that is 13B years old. Basically it means, the light from the star has been traveling for 13B years to reach us. You might say, then the star must be 13B light years away but that's not correct.
The thing is we cannot see such a star in visible light spectrum anymore. The light of the star has shifted to infrared or microwave. Why? Because while the light was traveling in spacetime, the spacetime itself expanded and stretched the light wavelength. When you take a redshift into account, you can correct the distance and figure out that the star must be, say, 40B light years away.
By the way, note that this is all about the observable universe. There might be stars beyond our observation horizon that we might never see due to rapid expansion of the universe.
So does this mean speed faster than light is possible?
Two objects can move away from eachother faster than the speed of light as seen by a third observer.
The space between objects can also expand faster than light, the object itself can still not move faster than light. If the space between two objects is expanding faster than light then for all intents and purposes, the other objects no longer exists to another. There is no way to contact or observe that object other than the light that was send your way before space went FTL.
To give another example:
If you have a laser pointer and flick your wrist fast enough, the spot on the ground that your cat chases can move faster than light without violating any laws of physics.
That's perhaps easier to see, if you think of a giant laserpointer aimed at the moon.
Similarly, a shadow on the wall can move faster than the speed of light.
Wouldn't there be a delay between the time you flicked your wrist and the time the spot actually aligns, due to the limited speed of light?
Yeah, and observing this is a good way to see that information isn't actually being transferred faster than light.
Yep, definitely.
An outside observer that just sees the spot on the moon (but has no clue that it's produced by you flicking your wrists), just sees a spot that moves insanely fast.
No information, energy nor matter travels faster than light here, of course.
Gotcha, so it appears to be moving faster, but relative to its point of reference it is not. I am wondering tho, what laws of physics state that there just is _no way_ of moving faster than light? Apologies for the lack of knowledge, I genuinely don't understand / know where this limitation is coming from.
I'm not sure if there is any single law that states nothing moves faster than the speed of light.
Quantum mechanics limit the spread of information to the speed of light, most wave functions including the fundamental forces propagate at the speed of light and acceleration requires increasing amounts of energy the close your come to the speed of light.
I guess one way to look at where this comes from is to look at Conway's Game of Life. A cell in this game spawns if enough neighbors are present and with some trickery you can make things that move. But due to the rules of the game, nothing moves faster than 1 square per round.
There is no explicit rule that the speed of light in that game is 1 square per round, it's just that the way the rules work, the fastest thing could only possibly be that fast. Everything is limited to this speed that doesn't exist in the game.
If you changed the rules to allow cells to die or spawn depending on cells up to 1 square inbetween in distance, the speed of light would be 2 squares and none of the rules will explicitly state this limit.
The proper formulation of the light-speed restriction is that no information can travel between two points in space faster than light-speed.
The expansion of space itself doesn't violate that - you couldn't use it to communicate with someone in an FTL way.
The comparison that helped me understand it was seeing the Universe as a balloon.
My friend and I both live at different points on the surface of a balloon with distance A. I throw a ball at light speed while the balloon is being inflated, and the distance between us is B when the ball reaches him after T time.
To my friend it seems that the ball was thrown at B / T speed, which is faster than the speed of light.
You cannot travel faster than light in spacetime but spacetime metric can expand faster than light. There is no speed limit on the evolution of spacetime itself as far as we know.
That’s fascinating to think about.
As observed, it's only 13.8, though, right?
No, it's observed as 93 billion light years. That's why we call it the "observable" universe.
There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion.
But of course the meaning of "observed" is kinda strange here, since we're not directly measuring the distance but estimating it based on from other observations.
Where I supposed that "observed as <distance>" just means how much red-shifted its light is- we don't really have any other way to measure such distances. This in turn only measures how much the space has expanded between us and the original starting point, much closer than 13 billion light years away. So we can say that we're seeing a star that is "now" 46 billion ly from us, but must have been only a few billion ly away when it emitted the light we're receiving. Correct?
You're right. The star could not have been more than a few billion light years away when it emitted the photons that we're seeing. Otherwise, not only would the photons have not reached us yet, they will probably never reach us because the universe is expanding faster than the speed of light.
I feel so stupid when talking and hearing about physics... but we are not in any meaningful way "at the center" of the universe are we? Or is every point in some sense at the center (a point of reference thing)? I'm asking because why would it be 46.5 "each way"?
We are at the center of the observable universe. And not that's not special, any star/galaxy is at the center of it's observable universe.
Imagine we exist in a 2D universe, but one that happens to be the surface of a sphere. Any point you pick on that surface is "at the center of the universe."
Hmm, but it looks like the universe is flat, doesn't it?
(Or is it flat like a torus, so doesn't need any curvature to go loop on itself?)
Cosmologists talk about "horizons" a lot, and the analogy of standing on a sphere actually works quite well. Remember that horizons only make sense on curved surfaces. You can't see beyond the point where certain features of spacetime (black holes, expansion, or sheer distance) prevent signals from reaching you, just as you can't see beyond a mountain range or the curvature of Earth itself. Of course you'll need to extrapolate the analogy to three, four, or more dimensions, but the basic idea is the same.
I'm not talking about our own light cone. I'm talking about https://en.wikipedia.org/wiki/Shape_of_the_universe
Basically, ignoring wormholes and black holes and assuming that spacetime is locally flat everywhere and it's mathematically a manifold, my question is: what's the shape of the (global) universe?
Global as opposed to observable. So we might have a hard time answering that question. How would you be able to distinguish between the (n-dimensional equivalent of) a torus vs a flat infinite space, if you can't see the repetition?
You'd even have a hard time distinguishing a hypersphere from a flat infinite space, if the hypesphere was big enough so that we can't tell it's curvature apart from no curvature.
Or the universe might be weirdly shaped, and we just happen to live in the flat part.
So I guess the question comes down to:
* assuming no edges * assume Copernicus at least for space (we might have a special position in time) * What's the simplest theory about the shape of the global universe that satisfies our observations?
I suspect general relativity toys around with such questions, because I know that they sometimes look at cosmological (toy) models for the whole universe, and not just what's in the light cone of one particular observer.
We are by definition almost at the center of the universe that we can observe - we can see as far away in any direction.
Aah of course... somehow we should believe that at some stage something accelerated to an impossible - according to all current observation - speed and then decelerated, somehow dissipating this energy somewhere.
Just to fit a bunch of observations that wouldn't otherwise make sense for the current model.
I remember the story of a chap from Pisa having a hard time trying to budge a bunch of clerics to consider his observations proof that their model was broken. ;)
No, it is that spacetime itself is expanding. Nothing with mass accelerated to an impossible speed.
Ah right, of course. So what caused space time expansion to accelerate and then the opposite?
Seriously... what's the difference between 'universe expansion' and 'travel'? A particle can't travel faster than c but it can effectively if the universe expands in addition to its travel speed? Are there any established sci-fi concepts that replace the concepts of wormholes with 'universe compression'?
Yes, it's called Alcubierre Drive, where spacetime is compressed in front and expanded behind a ship to drive it through the spacetime like a gravitational wave.
https://en.wikipedia.org/wiki/Alcubierre_drive
Or the Krasnikov tube, where time is tilted such that travel through it (in one direction) takes you back in time (from a stationary perspective).
That's very similar to a wormhole, but without the topological flaw.
