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Could pre-big-bang evidence survive the crunch?

  1. Jan 5, 2013 #1


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    I understand that there are singularity and non-singularity theories of the big bang. Also that there are those that posit nothing (no time, no space) before the big bang and others that posit something.

    What I'm thinking is that if the big bang event was singular (or nearly singular) then it would have destroyed (nor nearly destroyed) any remnants, any evidence of the pre-big-bang universe. In other words, the entropy of the universe was (nearly) zero at time zero, and not even a single bit of information (something or nothing) could have survived the event.

    If so, then all pre-big bang theories would by definition be not testable, and therefore a scientific waste of time.

    Here are my questions.

    The second law says that the entropy of the universe must increase, does that imply that the entropy of the universe was zero at a singular time zero?

    Do the pre-big-bang theories allow calculation of the upper limit of entropy in the information theory sense, at the moment of maximum crunch (singular or not)?

    I guess my ultimate question is, "Is it likely that pre-big-bang theories will ever be testable?"
  2. jcsd
  3. Jan 5, 2013 #2
    Ive read an article recently that they looked for evidence of the bubble universe theory by examining the cmb for signs of a collision. They found no indication. As far as other tests I cannot answer.
    Last edited: Jan 5, 2013
  4. Jan 5, 2013 #3
    I seem to remember there's one cyclic model where information gets transferred between each cycle, but I can't remember the name.
  5. Jan 5, 2013 #4


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    It is unclear how low the entropy of the universe was at the time of the big bang. There is nothing I am aware of that inists it be zero. Here is a related paper - Inflation as a Solution to the Early Universe Entropy Problem? http://arxiv.org/abs/1212.1087
  6. Jan 5, 2013 #5
    anorlunda...seems quite likely the big bang was not so singular is is popularly thought...

    here is one discussion right now:

    Seems like LQC...cosmology not LQG gravity might have an approach....


    Short answer is that some evidence MIGHT survive...nobody knows for sure....

    The Turok Steinhardt cyclic model has something that might be detectable....I forget the details...

    In another, it was posited at least some black holes would survive a big crunch....
  7. Jan 6, 2013 #6
    There are a lot of different models that posit a pre big bang universe and also posit some testable signature of this that we might yet find.

    Heres a selection , the link in each case talks about some empirical test of the model in question.

    1)Loop quantum cosmology , suggests our universe bounced from a previous universe that collapsed. Proposed test: look for corrections to the B mode polarisation of the CMB:

    2) eternal inflation suggests our big bang is one of many in an never ending inflationary process . Proposed test: look for potential signatures of bubble collision in the CMB:

    3 Ekpyrotic Cyclic Cosmology: suggests the big bang was caused the collisions of two branes in a higher dimensional space known as the bulk . Proposed test; look for non guassianity in the CMb power spectrum:

    4 Conformal Cyclic Cosmology:
    Suggests the universe rescales itself to zero size once all matter has decayed to radiation. Proposed test , look for ring structures in the CMb, a consequence of black holes collidiing in the previous universe

    Theres are more than these four but hopefully thats enough to be going on with for the moment.
  8. Jan 6, 2013 #7


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    There are technical problems with defining thermodynamic concepts like temperature and entropy over the course of a bounce---they may be scale-dependent or observer-dependent. A very small observer may see a different temperature from that seen by a large observer, or different quantities of energy etc.

    In skydive's #1 case, stuff from the prior collapsing phase certainly does come thru the bounce, and has an effect. It's an exciting line of research and people are getting new results. There is a new paper by Wilson-Ewing about the effect of the matter that comes thru, that I think will turn out to be important. I put it in the current MIP poll https://www.physicsforums.com/showthread.php?t=661347

    Visible effects of bounce have been predicted. There are many papers, some are by Barrau and Grain. You might look up their papers on arxiv. There are references in the Wilson-Ewing paper in case you want more.

    It's a fastmoving field of research and many of the key issues are not finally SETTLED. You can find a kind of majority consensus among LQC (loop cosmology) researchers emerging but there is still not complete agreement about what features new CmB probes should look for. At least that I know of.

    The main thing is nobody expects the LQC to produce a blank slate. There should be a footprint in the spectrum of fluctuations that should still be detectable in the CMB (ancient light released by hot gas many thousands of years later, long after start of expansion).

    I hope that's right, and that closer examination of the ancient light does reveal such an imprint. Certainly would be cool if it did.

