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Question on the universe

  1. Jun 30, 2011 #1
  2. jcsd
  3. Jun 30, 2011 #2
    Yes it will, and I already answered this in the previous 5 or so repeats :biggrin:

    The big question is, will it be different.
     
  4. Jul 1, 2011 #3

    Chalnoth

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    I'm exceedingly skeptical of cyclic universe models. First, they definitely seem to be at odds with our current observations, in that it doesn't look like our universe will ever collapse in on itself. Second, in their most naive incarnations, they completely violate our understanding of entropy.

    I'm much more partial to ideas related to quantum fluctuations leading to the births of new universes.
     
  5. Jul 1, 2011 #4

    bapowell

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    There is no evidence that supports this comment.
     
  6. Jul 1, 2011 #5
    May be it's like this ,the universe is expanding from an infinitely small size to an infinitely large size and there was no beginning and there will be no end. i mean may be the big bang is still happening and we are all inside it and it will be happening!!! can this be true?
     
  7. Jul 1, 2011 #6

    Chalnoth

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    Basically, if you have any matter or radiation in the universe at all, then extrapolating back into the past, General Relativity says that everything had to be concentrated at one point sometime in the finite past.

    Though we don't expect that General Relativity is completely accurate here, this does say that something interesting had to occur to produce our region of space-time at some point in the finite past. The ideas for what that could be are rather varied, but it can't have simply been a universe that has been expanding smoothly forever.
     
  8. Jul 1, 2011 #7
    There is no evidence to refute it, either. The point is, how can we know. And if there is repeating, does it repeat exactly, or are we just referring to the theory that space will eventually collapse and bang out again (and are those 2 concepts even different things). If it repeats exactly as before, how can we ever possibly know. And what happens to the cone of influence. At this point, it's all wild conjecture and fun.
     
  9. Jul 1, 2011 #8

    Ryan_m_b

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    It is wild conjecture and it is neither fun nor the right way to go about things. Claiming "there's no evidence to refute this!" before speculating whatever you want is not appropriate. This is a science forum, speculation should be based on evidence and current understanding.

    As has been highlighted there is no good evidence that the universe will eventually experience a http://en.wikipedia.org/wiki/Big_Crunch" [Broken]. Discussing the available evidence for and against is perfectly reasonable, assuming one to be true then discussing how it could work is fallacious as it puts the cart before the horse. Without knowing the mechanism by which something could occur it is unreasonable to merely assume a mechanism.
     
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  10. Jul 1, 2011 #9
    There are 2 major problems with the cyclical Universe cosmological model.

    Firsty, the model assumes a classical spacetime contractionary preiod - current observations rule out a contracting Universe now, and at any point in the future. Expansion continues until final entropy and heat death.
    Secondly, A cyclical classical spacetime does not resolve the singularity, and as far as I am aware does not contain any modification to the EFE that results in a "cycle" and allows unification of QG and classical GR - theory still breaks down at the planck scale.

    In my humble opinion LQC is a far more elegant solution, this assumes we are on a current "bounce" from a previously contracting classical spacetime or from random quantum fluctuations. For me LQC is a more elegant solution as the quantum effect of gravity can modify classical GR to cause a "bounce" without need for the current "bounce" entering a future contraction period. Thus LQC requires less caveats and is more observationally accurate than a cyclical theory.
     
  11. Jul 1, 2011 #10

    Chalnoth

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    Except for the fact that the vast, vast majority of unevidenced assertions are wrong. Every once in a while you may come across a correct one, but those instances are quite rare.
     
  12. Jul 1, 2011 #11

    bapowell

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    Perhaps. But you said "Yes it will" happen, and I'm simply pointing out that there is no evidence to support this assertion. If there is no evidence pointing definitively in either direction, then we simply don't know the answer. Why not just say that?
     
  13. Jul 1, 2011 #12

    bcrowell

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    Penrose's CCC doesn't require a recontraction.
     
  14. Jul 1, 2011 #13
    Ah yes. I am not overly familiar with this cosmological model so I will look further into this.

    Many thanks for the information, do you happen to have any Arxiv or similar references? If not I will search for the relevant material.
     
  15. Jul 1, 2011 #14

    Chalnoth

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    True, but Penrose's CCC also has no physical justification. It's an idea without any physical mechanism whatsoever that suggests it is in any way plausible, and what we know about quantum gravity (that horizons have actual, physical sizes that have real meaning) strongly indicates that it isn't possible.
     
  16. Jul 1, 2011 #15

    Chalnoth

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    I wouldn't recommend it. As ideas go, this one belongs in the crackpot bin. To give you a rough idea of just how bad CCC is, in order to claim evidential support for it by looking at the CMB, Penrose and his co-author, Gurzadyan, mistook the word "random" to mean "uncorrelated". It turns out that the correlations on the CMB are what the entire field of CMB science is all about, and not knowing about statistical correlations is just plain sad.
     
  17. Jul 1, 2011 #16

    bcrowell

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    He wrote a popular-level book about it: Cycles of Time: An Extraordinary New View of the Universe. I learned a lot from the book that was interesting and that remains important and valid regardless of whether CCC is right.
     
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