Just because the universe is expanding at c doesn't mean everything inside it is getting bigger at c. Due to the way mass curves spacetime, mass stays more or less the same size (in comparison to the rest of the empty universe, anyway) - it is mostly only the EMPTY parts of space that get stretched out at c. This means that distances between things are getting bigger all the time... BUT things are still able to travel through space, so they get further away even faster.
Imagine two people (which are galaxies) walking in the same direction on a travelator (which is spacetime). Ordinarily one person would reach the end, and then in a predictable amount of time so would the other. But if the travelator is expanding evenly in both directions, then the distance between the two people will increase, even though they're walking at the same speed.
One of the interesting things is that if everyone of us did manage to travel almost at the speed of light then time stops and you can be in universe wherever you want in matter of your seconds. In other words, whole universe just becomes giant wormhole and you teleport from wherever to wherever in matter of seconds at whim. If you reach to the edge of time dimension, the space dimension basically seize the exist.
I'm not sure that's right since things are light-years away it means you also need to travel just as long (or is this just as far as outside obeservers are concerned?). But even so, the universe around you would still get older while you travel like that so in a few trips you will reach the heat death of the universe.
That is just for outside observers. With time and space dilation, things do become instantaneous at light speed. It is actually kind of fun to think about that in the frame of reference of a photon which ‘is’ constantly coming in and out of existence as it interacts with charged mass.
> what's the difference between 'universe expansion' and 'travel'?
Travel happens inside the universe and expansion doesn't? If a rock is sliding along a stretchy sheet, stretching the sheet will make other things on the sheet move farther away from the rock, potentially faster than the fastest that the rock can slide, even though the rock doesn't move any faster.
The first one means the space time metric changes
Is it possible that the universe stays the same size and everything within it shrinks?
Don't think there is a way to tell the difference without stepping outside the universe to look back on the system relative to another frame.
But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.
Not that i'm truly able to wrap my head around it, but at the moment of the big bang, time and space themselves exploded into existence.
The expanding balloon analogy is often used with the wrong audience and badly misleading there: the surface of the balloon is a 2d analogy for the universe and it's expansion, rather than the interior of the balloon being a 3d analogy.
But that would be a wrong analogy. Cool thing about a balloon surface is that it expands distances between fixed points on it (ants if you want). But that doesn’t happen exactly the same way to a space inside of a balloon, it just grows at its border without dragging any reference frames.
The “nothing” is the one thing I find fascinating ~ maybe it’s our human brains that can’t get it but it just doesn’t work for me!
There has to be something doesn’t there?
A way of thinking about it is that space isn't "something". Space is just the gaps between "something".
It seems like a lot of people think of the universe as a sort of container/volume that things exist inside, and then what is "outside the box" is a natural question, because there is a boundary to pass.
If you instead think of it as simply a description of the extent of a set of objects, it's easier. Picture a simulated universe with no fixed boundary, just a set of objects with coordinates.
"The universe" is just the set of objects, and it's volume is just the volume encompassing all of those objects at any time. The simulation doesn't contain any "space". Space in the simulation is just an absence of objects, and there's nothing special about "outside" or "inside" in that situation. If the objects move further away, the universe gets bigger. If you "travel" to the edge of the universe, and travel further, the universe gets bigger. If you try to traverse the current "edge of the universe", the edge moves with you.
There are theories where there might be something "outside" our universe in one or more dimensions, but there's no reason why something absolutely needs to have an "outside".
Does that include photons? Photons that have travelled the furthest are, by the above definition, at the boundary of the universe - an ever expanding boundary - which they will never exceed. If this is so and you were 'outside' the universe, would it look like a black hole? Does a universe have an event horizon?
Don't you find the fact there is something to be equally weird? I mean, first there was nothing and everything was as it should be. Then: something! And time!
This is why I think the notion of free will is wrong. The initial nothingness wasn't nothing. It was mass, but without time. The way the particles were arranged in this nothingness determines a future that has already been carved in mass.
> But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.
Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. (But even if that's the case, that still has nothing to do with the expansion we've been talking about.)
> Eternal inflation, if correct, states that there is indeed something which our universe is expanding into.
I don't think that's right. The fact that space gets bigger doesn't mean that space is inside something, it just means space gets bigger.
Disclaimer: not an astrophysicist
Space does get bigger, but that's separate. What eternal inflation is claiming is that some fourteen billion years ago there was a false vacuum collapse, and we're living inside the bubble that resulted, but said bubble is itself still expanding.
It won't ever get anywhere, since space outside the bubble is inflating much, much faster than inside. It means there's a border to our universe, which is expending at lightspeed, but it's far outside the Hubble horizon and thus entirely unreachable.
I think those are just two equivalent ways of thinking of the same thing.
The standard way is probably much easier to reason about.
So, is it possible for an event in the Milky Way to ever affect GN-z11 (or vice-versa)? Or are the galaxies permanently 'read-only' to each other?
There is no such thing as “read-only” in physics. If we can observe the other galaxy, that means it is affecting the particles in this galaxy. Even if it’s just a single molecule in a telescope responding to a photon that hit it from the other universe.
I guess that makes sense, thanks. I was wondering if it might have been something like the theories of what might happen much later in the universe's life, where things eventually get so far apart while still accelerating that light can't travel between them. But it couldn't be something like that if we can see the other galaxy, right?
We can see light that was emitted from it billions of years ago, before we split apart. Similarly, they can see light that ours emitted billions of years ago.
They can never be affected by anything we do now, or vice versa. This doesn't make it "read-only", though -- we can't read it either. All we're seeing is their distant past, back when we were mutually able to reach each other.
At some point in the far distant future the night sky will be black, with everything being beyond our visible edge of the universe.
That still holds true, eventually nearby clusters will go beyond our "observable" view.
True, but we can see galaxies that are already over the "light horizon." In other words, we can see light emitted from those galaxies when they were within the light horizon (the point where they were receding from us at less than c), and that light has a causal effect on us. Light emitted from the galaxy now, however, will never reach us because it is already receding from us faster than the speed of light.
This still doesn’t add up. For one, now it’s saying that when the universe was .4 billion years old we were already 2.6 billion light years apart, so now you have to explain that first.
Short answer: it's only older than the universe if you ignore the error bars of the estimate.
I thought the answer was more like "all our figures are quite rough currently and a lot of work needs to be done".
They mention that the current universe age fits "within the bars", for example but also mention arguments for universe being younger as well, which would again put the measurements at odds.
Basically, both of these values involve a string of assumptions and while getting every single one of them to high accuracy is a remarkable achievement, the chance of some glitch in either measurement is significant.
And it's only older than the universe if you assume the Big Bang was the beginning of the universe, which there's really no evidence to suggest. Pretty much everything else in the universe all seems to suggest that it all originated from a similar point in time and space and that it rapidly expanded outward. Okay. But what was going on in this particular space back then is completely unknown, so... could there have been some stars forming already from matter that wasn't part of the Big Bang? Why not?
Quark Gluon Plasma (observed at the RHIC), at really early in the universe there was too much temperature and pressure to have stars or even atoms. At least that's my understanding.
Yes but Quark Gluon Plasma existed immediately following the Big Bang. We arrive at the Big Bang by reverse engineering lots of evidence we see in the universe. What evidence is there that nothing else currently inside the bubble of the observable universe could possibly have been there, very far away from the Big Bang, at the time of the Big Bang?