    Not sure what you mean by "near singular". A singularity is where equations blow up and give infinities that are meaningless---it is the failure point of a manmade equation.
    What does it mean to be NEAR infinite density? How do you measure nearness to infinite density? I can picture an infinitely long line or an infinite volume, but I cannot imagine infinite density or curvature. The conventional interpretation is that it's a symptom of a failed equation, that needs fixing or replacement. So "near singular" means near technical failure of some particular equation (vintage 1915 classic GR in this case.)

    In LQC bounce, the density gets very high but remains finite. Doesn't fail. It will be interesting to learn more about the behavior of matter and geometry at these extreme densities. It already seems (in the LQC analysis) that gravity becomes universally repellent, instead of attracting. Stuff repels other stuff, highly curved spots like BH tend to flatten out, at extreme density. According to this type of model, these equations. It's peculiar, has to be verified, checked by observation (will they find predicted effects in CMB?)
    The era of very high but still finite density is, of course, very brief. There is a rebound and very quickly you are back to lower density and more normal classical behavior.

    If you know the Friedmann equation you can see the quantum-corrected version (e.g. in Wilson-Ewing's paper) and get an idea of how high density has to be for the quantum effects to dominate.
    Last edited: Jan 6, 2013
  9. Jan 6, 2013 #8


    Staff: Mentor

    Thanks for the informative replies.

    I was thinking in terms of information when I said near-singular and nearly infinite. I thought that even if it might be theoretically possible for some information to survive the crunch, that so much information would be destroyed, that practical chances of seeing a signature today would be nil.

    I guess my question had more to do with optimism/pessimism than physics. It would be very frustrating to see something like LQC theory be developed and then find that it can't be confirmed by observation.

    I also thought that the information content of the initial universe would be an interesting research topic by itself.

    Certainly science fiction writers could have fun with the idea of discovering a message sent by pre-crunch inhabitants of a predecessor universe.

    Based on the answers, it sounds like hope for observable evidence exists. Hooray for that.
  10. Jan 6, 2013 #9


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    There are two separate and basically unrelated questions: (1) was the big bang singular?, and (2) was the big bang low in entropy? In classical models based on GR, the big bang is singular (the Hawking singularity theorem requires this for realistic models), but it can be either low in entropy (as in FRW models and in our actual big bang) or high in entropy (with the gravitational degrees of freedom activated, as in Misner's mixmaster universe: http://en.wikipedia.org/wiki/Mixmaster_universe ). Even in models where the big bang was low in entropy, there's no particular reason to expect its entropy to be zero. If the universe is infinite, then I suppose its total entropy is infinite no matter what, although people may have in mind the entropy of whatever part of the big bang was within our own past light-cone.

    Tolman ruled out cyclical cosmologies on thermodynamic grounds in 1934. Penrose argues that thermodynamics also rules out a bounce, because the previous universe would have had infinite time to equilibrate, so gluing it on to the post-bounce universe results in a violation of the second law and a mismatch in the physical characteristics of the spacetime (sudden suppression of gravitational degrees of freedom). These arguments tell us that any model that includes a bounce has to include exotic physics of some kind that was not considered by Tolman or Penrose. Since we don't have any solid idea about how quantum gravity works, I suppose you can just invoke that and say it's so exotic that it evades these objections.
  11. Jan 6, 2013 #10
    Nice articles skydivephil.

    This topic reminds me of a time when I was around 15. I asked myself and a friend the following. " If everything must have a beginning and an end. What is the beginning of everything "
    The only answer we could come up with made no sense whatsoever.

    " Nothing plus nothing equals something "

    Like I said makes no sense. From what I see from various posts there are several probable tests we can derive from the Cmb. Ive heard multi media hype that due to the uncertainty principle that everything could have staryed from nothing but have not been able to reconsile that statement with what I understand on Heisenburg.
  12. Jan 7, 2013 #11
    Fortunately, the general trend of science has been that, over and over and over again, methodologies for finer levels of detail of experimental observation have been developed.

    Consider for example, Wolfgang Pauli on hypothesizing the neutrino:

  13. Jan 7, 2013 #12
    and of course there was Compte who said:
    "On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are ... necessarily denied to us. While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never be able by any means to study their chemical composition or their mineralogical structure ... Our knowledge concerning their gaseous envelopes is necessarily limited to their existence, size ... and refractive power, we shall not at all be able to determine their chemical composition or even their density... I regard any notion concerning the true mean temperature of the various stars as forever denied to us."
  14. Jan 7, 2013 #13


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    Both really beautiful quotes! :biggrin:
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