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Is there a problem in assuming the universe has a boundary?

  1. Aug 25, 2012 #1
    I hear about the balloon analogy, and that there is no need to say that the universe has a boundary, but is that the only reason or would it be problematic to assume that space-time has a volume and a boundary?
     
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  3. Aug 25, 2012 #2

    Drakkith

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    When we get down to it the theory and models don't care whether there is a boundary or not for the most part. (If there is it MUST be significantly further away than we can see, otherwise it would be noticeable)

    IF the universe is finite in size, then it is very likely that it is "boundless", which just means that you can keep going forever in a direction, but you will keep passing by the same places, like going off the screen on an old arcade game only to come back in from the other side.
     
  4. Aug 25, 2012 #3
    thanks for the quick reply :)
     
  5. Aug 25, 2012 #4

    HallsofIvy

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    An interesting question would be- what would the boundary be made of?
     
  6. Aug 25, 2012 #5

    phinds

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    What would happen to physics as we know it at the boundary? How could our physics possibly deal with an "edge" to the universe? That of course does not PROVE the lack of a boundary, but you asked if it would be problematic. How about a total breakdown of all of our known cosmology? You lose isomorphism, you lose isotropy ... basically you pretty much lose your mind trying to figure out how do model a boundary.
     
  7. Aug 25, 2012 #6

    bcrowell

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    Since we don't see any evidence for a boundary, we have no evidence on which to base any theory of how physics would work at a boundary.
     
  8. Aug 27, 2012 #7
    It would be problematic because we don't see anything observationally that looks like a boundary. If there were a boundary then it would be so far away that we wouldn't see any evidence for it, and do for the purpose of doing calculations, it's assumed that the boundary is irrelevant.
     
  9. Aug 27, 2012 #8
    As a theorist, I don't think it would be that difficult to come up with something. If we actually did come up with something that looked like a boundary to the universe, you'd just look at it and see what happens there.
     
  10. Aug 27, 2012 #9

    turbo

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    There could be some kind of a boundary, but we can't see it. Once we have an orbiting telescope that pushes farther into the IR, I expect that we will see further "back" in terms of BB cosmology, but I don't expect to see an edge. Any boundary at this point would probably be an artifact of our inability to detect fainter and fainter objects. I could be wrong about this, but I doubt it.
     
    Last edited: Aug 27, 2012
  11. Aug 27, 2012 #10
    But wouldn't having a boundary be in conflict with the isotropy, ie. the assumption/measurement (which is it?), that the relative velocity of matter would look the same any where in the universe and that there is no center of the universe?
     
  12. Aug 27, 2012 #11

    Drakkith

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    If it does, and we find a boundary, then we'd have to re-examine our ideas about the universe.
     
  13. Aug 27, 2012 #12
    we may lose our minds but it'll sure be exciting! :biggrin:
     
  14. Aug 27, 2012 #13
    The expansion of space causes a natural boundary.

    Any objects separated in space by a distance that the expansion between them exceeds speed c then they become causally disconnected. In short our visible universe is surrounded by a collapsing event horizon beyond which the known laws of nature cannot communicate.
     
  15. Aug 27, 2012 #14

    phinds

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    I don't think that's what the OP was asking about at all. I'm not arguing w/ your statement, just saying that it is irrelevant to this discussion.
     
  16. Aug 27, 2012 #15

    bcrowell

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    Yes, "boundary" has a technical definition here http://en.wikipedia.org/wiki/Manifold_with_boundary#Manifold_with_boundary , and whether the OP knew it or not, #1 was using the term in a way that was consistent with that definition.
     
  17. Aug 27, 2012 #16

    phinds

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    Yes, that definition is consistent w/ how I thought about the term, but I don't see how the event horizon of our observable universe can be what the OP had in mind, as QuantumHop seems to think.

    Again, I was not arguing at all w/ what QuantumHop was saying, just about the relevance to the OPs question, which I took to be a question about the "whole" universe, not the observable universe.
     
  18. Aug 27, 2012 #17

    rbj

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    stretchy latex.
     
  19. Aug 27, 2012 #18

    Friedmann–Lemaître–Robertson–Walker cosmology requieres a connected universe hence a non-trivial topology.
     
  20. Aug 28, 2012 #19
    Would I be right in thinking that a boundary to the universe is an absolute, and that physics does not deal with absolutes.
     
  21. Aug 28, 2012 #20
    When people refer to the balloon analogy, they are actually making a dimensional analogy. The surface of the balloon is a two dimensional plane expanding in a three dimensional way, which is meant to represent the three dimensional space expanding in a four dimensional way, if that makes any sense.
     
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