I don't see why I'm being down-voted because as of yet no one has answered that question. I'm not saying the Big Bang didn't happen. I'm saying we have no observations that give us any confidence we'll never encounter something that wasn't part of the Big Bang. This star actually pre-dating the Big Bang is extremely unlikely, I agree. The simpler explanation is the uncertainty in it's age estimation. But equating "the observable universe" with "stuff that originated in the Big Bang" is an assumption we simply don't have much reasoning for, other than we haven't seen much other stuff. But then... we also don't see many stars this old.
Mostly because space and time are most likely a result of the Big Bang. For all we can tell, "the beginning" was an infinitely dense singularity of space time - there is no "very far from the Big Bang", because, there's no space.
Yes, you can somewhat debate that - we're not entirely clear what happened in the first 10^-33 seconds - but we don't know anything about that.
So you might as well say "what evidence is there that the universe wasn't the spawn of two extremely short-lived green bunnies", and the answer is "none", too.
THat's one theory. Another is Eternal Inflation.
I thought that theory had issues with quantum gravity?
But either way, inflation would happen in that 10^-33 span I mentioned above. We simply don't know.
And even if we assume it holds, inflation is an extremely rapid expansion of space-time, so the idea that there were things "very far from each other" still does not really hold.
And IIRC, Penrose and Hawkings disproved the idea that inflationary models can avoid an initial singularity[1]. I'd be somewhat surprised to see them disproven.
[1] https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singul...
All the evidence suggests that Big Bang did not happen at one identifiable point. Instead it happened everywhere simultaneously. So there is no place in our universe "very far away from the Big Bang".
Not the above poster, but I want to understand. I have half-understood thoughts that conflict with each other. Any help showing where I misunderstand is appreciated, but this is probably a failure of my brain to be able to cope.
* All space was contained in the big bang, so "everywhere simultaneously" makes sense from that perspective.
* ...but the idea that space is expanding implies an outer "edge" (, which doesn't make sense if space isn't infinite. (or is it infinite, and just getting "more" infinite, in the way that the space on the number line between 0 and 1 is infinite, but "less" infinite than between 0 and 2?
> ...but the idea that space is expanding implies an outer "edge"
No, it doesn't.
For example: consider an infinite number of marbles in an infinitely long line, all of them are touching each other.
Now, get an infinite number of helpers stationed along the infinitely long line, and have them all move the marbles in front of them to the right and stagger them out in the process, so that there's now an inch between each marble.
You now have an infinite number of marbles an infinitely long line, but it's a longer line than it started out, because there's now an inch between all the marbles.
You will need infinite amount of energy to insert spaces between these marbles. Can you point us to source of this energy?
The theory of the Big Bang (or at least the variants with an infinite universe) includes infinite energy as part of the initial singularity.
A less brain melting way to imagine the "expansion without edge" thing is the classic balloon system. Imagine your entire universe exists across the plain of a semi-deflated balloon and then someone slowly begins to blow it up. The "space" in your 2D world is expanding, but there are no edges.
Which leads to the follow up question: if you travel far enough in one direction do you end up where you started? (well, no, not if space expands faster than c).
If it wasn't for the expansion, and if the universe had positive curvature, then yes.
As it stands, there's not only the expansion, but the universe also seems to be flat -- meaning it's spatially infinite, not just borderless. That observation is also compatible with a very, very small curvature, but in that case the size would still be so large that it wouldn't be practically possible to circumnavigate even if it wasn't expanding.
> * ...but the idea that space is expanding implies an outer "edge"
Space expension just means that the metric used to measure distance increases with time. It doesn't imply a border.
Consider the function equivalent to the Euclidian distance on a line : d(x1,x2) = sqrt((x2-x1)^2)
Now, you can consider the function : d2(x1,x2,y) = y * d(x1,x2)
If you view x1 and x2 as positions on one axis and y as a position on the other one, you can think of d2 as the distance between x1 and x2 expanding as you go higher in the plane.
Space expansion works in the same way but y is actually time.
Others have mentioned the "expanding ballon" analogy. But personally I find this more intuitive:
Imagine you are baking sweet rolls with raisins in them. When you are proofing your dough, the yeast makes the dough expand in each point, so all the raisins become further apart.
Now imagine starting with an infinite dough. As it is proofing, it will still be infinite, but there will be a measurable increase in the distance between raisins, and the speed of how quickly a specific raisin moves away from another one increases linearly with how far they are apart when you start measuring.
That's all "expansion of space" really is. We can see other stars moving away from our own, and the stars far away are moving away faster than those close to us. And since looking outwards is looking backwards in time, we can only see as far away as corresponds to the first stars that were formed.
And maybe you've heard that we can observe "expansion is slowing down". That is actually the fact that we measure a deviation from linearity in speed vs. distance.
Layman here. To your second bullet, yes, the universe can be infinite but also be expanding. A rough analogy I've heard is that there are infinite points on the surface of a balloon, but if you blow it up more, the distance between any two given points will be greater than it was, so it has expanded. I don't think we know if the universe is infinitely large or not though.
We don't know, but the universe appears to be perfectly flat, which would indicate it's infinite.
That could also just be due to inflation, mind.
[0,1] is not less infinite than [0,2]. Also the less does not need quotes, there is an ordering of infinities.
Edit: typo
I don’t care where you have your Big Bang, Just as long as there’s no Big Bang, nuclear power, or homeless shelter in my back yard.
You are probably being downvoted because the big bang, our ability to estimate the age of the universe, etc is believed to be a fact by many including astronomers and astrophysicists. And yet there are so many unanswered questions that it is reasonable to question those things. In this case, it calls into question our age estimation for both planets and the universe.
That's great, but The Big Bang is more or less chasing a status as the canonical answer to an intensely desirable question.
It wants to be the answer, yet the facts don't really align with the premise of the question. We just shoe horn a litter of observations into some post-hoc rationale for how the universe "began" and then we retcon anything that doesn't make sense, when we think more deeply about it.
It's fine to have cosmic background radiation, and elemental distributions and ratios, and sure that all kind of lines up, pointing to some large, highly influential event in the past.
But just because we can chart certain facts doesn't mean the story stops where our charts end.
I'll concede that something really important happened 13-ish billion years ago, and that it represents a significant temporal horizon, beyond which we cannot draw conclusions, yet because we cannot see beyond the peaks of some mountain range does not mean the edge of the world resides on the other side.
All the mental backflips required to rationalize The Big Bang, and then from the other side of your mouth, murmur that it actually wasn't a "bang" but instead an "expansion" really just result in a colossal defrocking of the theory.
Sorry, but if it's not actually a bang, don't call it a bang.And if it seems that we don't really know what was going on, as we try to recreate a march back inward to the hypothetical origin singularity, and every time we try to walk events backward, they stop making sense, we just need to say we don't really know.
The Big Bang is a smelly theory, and it smells wrong. There's a lot of evidence of a really powerful event some 13 billion years ago, and it may have been transformative, but there's no basis to the claim that it was truly event zero.
Just as possible is that we're the fallout of some localized very large bang, that cleared out a radius of space time that is more distant than we can observe. This idea is not interchangeable with the big bang origin story of the universe. It's not an Occam's razor victory in favor of the big bang.
If we are stuck within a spatial temporal crater as the result of a non-unique very large bang, it would mean that we can look differently at how we model the bang, and come up with saner conclusions than "oh, it just popped into existence, and then went kablooey, and that's okay because the very idea of time is bound to that ever even happening at all."
Like, sorry, that hand wavey whitewash doesn't adequately paper over the possibility of retconning some nonsense into the narrative, because it would be a threat to academic authority to say "we don't know" regarding the nature of a hypothetical origin singularity.
