How large might the Universe be

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Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?
 

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  • #3
phyzguy
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I think it is not possible to say. Since only the observable universe is observable, it is impossible for us to have "indications" or any other information on anything which is not part of the observable universe.
 
  • #4
kimbyd
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Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?
The only indications that exist are very indirect. For example, if we measure the spatial curvature of our observable universe, and extrapolate that curvature outward, we get a universe that is at least 250 times as large as the observable universe. But that's an extrapolation: there are other effects that could lead a flat universe to be smaller. For example, if our universe were torus-shaped, then it could be only a little bigger than the observable universe but still be flat. It would still have to be bigger than the observable universe because the torus-shape would leave evidence would observe otherwise. Or it could just be that our observable universe is unusually flat, and the rest of the universe is more tightly-curved, leading to a smaller overall size.

In the end, we just can't be sure. The universe is probably much bigger than the part of it we can observe, but nobody can say just how much bigger.
 
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  • #5
phinds
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Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?

@PeterDonis , post 26 in https://www.physicsforums.com/threads/is-the-universe-infinite-or-not.898676/page-2 says:
Then all your questions boil down to one: what, if any, evidence do we have that bears on the question of whether the universe is infinite or not? The answer to that is that we have plenty of evidence that constrains the parameter space of the standard hot big bang model of cosmology to a pretty narrow range, a range which makes it very unlikely, based on that model, that the universe is spatially finite, and very likely that it is spatially infinite. AFAIK there are no other models in serious contention with the standard hot big bang model in this area.

Peter has made similar statements in other threads, e.g. https://www.physicsforums.com/threads/infinite-versus-finite-space.924230/#post-5832649

And by the way @Cato obvious questions like this one have been asked here hundreds if not thousands of times. A forum search is always the best place to start for such questions.
 
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Adding to what @phinds has said, if you look at the end of this page you’ll see links to other related PF pages that may address your question.

See similar discussion links below.
 
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And by the way [USER=469185]@Cato obvious questions like this one have been asked here hundreds if not thousands of times. A forum search is always the best place to start for such questions.[/USER]
Sounds like this is a good candidate for the Astronomy and Cosmology FAQ (which is actually hidden in the Astronomy forum). Even when these questions are repetative, I always enjoy reading interesting and fresh perspectives like the one from @kimbyd.
 
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phinds
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Sounds like this is a good candidate for the Astronomy and Cosmology FAQ (which is actually hidden in the Astronomy forum). Even when these questions are repetative, I always enjoy reading interesting and fresh perspectives like the one from @kimbyd.
upload_2017-10-2_20-46-38.png
 
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  • #9
Drakkith
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Well, I guess this is one of the unanswered question till now. How big is the universe is really very difficult to know. It may be so big that even light has not had that much time to cross it in nearly 14 billion years and it is still getting bigger all the time.

Indeed. It may be infinite in size. Or it might not be. It is likely that we will never know.
 
  • #10
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It may in fact even be much smaller than what we think. If the universe is flat with a periodic boundary in each direction, most of the galaxies would be duplicates of other galaxies but seen from another direction and at another time.
 
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Drakkith
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It may in fact even be much smaller than what we think. If the universe is flat with a periodic boundary in each direction, most of the galaxies would be duplicates of other galaxies but seen from another direction and at another time.

Sure. But I think we've looked and so far haven't found any patterns that would be suggest this as being likely.
 
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kimbyd
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Sure. But I think we've looked and so far haven't found any patterns that would be suggest this as being likely.
Yup. Repetition would produce a pretty clear signal on the CMB. If our universe does wrap around on itself, it must do so outside of the observable universe. So it could be smaller, if that were the case, than the upper limit mentioned earlier in the thread of 250 times larger than the observable universe. But it's definitely larger than the observable.
 
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Yup. Repetition would produce a pretty clear signal on the CMB. If our universe does wrap around on itself, it must do so outside of the observable universe. So it could be smaller, if that were the case, than the upper limit mentioned earlier in the thread of 250 times larger than the observable universe. But it's definitely larger than the observable.

Not necessarily. The universe can be an expanding box of flat space with periodic boundary conditions. Such a scenario is clearly not inconsistent. Just think of Pacman ;-).
 
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phinds
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Not necessarily. The universe can be an expanding box of flat space with periodic boundary conditions. Such a scenario is clearly not inconsistent. Just think of Pacman ;-).
But isn't it inconsistent with known physics? It assumes "boundary conditions" not in evidence.
 
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But isn't it inconsistent with known physics? It assumes "boundary conditions" not in evidence.

Not really. It's true that there's no evidence any cyclic repetition in space but there is also no evidence against it, at least if the scale is sufficiently large. The CMB only tells us that the large scale curvature is consistent with zero.
 
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kimbyd
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Not necessarily. The universe can be an expanding box of flat space with periodic boundary conditions. Such a scenario is clearly not inconsistent. Just think of Pacman ;-).
That's more or less the scenario I was thinking of. This is the toroidal universe. It still has to be larger than the observable universe or else there are large effects on the CMB.
 
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phinds
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Not really. It's true that there's no evidence any cyclic repetition in space but there is also no evidence against it, at least if the scale is sufficiently large. The CMB only tells us that the large scale curvature is consistent with zero.
Well, the "evidence against" is just that it would require new physics at the boundaries and there's just no reason to make such an "out of left field" assumption.
 
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kimbyd
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Well, the "evidence against" is just that it would require new physics at the boundaries and there's just no reason to make such an "out of left field" assumption.
I'm not sure that's entirely true. PBC is basically just a toroidal topology. I don't think you need any new physics to describe our universe using a toroidal topology, as General Relativity allows this possibility, but it would raise questions of how such a topology could have been generated when our observable universe first formed.
 
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phinds
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I'm not sure that's entirely true. PBC is basically just a toroidal topology. I don't think you need any new physics to describe our universe using a toroidal topology, as General Relativity allows this possibility, but it would raise questions of how such a topology could have been generated when our observable universe first formed.
What is "PBC" ?
 
  • #20
kimbyd
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Periodic boundary conditions.
 
  • #21
phinds
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@PeterDonis is this "periodic boundary" model viable? (start w/ post #13). It doesn't seem so to me but I never let a little thing like total ignorance get in the way of having an opinion.
 
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PeterDonis
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is this "periodic boundary" model viable?

It depends on what you mean by "viable". :wink:

If you mean, is it mathematically consistent, yes, it is.

If you mean, is it consistent with the data we have up to now, yes, it is, but only if the periodicity is very, very much larger than the size of our observable universe. (This is similar to the way that a model with positive spatial curvature is consistent with the data we have up to now, but only if the curvature is very, very small, so that the spatial volume of the 3-sphere universe now is very, very much larger than the size of our observable universe.)

If you mean, is it likely to be true, that's a matter of opinion. I personally think it's unlikely; it seems to me like an ad hoc suggestion which, while mathematically consistent, doesn't have any real physical principle to back it up. But as I said, that's just an opinion. We don't anticipate having any way of testing the matter experimentally any time soon.
 
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phinds
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Thanks.
 

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