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Is the Universe circular?

  1. May 15, 2012 #1
    I am assuming the answer to this is no but having view to the balloon analogy in the neighbouring thread (that I don't want to dilute with this probably trivial question by joining)
    I just thought I might ask whether a photon (or any other massless particle which seem to move forever at a continuous speed unless they meet an obstacle) might eventually end up where it had started from?

    Otherwise stated would we see the backs of our own neck if we looked far and well enough?

    I thought of this when I looked at the balloon diagram but I assume that this is just one of the misperceptions that the balloon analogy can give rise to (I know we have been told repeatedly to view it with a pinch of salt).
     
  2. jcsd
  3. May 15, 2012 #2
    I don't know if 'circular' is the proper term to use.

    Yes, there is a good possibility the universe is finite. The same chance that it is infinite, there is no reason to suppose either is correct. However, it most definitely does NOT have a boundary if it is finite.

    It's actually the whole purpose of the balloon analogy - to show that the universe can expand without having any edge or center.
     
  4. May 15, 2012 #3
    How can it be infinite if it had a start ~13.7 billion years ago?
     
  5. May 15, 2012 #4
    Yes, I think here you are trying to take the balloon analogy beyond its scope, the balloon analogy is mostly to show how something with no edge or center can expand causing arbitrary points on the balloon to move away from any other arbitrary point.

    This one is easy, there is no proof either way, but to be infinite the universe simply had to be infinite at the start.
     
  6. May 15, 2012 #5
    As Some Slacker pointed out, it could have began infinite. Remember that the expansion of the universe is a metric expansion - the universe expands from every point.
     
  7. May 15, 2012 #6
    An infinite universe coming out from nothing simultaneously all at once ?
    (simultaneously = in some billion years)

    If the universe came out from nothing with a "diameter" other than zero, then even before there was a metric in the nothingness.
    But how can nothingness have a metric ?

    (Scary to imagine)
     
  8. May 15, 2012 #7
    Considering we don't know what kind of physics was obeyed at the Planck Time and before, the most we can, as of now, speak about factually, is the universe after that time. So whatever went on then, it is possible (and preferred by a large number of cosmologists) that the universe was infinite in size.
     
  9. May 15, 2012 #8
    In the absence of dark energy, in a closed universe, I believe the photon would get exactly half way round in the time between the big bang and the big crunch. Two photons launched in opposite directions would just meet up as the universe ended. :-(
     
  10. May 15, 2012 #9

    Drakkith

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    Staff: Mentor

    If the universe is infinite in spatial extent, aka size, that does not require that it is also exist for an infinite amount of time.

    We don't know how the universe was formed or whether it is infinite or finite. We do know that the OBSERVABLE universe was packed into a very very tiny volume, but this is ok because the observable universe is definitely not infinite.
     
  11. May 16, 2012 #10
    Is anyone else of that opinion? Since my question was based on an apparently erroneous interpretation of the balloon analogy would you be coming to that supposition on a different basis?
    Maybe it wasn't meant seriously (it wouldn't be hard to go over my head)
     
  12. May 16, 2012 #11
    It is something I remember from many years ago but there may be a subtlety I missed, see the post from pervect in this old thread:

    https://www.physicsforums.com/showthread.php?t=42599
     
  13. May 16, 2012 #12
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