I get that there's no evidence beyond the reaches of our deepest observations, but it's lazy to put forward that because we can't grasp at evidence beyond certain terrestrial limits, then we get to say that we know conclusively that the observable limits are known to be the actual limits and thus the assured origin of all space time.
What we should say is that there is a depth beyond which we cannot summon further evidence, and while there seems to be some tumultuous event back there, we don't know what happens prior to that or where that limit comes from.
Because we really don't know.
So your chief complaint is that people call it a bang instead of an expansion? Obviously from my comments you'll see I agree we're a little over-confident in all the specifics, but the question isn't "was it event 0", which I don't think anyone has really claimed, but could events, specifically this star, that occurred outside of the horizon of that expansion now have moved into our visible universe.
If that's not what you're saying I really cannot see what point you're making.
But why would that matter? If, like you said "...all originated from a similar point in time and space and that it rapidly expanded outward", any star forming next to it would be pushed out of the observable edge.
Hence, no one would ever know if a star was there in the first place.
Our universe in it's recent state is incredibly sparse. What would have pushed a star out of the observable edge, exactly?
The Big Bang was not (or not only) an expansion of matter, it's an expansion of space itself; the known universe is an enormous place that expanded out of a microscopic one. If there was space before the Big Bang, presumably it's been displaced by the sudden appearance of the entire universe as we know it. Not just the things in it but all the empty parts too, especially the empty parts.
Even ignoring complicated questions about gravity and space metrics and their evolution over time, if a big bang occurred in an occupied region of space, we'd see the results of these two things crashing together. We see similar, but much smaller, such things all over the place in space. We do not see any collisions best explained this way.
As I've said in another comment, in most of the time since the Big Bang space was been extremely sparse. If there's 1 example of a star that could maybe be older than the rest of it, what makes you think we'd be seeing collisions more frequently if some stuff existed at the time of the Big Bang. I'm not even suggesting there was a second Big Bang that immediately preceded the one we already talk about. But other than theory, we know nothing about pre-Big Bang conditions, so I don't see how we can reason with confidence about how nothing within the current bubble of the observable universe could possibly have existed outside of that bubble at any point. Maybe with the star's current direction it's consistent with the same Big Bang, but we seem awfully confident about conditions we've never observed.
Because as "extremely sparse" as it may be, colliding gas clouds still light up. To try to express in English what properly ought to be math, space is really, really, really empty... but it isn't really, really, really, really empty. Everywhere there's a bit of at least hydrogen gas, and we'd probably see it if even two of the emptiest regions of space were colliding.
I'm presuming this is happening at some non-trivial relative velocity on the grounds that it's hardly a "big bang" if it's actually in the same velocity vector as the location in which it occurs, and doesn't have any new material coming out.
You're kind of overextrapolating the "we know nothing about pre-Big Bang conditions" based on the English phrasing of a more mathematical idea. We can eliminate some things, like as we are discussing, the possibility that the Big Bang occurred in an already-existing universe in some way that it could create a visibly older star showing up a mere 100-200 light years from us. I can't eliminate the possibility that there's some "collision" happening out of our past light cone, or that a Big Bang occurred and that taking into account metrics of space this time, that it simply "blew" all the old universe out of our past light cone as our universe inflated. But I can eliminate the idea of some older universe being something we could co-exist in local space with. It would produce visibly obvious results that we do not see, a fairly violent cataclysm that would have results that we could see. As we parse the Cosmic Microwave Background, we're parsing twitches and fluctuations far, far smaller than such an event would have created. This would be writ large on the sky, not something that could sneakily inject a star into our galaxy that we only notice after hundreds of years of astronomical observations.
We can't make very many positive assertions about pre-Big-Bang conditions, but we can make a few negative ones like this.
I'd also point out it's not an intrinsically crazy idea. Some variants of String Theory hypothesize situations in which it would be possible for a "Big Bang" to occur in an existing universe like ours, due to membrane collisions. It could well have happened elsewhere. It just doesn't seem to have happened in our past lightcone.
What would it even look like if it was occupied space? Uneven expansion?
Colliding bits of material produce light. Even colliding hydrogen clouds produce detectable signatures. If, for the sake of argument, two big bangs occurred and we could see where they met, there would be a large visible region in the universe putting out some sort of photons. (Not necessarily visible ones.)
More subtle things would be visible too, like, yes, interacting gravity fields, but it wouldn't take such clever analysis; the impact points would literally light up in the sky.
(That) Space was also created by the big bang.
What's the evidence of that? I'm not arguing, but between cosmic background radiation, doppler shift of stars and the relationship between speed / distance, all the evidence of the big bang I've heard doesn't suggest anything about space itself.
The theory of cosmic expansion states that space is being created between objects in the universe (rather than them simply moving away from each other). Therefore, all space in the universe, or at least the ~90 billion light year observable universe, was created by the big bang itself.
Okay - that makes sense. Upon reviewing the Wikipedia page, that sounds to be a much more accepted consensus than I thought it was, with more reasons to accept it than I thought.
The universe is expanding. This started at some point, and I suppose the theory says that it started at the point where there was no space.
Correct me if any of the following is wrong, though: when we say the universe is expanding, we mean that matter is spreading out. The further apart the matter is, the faster it's spreading out. If we back-trace the position of stars, they all coincide at roughly the same point in time and space. This led us to believe that at some point in time all of this matter was together, incredibly dense, and in a very "hot", high-energy state. This led to the prediction that has since been confirmed, that from the furthest reaches of the observable universe, we would see cosmic background radiation. And as time goes, that cosmic background radiation is getting further and further away, so the observable bubble is expanding.
Maybe I have some of that wrong, but that's my understanding. None of that means that the fabric of space (as in space-time) is actually expanding. And even if it did, none of that means that nothing could have existed outside of the observable bubble billions of years ago, and could have since been passed by the observable edge and now be inside that observable bubble. Unless maybe the source of the background radiation was still dense / high-energy enough to absolutely obliterate everything in it's path. And maybe it was? But I just don't see how we can reason about what was outside of the observable bubble.
Perhaps a big bang that's 'just' an explosion in spacetime, rather than of spacetime, would not work with known physics because that concentration of that much matter would be a black hole? Until now, I never thought to ask anyone.
It wouldn't work out because it wouldn't match what our telescopes are observing. This image has a good example of what's happening
https://upload.wikimedia.org/wikipedia/commons/c/c7/Space_Ex...
You can see that from any given point:
1. Every other point is moving away from you
2. Points further away are moving away at a faster rate
This could only work out if space were expanding. If it were an explosion instead, everything would be moving away from a fixed central point.
That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region.
Furthermore, our telescopes have only observed from the vicinity of the sun - we have not actually seen what the universe looks like from another star. Of course, we can figure out what it would look like, but if that is your argument, then it is circular.
> That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region.
The image isn't enough to demonstrate the difference. But the speed of light is, along with the way acceleration and redshift work.
> Of course, we can figure out what it would look like, but if that is your argument, then it is circular.
A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular.
You might have some specific circular argument in mind, but there are non-circular arguments for what GP is saying.
> The image isn't enough to demonstrate the difference...
Precisely.
> ...But the speed of light is, along with the way acceleration and redshift work.
Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.)
> A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular.
It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)
> Precisely.
Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all.
> What set of measurements and calculation do you have in mind that avoid this circularity?
