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Is Bojowald's Bounce unique?

  1. Sep 27, 2007 #1
    Is Bojowald’s Bounce unique? (In a sense that the BB’s bounce is the one and only).

    I foresee 3 possible answers:
    1) Yes. But IMO there is no evidence (theoretical or experimental) for this answer.
    2) Physics can’t say anything about it. But because of 3) also this answer is wrong.
    3) No, this seems to be the reasonable answer. Relativity taught us that we are not the centre of our planetary system, or of a cluster of stars, nor of our observable universe. So why should we be the centre of the universe. That would seem to be the same mistake the catholic church made before Copernique.
    The consequence of this reasoning IMO is that there might be an infinite number of bounces in many, many, "verses".
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  3. Sep 27, 2007 #2


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    I think you are invoking the Copernican Principle, also called the Principle of Mediocrity
    and it's a good idea to keep that in mind as a guide, it has worked so well in the past

    At present the bounce models are getting more detailed, and simplifying assumptions are being gradually relaxed. I suppose some of the Quantum Cosmology models that people are working on will eventually give rise to testable predictions. IF a QC model survives testing is a big IF.

    But I feel pretty optimistic about the bounce idea. It is simple and robust.

    So your idea is IF there was a bounce at the start of our spacetime's expansion,
    then it is probably not the ONLY such bounce
    (and Copernicus would tell us that.)

    To me what you are saying suggests two pictures:

    *A*. Something like the Smolin CNS picture, where some (if not all) black holes lead to a bounce and a new expanding region of spacetime.

    Expanding spacetime regions are processes which proliferate----the black hole plunge is the branch point. In the whole picture, time evolution is somewhat like a very branchy tree.

    If it were like that then what you say about bounce being common would be true. Bounce would be very usual.

    *B*. The other picture is linear----a series of bounces. A kind of necklace, or string of pearls.
    To fully apply the Copernican Principle, one would have to assume that our universe will eventually collapse.
    It could not be the LAST one in the sequence---the only one which is destined to expand indefinitely----because that would be a kind of uniqueness.

    But people would object to that because there is currently no evidence that our universe is ever going to totally collapse. It makes little black hole collapses all the time---but the whole region looks like it will expand indefinitely.

    So the Copernican Principle seems to favor picture *A*.

    However the picture *A* has some puzzles, like where does all the energy come from to make the new universe, when there is a black hole bounce? there are some tentative answers---inflation, and the Guth "free lunch", are invoked---but it is unsettled.
    Last edited: Sep 27, 2007
  4. Sep 28, 2007 #3
    Dear Marcus,

    Thank you for your immediate reaction.
    I am glad with this discussion and you might understand that I will continue with it.
    Let me start with two introducing remarks.
    1) If stuff (energy and the like) falls into a black-hole, then, according to Schwarzchild:
    - The B.H’s radius will grow
    - And its average density will slow down.
    If our “verse’ is a BH, then, no wonder, it expands in such a condition and certainly will do our observable universe, (which as a kernel could even more rapid expand than its BH’s radius). I know very well that there are cosmologists denying the possibility of our observable universe being part of a verse which is a BH. If have seen Mr. Baez article and I don’t agree with his argument.
    2) I.M.O. Space and time should (can!) physically not be seen as two separate entities.
    For me it is evident that many, maybe even most or all physical things can only be seen in their totality, time and space are then only mathematical parameters, but physically it was nor will ever be possible to discover what time and space really are.
    I.M.O. Space-time and energy-density are belonging together. If energy density locally diminishes then space locally expands.
    Space-time and energy-density belong to each others as e.g. a particle and its wave belong to each others. Thus I.M.O. physically don’t separate those things.

    Now coming back to your pictures *A* and *B*.
    First of all I understand that you agree with the only possibility of the many bounces?!
    But in your picture *A* I got the idea that you are still suggesting one starting bounce?
    Once (already several years ago) I have read L.Smolin’s “Life of the Cosmos”. By then I already could not accept the idea of all his nested verses in the original one. And indeed where should then all the energy for each new “verse” could come from?
    As we already agreed, in a previous discussion in your earlier thread about Bojowald’s bounce-theory, we don’t accept inflation theory nor does Smolin, as I have read in his excellent book “The trouble with physics”. For me the infinite universe might “contain’ an infinite amount of bounces which are continually coming up or resolving (Hawking radiation) more ore less. In that case also your picture *B* seems not necessary to me.
    Some ‘brainstorming”:
    By the way the life-time of a BH (i.e.) depends on its energy-content at its bounce. Maybe testability of mini and or micro BH’s with there respective bounces can come into vision when new particle accelerators like LHC are coming into use or by using new detectors gazing at the universe. What kind of devices really are nuclear bombs? It might be interesting to categorize and select suitable candidates with lifetimes we can handle. I fully agree with L.Smolin’s view that in many case physics is missing, or don’t investigate, phenomena which are just in front of us.
  5. Sep 28, 2007 #4
    In his Dark Side of a Patchwork Universe, Chapter 4, Conclusions, Bojowald says that further effort “would allow one to predict the far future of the universe.” Either “patch size remains nearly constant during expansion and there will be no dramatic changes in the future evolution.” or else “patch size increases with expansion & dark energy will die off.” However, he says, if “patch size decreases...the behavior of the universe would change dramaticallly through quantum effects on large scales. This case .. is ...not supported by current constructions of LQG.” I'm just a layman, do I understand that at this time, he's not predicting a reversal ( a future crunch) at the end of this expansion period?
    "Do not ask what came before the Big Bang!" (a quote from the previous pope)
  6. Sep 29, 2007 #5


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    Grosquet, you understand as well as I do about this, I think. He is NOT predicting a reversal of expansion.

    Something I like about Martin Bojowald's style is that he seems to think very methodically. He tends to patiently explore every logical possibility whether or not it is appealing to one's prejudices.

    this can sometimes make his writing confusing and tedious. He does not emphasize what he may suspect is probably right, or even obvious, at the expense of other possibilities. But in the long run I appreciate this slow methodical thoroughness.

    "Do not ask what came before the Big Bang!":biggrin:

    Grosquet, that is a very amusing quote! Do you happen to have the source? and do you have a sentence in the original Italian? (It would be too much to expect that it was said in Latin--that would be too good to be true.)

    If you scroll to the top of the PF page, over to the left you will see the "User CP"---the so-called "control panel"---and clicking there you will see "Edit signature". I think you can use this to make whatever you wish appear at the bottom of each post without your having to type it in each time.

    By the way, welcome! I see the above was your first post.
    Last edited: Sep 29, 2007
  7. Sep 29, 2007 #6
    Thank you for your reply & your welcome. I have been learning quite a lot from your posts for some time now. If my memory serves me right, years ago, John Paul II convened a few scientists at the Vatican, including Stephen Hawking, & at the meeting's end, he gave them this admonition, which I could not forget! (I believe he said it in English.)
    "Do not ask what came before the Big Bang!"
    Last edited: Sep 29, 2007
  8. Sep 30, 2007 #7
    Yes, thank you Grosquet, I am fully with Marcus, your quote is super.
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