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Did the universe have a beginning?

  1. May 2, 2012 #1
    That's title of the paper i've linked below. It's small paper that argues against an eternal universe.
    http://arxiv.org/pdf/1204.4658v1.pdf

    I'd like some opinions on the paper. It seems interesting. although the argument for the geodesic incompleteness of the eternal inflation model is not new to me, the concept of the emergent universe is new to me
     
  2. jcsd
  3. May 2, 2012 #2
    An assumption in this paper is that entropy always increases through cycles.
    However I believe there are several papers that challenge that asusmption. Penrose argues entropy is erased in his CCC model.

    I think Bojowald
    and Baum/Frampton argue something similar. It seems the Caroll/Chen model also is not discussed
    I would very much like the authors to dsicuss these models and create a dialogue between the authors. Although in all hosently I doubt how any of this could ever be tested. But who knows maybe we will be pleasatnly suprised.
    Some references:
    Bojowald paper
    http://arxiv.org/abs/0803.4484
    Penrose paper
    http://accelconf.web.cern.ch/accelconf/e06/PAPERS/THESPA01.PDF

    Caroll Chen
    http://arxiv.org/abs/hepth/0410270

    Baum Frampton
    http://prl.aps.org/abstract/PRL/v98/i7/e071301
     
  4. May 3, 2012 #3
    I find the results concerning emergent universes very plausible. If you think about it in a hand-waving way, if you have infinite time to quantum tunnel, the odds for it are quite good (one) even if the probability of a single event is very low (but not identically zero).
     
  5. May 3, 2012 #4
    It's a minisuperspace based approach:

    basically you pick one degree of freedom and do all your quantization based on that. In this (and Hawking's) case they pick the radius of the universe. The problem with holding the other degrees of freedom fixed is that this implies the uncertainties in the variables *and their conjugates* are zero, which is problematic (think HUP).

    Now it's not surprising they have to do this - it's a difficult problem after all! However, it should be treated as only a toy model which may contain some *hints* of the right approach
     
  6. May 3, 2012 #5

    phinds

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    Uh ... what is the radius of the universe ???
     
  7. May 3, 2012 #6
    Different plausible models emerge from different assumptions.
    Somebody will have to determine actual initial conditions to decide which model fits.....if any.

    I've read Steinhardt and Turok's 'Endless Universe' which explains their cyclic model...seems as likely, but not as popular, as a big bang scenario.
     
  8. May 3, 2012 #7
    Sorry I meant the scale factor "a" that multiplies "r" in the FRW model
     
  9. May 4, 2012 #8
    The big bang is supported by many independent lines of evidence., Turok and Steinhardt's model is not. However to be fair to them they are not seeking to replace the big bang. When the branes clash in their model you get a big bang, its just re interpteting what the big bang means, not replacing it.
     
  10. May 4, 2012 #9
    My problem with their analysis of what they call the 'emergent universe' scenario is that they claim it must be static. This usually isn't the case, it's perfectly viable that a high entropy de Sitter space could used (that is, a space with a positive cosmological constant).

    IMO, the big problem with cosmological models that have a an absolute beginning is the assumption that entropy was extraordinarily low - unless you allow for a future low entropy boundary condition, it seems rather unwarranted to assume such fine-tuning. And that scenario seems unlikely, as modern cosmology holds that the universe will expand forever.
     
  11. May 6, 2012 #10
    sky:
    According to the authors, there is no evidence for or against their model yet. Their model is a cyclic model with a finite bang....very different from a 'one of a kind big bang' where any current model breaks down. It is, according to them, constent with all observational evidence [as is the big bang inflationary model] except with regard to tiny, difficult to observe, CMBR fluctuation characteristics.

    Can you post any such 'line of evidence' or a contrary source that contradicts their model?? I'd be interested to see it. I have previously posted about the observational 'evidence' they expect to be forthcoming that will confirm or refute their model....If you are interested I'll see if I can find it.

    edit: no I won't...seems like earlier discussions have all been locked...presumably some officials here find cyclic models 'speculative'....
     
    Last edited: May 6, 2012
  12. May 7, 2012 #11
    According to numerous researches carried on in cosmology, the Universe began with a huge Big Bang.Since then the hot gases and matter began expanding and time and space were created.Different studies also indicate that the expansion of universe is continued in the shape of a balloon.
     
  13. May 7, 2012 #12

    Chronos

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    Well, even cosmologist admit everything about the big band is pure speculation.
    Comparing different versions of pure speculation is not very scientific.
     
  14. May 7, 2012 #13

    phinds

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    This is totally contrary to everything I've ever heard or read. Can you site any references for this claim that the "big bang" was a point-source, which it would have to be if the ensuing expansion was spherical.

    I think you have misunderstood something you read somewhere.
     
  15. May 9, 2012 #14
    NO Im not aware of any evidence that contradicts their model. But thats not really the point, they have to shopw their model is correct, the burden of proof is on them. As I udnerstand it they predict a weaker gravitational wave spectrum and one that is not scale invariant. Inflation predicts scale invaraince. But it will probably be a very long time before we get this data. Unless PLanck throws us a suprise that is.
     
  16. May 9, 2012 #15
    Even if the theory does not make any strong positive predictions, that doesn't mean it's necessarily wrong. I understood that the GW spectrum they predict is so small it's virtually unobservable with current technology. So you could rule out the model by measuring a large r, but finding positive confirmation is far into the future.
     
  17. May 9, 2012 #16
    Im not claiming its neccassarily wrong. Im not saying it should be taken off the table as one of many interesting ideas. Im just saying that if one wants to graduate from interesting idea to standard model of cosmology were going to need some evidence. That evidence would be more forthcoming if we had a grav wave spectrum, but right now we dont.
     
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