Any measurements that would actually work, I think.
You're the one asserting circularity, so I think it's up to you to explain what specific argument would be circular. Especially when you're apparently talking about theoretical arguments that swebs might have.
At best, you are claiming that swebs' argument was correct because there is a different argument for the point he claimed has been demonstrated - but that is, as you say, "a very different thing." My original objection to that argument stands, and you have agreed that it does!
> Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all.
Who is making any claim about demonstrating the opposite? You can see from my original post that I agree that spacetime is expanding; I merely disagree with swebs' argument for claiming to know that it is.
>> What set of measurements and calculation do you have in mind that avoid this circularity?
> Any measurements that would actually work, I think.
In that case, you will have no difficulty in stating, or providing citations for, the actual measurements and calculation that get the job done - if any such set of measurements exist, that is. Otherwise, your reply is equivalent to "I don't know."
My point is that I do not think that there is any set of deduced remote measurements that a) show, as a certainty, that space itself is expanding, and b) can be deduced without implicitly or explicitly assuming a position on the issue. Current cosmology expects that the large-scale measurements will show the same results as from Earth (e.g. the same Hubble constant, if and only if you derive it using the same assumption about space itself expanding), but the absence of a difference between there and Earth would fail to satisfy a).
Furthermore, I notice that you have still not made clear which redshift measurements support your claim that the issue has been resolved experimentally. A citation would be sufficient.
> Who is making any claim about demonstrating the opposite?
I guess I misunderstood when you said "That actually demonstrates the opposite"...
I probably could have been more explicit. To be clear, the opposites here are 'the big bang is an expansion of spacetime' and 'the big bang is an expansion in spacetime.' Swebs claimed the diagram showed the former, but it acually literally shows an expansion in space (which, it so happens, also works for an expansion of space, and so does not, as it stands, show a way to choose between the two.)
Let's go over the circularity issue in more detail. It starts with Swebs' statement that "[The big bang as a uniform expansion in spacetime] wouldn't work out because it wouldn't match what our telescopes are observing." Here we have a claim that the 'uniform expansion in spacetime' hypothesis (UEIS for short), which we all agree is wrong, has already been ruled out by astronomical observation.
Next, Swebs offers a specific argument for that claim, in the form of a diagram which shows that, in a uniformly expanding universe, each point appears to be that from which the expansion is ocurrring. One problem with that argument, which we both apparently agree on, is that this would be true for UEIS (for one thing, the image can be taken as a diagram of a demonstration using marbles on a table, which would literally be a UEIS.)
There is a second problem with it, however: none of our telescopes have observed the universe from the perspective of a distant galaxy, so we cannot say that our telescopes have shown that everywhere seems to be at the center of the expansion. We can deduce what it looks like from distant galaxies, if and only if we make some assumptions about the dynamics of the universe (there may be other observations from Earth that have already ruled out UEIS empirically, in which case the rest of this comment is moot. You have claimed that there are, but so far, you have have not presented any details.)
There are two possibilities: either it looks the same as from Earth (putting aside details local to the specific point of observation) or there is some difference. In the former case, which would hold if the expansion we observe is a uniform expansion of spacetime (UEOS), and which therefore is what we all, and cosmology in general, assumes to be so, I do not think these observations would rule out the possibility of UEIS. Therefore, for observations made in a distant galaxy to empirically settle the UEIS/UEOS issue, they must differ from those from Earth, in a way that is diagnostic of, and therefore causally dependent on, what form the expansion takes. Furthermore, because the local and remote results would differ in this case, UEOS would be ruled out.
So, when we try to deduce what can be observed from a distant galaxy, we must either assume that it is the same as from Earth (subject to local corrections), which will fail to resolve the issue, or we must deduce that there is some difference - but what difference? As shown in the previous paragraph, the relevant differences must be causally dependent on what form the expansion takes, so you have to make assumptions about the latter in order to deduce the former. There's the circularity, and it is general - i.e. not dependent on what specific measurements are being considered.
> You have claimed that there are, but so far, you have have not presented any details.
You can measure stars and watch the distances accelerate, or look at how the redshift of a star changes over time instead of staying static. These might take a while to do, but this is about what we can measure, not what we already have.
> As shown in the previous paragraph, the relevant differences must be causally dependent on what form the expansion takes, so you have to make assumptions about the latter in order to deduce the former.
This is that part where I see a breakdown in your logic. It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy. In that case, there would be no circularity.
Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
> This is about what we can measure, not what we already have.
That is not so, this discussion started with a claim that we already have observational proof that distinguishes between the two scenarios. I have already suggested that the Hubble constant could, in principle, be measured without cosmological assumptions, but we do not, AFAIK, have good enough data, and you have not offered anything to alter that opinion.
>... look at how the redshift of a star changes over time instead of staying static.
Up until now, you have been very vague about what sort of measurements you have had in mind. This is getting closer, but under which scenario does the redshift stay static?
> This is that part where I see a breakdown in your logic...
All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
> It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy.
There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
> Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
Note that there are some conditionals here that you have to be careful of. What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)
There's definitely some deep miscommunication going on. I'm just going to try to clarify a couple parts...
> All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
It's circular if we assume the form of the expansion. If the real form influences our measurements, and we use that to figure out what telescopes would show, which lets us calculate the real form, then nothing is circular.
> Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
No, those are very different statements. "We can tell from here what telescopes would show" could be part of a valid argument. "We need to go there to know" is not necessary.
> There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
I'm talking about the "none of our telescopes have observed the universe from the perspective of a distant galaxy" stuff.
Okay, look, this is clearly not working. If you want to format your argument as a numbered chain of logical statements, I can probably give you a response you'll understand. Otherwise I'm giving up. Big blobs of paragraphs are not conducive to debating whether there "must" be certain assumptions.
I had written a long response to your various points, but this seems to get to the crux of the matter:
> It's circular if we assume the form of the expansion...
Yes.
> ...If the real form influences our measurements, and we use that to figure out what telescopes would show...
In what way is that not exactly the sort of circularity that we just agreed about above, given that the real form is the issue to be decided?
> In what way is that not exactly the sort of circularity that we just agreed about above, given that the real form is the issue to be decided?
Because measurements are not assumptions.
If we use assumptions about UEOS/UEIS to figure out what telescopes show, and use that as evidence for UEOS/UEIS, that's a circular argument.
If we use measurements to figure out what telescopes show, without any assumptions about UEOS/UEIS, and then use that as evidence for UEOS/UEIS, that's not a circular argument.
(And obviously there are always some assumptions when being sufficiently pedantic. It's an assumption that the sun still exists, etc. That's why I'm specifically saying "assumptions about UEOS/UEIS".)
> If we use assumptions about UEOS/UEIS to figure out what telescopes show, and use that as evidence for UEOS/UEIS, that's a circular argument.
Exactly, and that is what my comment to Swebs was intended to forestall (I have already made this point, several posts back.) That comment, being a reply to Swebs, must be read in that context (obviously, as you had not even joined the thread yet), and in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation. In other words, the resolution of UEOS/UEIS was, in fact, the conclusion that Swebs was claiming to have observational proof of. Using a pre-existing resolution of UEOS/UEIS to deduce what observations might be made elsewhere simply didn't enter into the discussion, as in that case, the issue being debated would have already been resolved.
> Because measurements are not assumptions.
I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
> in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation.
At most that was just being wrong, not having a circular argument.
