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B How large might the Universe be

  1. Oct 2, 2017 #1
    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?
     
  2. jcsd
  3. Oct 2, 2017 #2

    jedishrfu

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  4. Oct 2, 2017 #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.
     
  5. Oct 2, 2017 #4

    kimbyd

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    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.
     
  6. Oct 2, 2017 #5

    phinds

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    @PeterDonis , post 26 in https://www.physicsforums.com/threads/is-the-universe-infinite-or-not.898676/page-2 says:
    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.
     
  7. Oct 2, 2017 #6

    jedishrfu

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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.
     
  8. Oct 2, 2017 #7
    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.
     
  9. Oct 2, 2017 #8

    phinds

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    upload_2017-10-2_20-46-38.png
     
  10. Nov 3, 2017 #9

    Drakkith

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    Indeed. It may be infinite in size. Or it might not be. It is likely that we will never know.
     
  11. Nov 3, 2017 #10
    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.
     
  12. Nov 3, 2017 #11

    Drakkith

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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.
     
  13. Nov 6, 2017 #12

    kimbyd

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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.
     
  14. Nov 6, 2017 #13
    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 ;-).
     
  15. Nov 6, 2017 #14

    phinds

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    But isn't it inconsistent with known physics? It assumes "boundary conditions" not in evidence.
     
  16. Nov 6, 2017 #15
    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.
     
  17. Nov 6, 2017 #16

    kimbyd

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    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.
     
  18. Nov 6, 2017 #17

    phinds

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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.
     
  19. Nov 6, 2017 #18

    kimbyd

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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.
     
  20. Nov 6, 2017 #19

    phinds

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    What is "PBC" ?
     
  21. Nov 6, 2017 #20

    kimbyd

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    Periodic boundary conditions.
     
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