> In other words, the resolution of UEOS/UEIS was, in fact, the conclusion that Swebs was claiming to have observational proof of.
Yes, Swebs was making a conclusion about UEOS/UEIS. But both the circular and non-circular arguments do that. Swebs was not using assumptions about UEOS/UEIS to reach that conclusion.
> I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
That doesn't matter.
If I say "A implies B", and I have no evidence for A, then I have not proven B. But it is not a circular argument.
You can have an argument that is both valid and unproven.
The argument "Measurements will/might/do tell us UEOS/UEIS" is not circular.
>> in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation.
> At most that was just being wrong, not having a circular argument.
I did not call Swebs' argument circular. I will repeat, for at least the third time now, that my comment about circularity was intended to forestall the use of a circular argument, of the sort that you have acknowledged is possible, in attempts to correct it.
>> I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
> That doesn't matter...
It is not intended to be an argument for the circularity of anything, it is simply a reply to your comment about measurements, pointing out the lack of them in this discussion.
> I did not call Swebs' argument circular.
I know. Swebs has not proposed a full argument. We're talking about hypothetical arguments that swebs, or someone else, could propose. Right?
> I will repeat, for at least the third time now, that my comment about circularity was intended to forestall the use of a circular argument, of the sort that you have acknowledged is possible, in attempts to correct it.
Here are the things you said:
"Of course, we can figure out what it would look like, but if that is your argument, then it is circular."
"you have to make assumptions about the latter in order to deduce the former"
"I do not think that there is any set of deduced remote measurements that a) show, as a certainty, that space itself is expanding, and b) can be deduced without implicitly or explicitly assuming a position on the issue."
"What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)"
So, correct me if this summary is wrong, "Any argument based on figuring out what it looks like has to be a circular argument."
Do you stand by that statement, or am I misinterpreting something?
Because that's what I disagree with. I think it's possible to make an argument, based on figuring out what it looks like, that is not circular. This argument would use measurements as the basis for its figuring/deducing.
> circular argument, of the sort that you have acknowledged is possible
It's always possible to make a circular argument. What matters is whether it's possible to make a non-circular argument.
I am assuming that, in your statement of my position, where you wrote "figuring out what it looks like" you meant "figuring out what it looks like elsewhere." I also want to make it clear that I take "based on" to mean that the deduced remote measurements are a necessary part of the argument. Nevertheless, it is still not quite there: as it stands, even after these adjustments, it leads to the conclusion that Swebs' argument is circular, yet we are both agreed that it is not.
This goes back to the part of Swebs' argument that says, in effect, "if we were to observe the expansion of the universe from a distant galaxy, it would look like it does from here" (specifically, that we are at the center in both cases.) My position is that I do not think one could make any successful argument of that form - i.e. which depends on comparing what one would measure at a remote location with what one observes here - without it being circular. My reason for thinking that is so is that I suspect that, if the argument is valid (Swebs' was not), you cannot deduce what those remote measurements are without begging the question.
Now we get to the critical part: my reason for thinking it would be begging the question is that, if the measurements are capable of resolving the UEIS/UEOS issue, then I think they must have a causal dependency on which way UEIS/UEOS goes, and thus, on account of that dependency, one could not deduce what would be measured remotely without first picking one or the other.
Update: Let's look at it from the other direction: if your deduced remote measurements do not differ depending on whether you assume UEIS or UEOS, then how are they going to resolve the issue? This is what makes Swebs' argument an invalid one.
Note that this does not rule out UEIS/UEOS being settled by local measurements, and in fact, early in this thread, I mentioned what I thought might be a possible candidate: directly determine the Hubble constant without reference to redshift, through measurements over time of objects whose distance can be measured by other means. Such methods would not depend on comparing local and deduced remote measurements, and would render moot the whole issue of whether we can deduce relevant remote measurements, as well as all arguments that rely on doing so.
So my position is that I do not think there are any arguments that are both valid and non-circular, that depend on comparing deduced remote measurements to local ones, that show we already have observations that resolve the issue, and I will stand by it until I am persuaded otherwise. One valid counter-example would be sufficient.
Okay, I understand everything you're saying, so let me put this as simply as possible.
Let's posit that we're taking local measurements and using them to determine UEIS/UEOS.
We know it's possible to do a whole bunch of abstract math and get an answer, without involving remote galaxies.
But maybe that's hard math to do.
What if it's easier to extrapolate our measurements to a remote galaxy?
So we calculate what telescopes would show in a remote galaxy as an intermediate step.
And from that we actually determine UEIS/UEOS.
That would be a non-circular argument, in which the deduced remote measurements are a necessary step.
It's not the only argument we could have made, but it's a useful and valid one.
Is that hypothetical argument detailed enough to satisfy you? It's something that should apply to just about any measurements. There's an unlimited number of ways to prove anything. There's no reason "deduced remote telescope measurements" can't be an intermediate step for all sorts of proofs that look at stars for data.
> Update: Let's look at it from the other direction: if your deduced remote measurements do not differ depending on whether you assume UEIS or UEOS, then how are they going to resolve the issue? This is what makes Swebs' argument an invalid one.
I know that my local measurements will differ based on UEIS/UEOS. But I don't know specifically how. Just looking at my local measurements by themselves, I have no idea which one they support. And in the process of deducing the remote measurements, I don't assume either one. I just plug in the numbers.
If you want a math analogy, then uhhh think of the local measurements as a composite number, and factoring it to deduce the remote measurements? And the question we ultimately want to answer can easily be solved with the prime factors.
It would be possible to calculate the answer without factoring, but much harder.
And factoring does not depend on assuming the answer. It's just factoring. It depends only on the composite number.
I am not sure that I have made every implication of my position clear to you, as the arguments in the paragraph beginning "now we get to the critical part", and even more so in the update following it, imply that if there is some calculation where one can just plug in the numbers without explicitly or implicitly taking a position on UEIS/UEOS, it will not yield a result that resolves the issue (unless it merely duplicates a set of measurements that could just as well have been made locally, in which case the excercise would have been pointless.)
I think the best thing we can do at this point is to agree to disagree, especially as we are disagreeing over a hypothetical set of measurements and a calculation that neither of us can specify.
> I think the best thing we can do at this point is to agree to disagree
I would say yes to this except for your parenthetical there. Reading that, I think we're finally on the same page.
> unless it merely duplicates a set of measurements that could just as well have been made locally, in which case the excercise would have been pointless
Yes, it does duplicate the measurements that could have been made locally. But to whoever is writing the proof it's easier to use them in the 'remote galaxy' form. So it's not pointless. It's a critical part of that particular proof.
So with that noted down, I think everything is resolved?
I'm fine with that.
> I do not see how differing telescopes have anything to do with the issue.
How our telescope's observations would differ if they were placed in a different galaxy.
> it would have to be done without any tacit assumption about whether UEIS or UEOS applies
Yes! Now we're getting somewhere.
When you say "the argument is circular" that depends on all possible arguments using assumptions about UEIS/UEOS. I'm saying that you can make arguments that don't make assumptions about UEIS/UEOS. It's possible to make a circular or non-circular argument.
> In what way is that not exactly the sort of circularity that we just agreed about above, given that the real form is the issue to be decided?
Because I'm talking about measurements! You know, the same thing telescopes do. They measure. If you make measurements instead of assuming, then you don't have a circular argument.
> Mainly because you haven't said what measurements you think will substitute for Swebs' "the expansion appears to originate from wherever we are."
I don't have any in mind. I'm not saying there are any. I'm just saying that if the argument exists, it can be expressed in a non-circular form.
> point out the first statement in there that you disagree with
"We can deduce what it looks like from distant galaxies, if and only if we make some assumptions about the dynamics of the universe"
If "some assumptions about the dynamics" is supposed to include UEIS/UEOS assumptions, then this is where I disagree. We do not necessarily have to make those assumptions to make those deductions. You have not given any proof that all possible ways to deduce what it looks like will require those assumptions. We may be able to remove all of those assumptions with the right local measurements. Then we would have a non-circular argument.
I think string theory approaches this idea with the holographic principle: https://en.m.wikipedia.org/wiki/Holographic_principle
I'm not a physicist but I suspect this comment hits the nail on the head. You can only "undo" a black-hole by expanding the space it is embedded in.
> when we say the universe is expanding, we mean that matter is spreading out
It's been a while, but I recall reading in a book that's actually the space between everything that's expanding, much like you state in your second paragraph. For example, if you blow up a balloon, two points on it will appear to be moving apart from each other even though they're stationary.
I believe this Forbes article is in agreement: https://www.forbes.com/sites/startswithabang/2018/02/10/ask-...
It's also been described like a loaf of bread expanding, where stars are like raisins, except the loaf might be infinitely big. See https://en.wikipedia.org/wiki/Inflation_(cosmology) and https://en.wikipedia.org/wiki/Shape_of_the_universe
But that's also somewhat misleading as it gives the impression that it's just interstellar space (reinforcing the non-scientific connotation of space) that inflates. At some point even atoms may not be able to hold together. See https://en.wikipedia.org/wiki/Big_Rip Whether the Big Rip is true or not, it's useful as a supplemental illustration.
To be clear, the big rip is only if expansion gets massively faster. At the current rate, the disturbance on matter is ridiculously negligibly small.
Space itself is actually growing as well as celestial bodies having some relative motion that on average moves away from a common point. The growth in space itself is called inflation. Last time I studied cosmology (university) this was an accepted fact although the precise mechanism that explains this inflation was not agreed upon (scalar field, dark energy etc). I think there is some debate about whether inflation is speeding up or slowing down or how long it has occurred for but AFAIK the concensus is that it is a real thing.
To make an analogy, it is as if you had an elastic sheet with objects moving on its surface. The elastic sheet can be expanded which causes the distance between any two fixed points to grow.
(it's been many years since I studied physics so any corrections to my comments are appreciated)
The concept of the "big bang" is not an 'explosion' in pre-existing space, but really is space itself expanding.
We've been here before, with stars older than the age of the universe. Last time, we thought that there was only one kind of Cepheid variable. This lead to underestimating the age of the universe. Later, astronomers realized the nearby Cepheid variables (used for calibrating the period/luminosity relationship) differed systematically from the bright Cepheid variables that we could see in distance galaxies.
Understanding the difference between Type I and Type II Cepheid variables lead to a greater estimate for the age of the universe, resolving the difficulty. So I expect some interesting astronomy to come out of tracking down the cause of this discrepancy.
A great cosmology faq, answering this and a lot of other questions: http://www.astro.ucla.edu/~wright/cosmology_faq.html
> It's composition meant the star must have come into being before iron became commonplace.
Hypothesis 1: the star formed in a way that did not involve or produce a lot of iron.
Hypothesis 2: the star is older than the universe itself.
Clearly, hypothesis 2 is the more likely.
Hypothesis 3: Due to inherently incomplete information, our understanding of the age of the universe is not yet perfectly correct, and the universe may be older than current models and theory.
Strap yourselves in folks, maybe The Big Bang is but one event woven amid a deeper tapestry of events, significant only in its capacity to occlude deeper periods of time.
It should surprise no one that we might be wrong about some centralized, focal aspect of the universe, given how wrong we've been before, about things we assumed to be the center or origin, previously.
3 our estimate of the age of the universe or of these stars need to be improved.
It's a hard sell to someone who has cursory interest on the subject had the title been your first hypothesis. Still, not great.
This remind me of a similar story with the age of the earth and the solar system. Scientists using new methods were starting to discover that Earth was far, far older than originally predicted...so old, that it seemed to predate our sun. It turns out it our estimates of the solar system's age were wrong, but it took some time to figure out why.
I imagine something similar is going on here. The article mentions that once they got an age older than the universe, they started looking at how to make the star younger...finally by acknowledging the margin of error in estimates. That sounds like bad science to me.
> The article mentions that once they got an age older than the universe, they started looking at how to make the star younger...finally by acknowledging the margin of error in estimates. That sounds like bad science to me.
Later (in section "Taking a closer look at the age of the universe"), they tell that your theory is also being looked into.
I think, by Occam's Razor, the most likely explanation is that there's a skybox surrounding the solar system that Voyager 1 may run into any day now, like Truman Burbank at the end of The Truman Show. It seems to be a more parsimonious explanation of the available facts that we're looking at the result of a super duper planatarium projector, as opposed to an actual vast and frequently inexplicable universe.
If you're going in that direction, you may as well say that the universe is a simulation. Planck limits on measuring time and space as a continuum would pretty useful for putting a hard cap on required rendering resources per volume of space-time...
Simulation doesn’t have to obey the rules. For non-scientists it is enough to “tell” them how it works without actually rendering it, cause they’ll never check. For scientists, you have to make their brain believe that all is consistent with complex computations all the way down.
Imagine that you know physics and math very good. But then simulation suddenly fails due to segfault and you realize that all your knowledge was just a gibberish nonsense and your work sessions and discussions were dream-like experience. Otherwise it was just a pretty dumb 3d simulation slightly better than a modern AAA game.
Edit: I mean, a simulation argument opens a huge can of worms, if you consider the perspective of a lazy simulation developer them-self.
While a simulation doesn't have to obey the rules, any given simulation is likely to resemble a system that does adhere to rules, since a simulation is indeed simulating something else.
Even if some external observer can pause, rewind, intervene and violate rules, the reason a simulation exists is to model something else.
This "something else" will have some kind of rules, and the goal of the simulation will be similarity to the actual realm external to the simulation.
Meanwhile, a lazy replication of a model by an unmotivated author would likely impose finite quantities upon scales of interaction, since a simulation won't be able to recreate a real time version of something larger than the external reality itself.
We wouldn't notice a lagged simulation that tries to consume its footprint, but it's probable that an artificial creation would impose caps on aspects of a system to prevent runaway reactions that produce useless simulations. To us, those sorts of limitations would resemble extra physical laws, and while incontrovertible to the simulated entity, such limits might confront intuition in strange ways.
Reminds me of https://en.m.wikipedia.org/wiki/Century_Rain
I was hoping for some weird time-bending effect from general relativity, but no, it’s just that the ages are within each other’s error bars.
Still an interesting read and thought experiment.
This reminds me of Iain Bank’s Excession. Great book, strongly recommend it.
The relevant bit from Excession is the discovery of a mysterious entity that is 50x older than the universe.
What's the explanation?
Without giving too much away, it's not from this universe.
You really should read it, it's one of the best books in the culture series and Banks was a phenomenal writer.
It also contains the Affront, a species that are almost comically awful and coined the term "outside context problem":
"The usual example given to illustrate an Outside Context Problem was imagining you were a tribe on a largish, fertile island; you'd tamed the land, invented the wheel or writing or whatever, the neighbors were cooperative or enslaved but at any rate peaceful and you were busy raising temples to yourself with all the excess productive capacity you had, you were in a position of near-absolute power and control which your hallowed ancestors could hardly have dreamed of and the whole situation was just running along nicely like a canoe on wet grass... when suddenly this bristling lump of iron appears sailless and trailing steam in the bay and these guys carrying long funny-looking sticks come ashore and announce you've just been discovered, you're all subjects of the Emperor now, he's keen on presents called tax and these bright-eyed holy men would like a word with your priests."
You left out the best bit!
"An Outside Context Problem was the sort of thing most civilisations encountered just one, and which they tended to encounter rather in the same way a sentence encountered a full stop."
Shades of Douglas Adams there...
Why can't the star be older because of relativity? I imagine that, particularly in the early universe while everything was nearer to everything else, relative background gravity was higher (and thus relative time) is different than now. Would the star not age at a different rate if it's a remnant from violent beginnings of the universe?
I think, if I understand your question correctly, that a star couldn't age more, only less, from our perspective.
EDIT: I could be wrong though, and additionally I believe the star being referenced by the article is actually in a similar reference frame so relativity likely isn't the answer anyways.
No, but we (our planet, stuff in our vicinity) could age less.
What we'd need is most stuff (that gave rise to most stars and most of our measures of the age of the universe) to be in a higher-gravity region compared to the star in question. It seems unlikely, but I suppose it's possible...
[Edit: I thought you were talking about GR time dilation due to gravity. A re-read shows that I may have been in error.
Also, for a star 190 ly away, any relativistic explanation, either special or general, is probably wrong.]
I don't think there's such a concept as "higher-gravity region" in modern physics, unless you're thinking about "in the vicinity of a giant star, black hole, or something equally massive."
However, if most of the known universe is sitting next to a gigantic mega-blackhole (or enough of them to cover the whole sky), then surely we would have noticed by now...
> I don't think there's such a concept as "higher-gravity region" in modern physics, unless you're thinking about "in the vicinity of a giant star, black hole, or something equally massive."
I think at the beginning of the universe, there might be a lot of regional discrepancies. They might even just be "temporary" regional discrepancies from gravitational waves due to the inherent nature of the universe growing and everything still crashing together.
The cosmic microwave background tells us there were practically no discrepancies at all, actually. Not too surprising, since at the densities back then even a small local fluctuation would easily have become a black hole.
The very early universe wasn't perfectly uniform, but it was really, really close.
The star would need to be in a _lighter_ background gravity than us, to appear older to us.
The most likely answer is that either the measurement or the theory is wrong. Both are probably about as likely.
It reminds me of the same question asked in this book, "The Birth of Time: How Astronomers Measured the Age of the Universe" by John Gribbin. The bottom line was that the calculations always have some approximations, which can throws off a number by a large factor.
Did anyone read the article and get a feeling it was mostly just a fluff piece describing how we're refining our ability to tell the ages of stars?
Start over space theories all over again. Lets not build our assumptions on previous generation findings.
Only if we take path of how the previous geniuses went through we will have a flawless exploration. Instead of doing that, if we work on someone else's work, its like eating someone else's recipie and trying to remake the tase of it without understanding the ingredients in it.
Given the norm for question titles, the answer must be "It can't." Also simply based on what a universe is.
What we have are just different estimates for the age of our universe. And none of them involve simple measurement, obviously. So the challenge is finding the artifact(s) that generate the disagreement.
Does the microwave background come from the centre of the universe where the big bang was; and the light from stars on the far side, flying in the opposite direction to us, take longer to reach us than the microwave background does?
The Big Bang happened everywhere in the Universe, at once. Our CMBR is coming in from all directions, at all times, as more distant locations(eg further away from us at the start) finally reach us.
I would REALLY love to read this, but space.com is one of those asshole sites that wants to take a gigabyte of memory (I'm not joking) and wants a full 30 to 45 seconds of CPU just to load the page.
It loads almost instantly without javascript. I can understand not using adblocking but why are you complaining about loading times when not using umatrix or noscript?
crashed my mobile browser
What came first, the photon or the electron? Can a photon be emitted without a change in energy state?
I’m new here, so instead of answering questions is the protocol just to downvote?
Oftentimes. I gave you an upvote just because sometimes around here you will get downvoted because the gods of zeitgeist decide it must be so.
wow what an interesting article. Finding not one, but multiple stars that are much older than the predicted age of the universe means something is likely very wrong with one of our theories: dark energy, star aging, or using the cosmic background as a measure of the universe's age.
I was thinking maybe it was traveling a large fraction of c and there was a relativity thing going on
HN is making me wish I studied astronomy, such cool stuff.
Nonsense, the universe is only 6000 years old....
and it's a Libra.
Okay... This implies the light was stretched along with the space, right? Because otherwise, the photons would just... Never reach here if space between us and the light expanded faster than c.
Correct. See...
https://en.wikipedia.org/wiki/Horizon_problem
Which universe?
Tl; dr: They don't know either
> It's composition meant the star must have come into being before iron became commonplace.
Hypothesis 1: the star formed in a way that did not involve or produce a lot of iron.
Hypothesis 2: the star is older than the universe itself.
Clearly, hypothesis 2 is the more likely.A
Jesus, what an aggresive website: https://i.imgur.com/1aCo9DV.png.
A question - how common are such old stars? Because it is right in our backyard. 190 light years away are nothing. And if those stars are rare - that makes our neighborhood unusual - we have life and some of the oldest object in the universe in almost the same spot.
Almost everything of interest essentially happens in our backyard because that's the place we can see with the most detail. It's a little like looking out at the Universe and coming to the conclusion that we're in the middle of it because the "edges" are all around us.
Even if this was rare and there happened to be tonnes of very old stars only close to us, this could again be coincidental, especially if intelligent life is more common than we currently know.
This is precisely true, only very small stars last very long, and small stars are very dim. All the old stars that we see must be very close. The bright stars that we see that are far away only live for a few million years.
190 light years is in our backyard compared to the size of the universe, but I was curious about how many stars are that close to us. It turns out that we have around 100k stars within 190 light years.
0.120 stars/parsec^3 * 4/3 * pi (190 ly * parsec/3.26ly)^3
https://en.wikipedia.org/wiki/Stellar_population#Population_...
"Thus far, they have uncovered and studied in detail about ten ultra metal poor (UMP) stars"
https://en.wikipedia.org/wiki/List_of_oldest_stars
Here we go again... "scientists would look at the ripples in the fabric of space and time". Am I the only one who finds this expression very confusing? What fabric? There is no fabric. Space and time are coordinates. They do not exist physically at all. There cannot be a fabric of space and time.
It's a metaphor to help humans grasp it better. Space and time does exists physically.
Does a shortest path between you and your workplace exists that you can traverse? Is it a straight line? No, it's not, but there's one path that you as a being can walk along physically. The buildings, roads, obstacles force you to diverge from the ideal straight line. That's another analogy of what space & time is. Fabric is just shorter.
Are you sure that's how general relativity sees spacetime? What about graviational waves and vacuum energy?
> On 11 February 2016, the LIGO and Virgo Scientific Collaboration announced they had made the first direct observation of gravitational waves.
https://en.wikipedia.org/wiki/Gravitational_wave
https://en.wikipedia.org/wiki/Spacetime
Well, as farbas I understand quantum loop gravity, there is a fabric of space and time
Twists and turns like a fabric, waves like a fabric...