View Poll Results: Do you think our universe has started with a Big Bounce?

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  • No, it has rather started with point singularity

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  • Yes and I think scenario 1) sounds most reasonably

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  • Yes and I think scenario 2) sounds most reasonably

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  • Yes and I think scenario 3) sounds most reasonably

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  • Yes and I think scenario 4) sounds most reasonably

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Thread: What about 2nd law of thermodynamics in Cyclic Universe Model?

  1. #1 What about 2nd law of thermodynamics in Cyclic Universe Model? 
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    Not everyone likes the idea of universe ​​created from a point singularity, so recently grows in popularity cyclic model - that our universe will finally collapse, use obtained momentum to bounce (so-called Big Bounce) and become the new Big Bang.
    One might criticize that we "know" that universe expansion is accelerating. But it is believed to be pushed away by "dark energy", so accordingly to energy conservation, this strength should decrease with R^3 ... while attracting gravity weakens like 1/R^2 and so should finally win - leading to collapse.

    But it seems there is a problem with the second law of thermodynamics here - on one hand entropy is said to be always increasing into the future, on the other Big Bangs should intuitively 'reset the situation' - start new entropy growth from minimum.
    I wanted to collect the possible approaches to this problem and discuss them - here is a schematic picture of the basic ones (to be expanded):



    The age of thermal death means that there are nearly no changes, because practically everything is in thermodynamical equilibrium, most of stars have extinguished.

    1) The second law is sacred - succeeding Big Bangs have larger and larger entropy,
    2) It is possible to break 2nd law, but only during the Great Bounce,
    3) It is possible to break 2nd law in singularities like black holes - the universe may be already in thermal death, while the entropy slowly "evaporate" with black holes (I think I've heard such concept in Penrose lecture in Cracow),
    4) The second law of thermodynamics is not fundamental, but effective one - physics is fundamentally time/CPT symmetric. So Big Bounce is not only single Big Bang, but from time/CPT symmetry perspective, there is also second BB-like beginning of universe reason-result chain in reverse time direction. The opposite evolutions would finally meet in the extremely long central thermal death age, which would probably destroy any low-entropic artifacts.

    Personally,
    I see 1) as a total nonsense - thermal death is near possible entropy maximum (like lg(N)).
    Also 3) doesn't seem reasonable - hypothetical Hawking radiation is kind of thermal radiation - definitely not ordering energy (decreasing entropy), but rather equilibrating degrees of freedom - leading to thermalization of universe.
    2) sounds worth considering - physics doesn't like discontinuities, but Big Bounce is kind of special - crushes everything, resetting the system.
    And 4) is the most reasonable, but requires accepting that thermodynamical time arrow is not fundamental principle, but statistical effect of e.g. low entropic BB-like situation: where/when everything is localized in small region. To see that 2nd law can work in both time directions, there is nice thought experiment: Thermodynamical thought experiment with time reversing loop?

    Assuming our universe will eventually collapse, which thermodynamical scenario seems most reasonable? Why?
    Perhaps above list requires expansion?
    Did universe started in a point, or maybe something ends - something begins (like tonight)?


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  3. #2  
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    None of the above.


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  4. #3  
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    Neither was created from point singularity nor as Big Bounce?
    Or do you mean scenarios? Please expand - I would gladly expand this list of basic options by succeeding possibilities.
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  5. #4  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Jarek Duda View Post
    Not everyone likes the idea of universe ​​created from a point singularity
    I don't think anyone "likes" the idea of the universe created from a point singularity. I'm not sure anyone even claims that. The existence of the singularity is an indication that our theories break down at the point and so it implies "we don't know".

    so recently grows in popularity cyclic model
    Do you have any evidence that that theory is "growing in popularity"?

    One might criticize that we "know" that universe expansion is accelerating. But it is believed to be pushed away by "dark energy", so accordingly to energy conservation, this strength should decrease with R^3 ... while attracting gravity weakens like 1/R^2 and so should finally win - leading to collapse.
    My understanding is that dark energy is constant (independent of the size/density of the universe) and that is why it has "recently" won over gravity leading to an accelerating expansion (instead of the expected slow down). I don't believe you can use an argument based on energy conservation as that is (as I understand it) a purely local phenomenon (local in the GR sense) and so cannot be applied on cosmological scales.
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    I thought inflation theory assumes starting from a point? If it was something something larger, was there time passing then?
    Ok, I should ask a different question - do you think there was something before our Big Bang? If it would occur that universe will finally collapse, would it just vanish into a nearly point?
    I don't have any evidence that they are "growing in popularity", but looking at Wikipedia articles suggested it to me:
    Big Bounce - Wikipedia, the free encyclopedia
    Cyclic model - Wikipedia, the free encyclopedia
    Ekpyrotic universe - Wikipedia, the free encyclopedia

    About dark energy, so you don't think it is a form of energy or maybe that energy conservation can be violated?
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  7. #6  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Jarek Duda View Post
    I thought inflation theory assumes starting from a point?
    Don't confuse inflation with the big bang. Big bang theory projects back to a hot, dense state. How that came into being is unknown.

    Ok, I should ask a different question - do you think there was something before our Big Bang?
    Don't know. We don't have enough information (or theories that allow us to extrapolate). Perhaps we can never know.

    About dark energy, so you don't think it is a form of energy or maybe that energy conservation can be violated?
    As I understand it, you can't even define conservation of energy on cosmological scales, so violation is a bit of a non-issue.

    And, as we don't know what dark energy is then there isn't much we can say beyond the observed effects.
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    I'm not confusing inflation with Big Bang, but if before there wasn't Big Crunch/Collapse, the default answer seems to be inflation (?)

    We indeed rather cannot observe if there was something before (but e.g. Penrose claims otherwise: Penrose claims to have glimpsed universe before Big Bang - physicsworld.com ), but we should find model of reality which is self-consistent ... and combining cosmological hints into some complete picture of universe could be essential piece of these puzzle.

    About conservation of energy, Lagrangian mechanics has always some time invariance, which through Noether theorem leads to some conserved total energy ... it might be complicated to specify it in general relativity, but are you sure there is no some energy conservation there?
    And while talking about dark energy, we use density/mass (e.g. 74% of the mass of universe) and mass is energy - why do you think standard view on mass/energy doesn't really apply here?

    ps. This physicsworld article supports what I have written about what I think I've heard on Penrose lecture - scenario 3) :
    "Central to Penrose's theory is the idea that in the very distant future the universe will in one sense become very similar to how it was at the Big Bang. He says that at these points the shape, or geometry, of the universe was and will be very smooth, in contrast to its current very jagged form. This continuity of shape, he maintains, will allow a transition from the end of the current aeon, when the universe will have expanded to become infinitely large, to the start of the next, when it once again becomes infinitesimally small and explodes outwards from the next big bang. Crucially, he says, the entropy at this transition stage will be extremely low, because black holes, which destroy all information that they suck in, evaporate as the universe expands and in so doing remove entropy from the universe. "
    Last edited by Jarek Duda; December 31st, 2011 at 10:30 AM.
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  9. #8  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Jarek Duda View Post
    I'm not confusing inflation with Big Bang, but if before there wasn't Big Crunch/Collapse, the default answer seems to be inflation (?)
    I see what you mean; if there was a "bounce" then there may be no need for inflation. However, there may be no need for inflation anyway:
    The Inflation Debate: Scientific American

    We indeed rather cannot observe if there was something before (but e.g. Penrose claims otherwise: Penrose claims to have glimpsed universe before Big Bang - physicsworld.com )
    Yes, Penrose's ideas are interesting, if speculative.

    but we should find model of reality which is self-consistent ... and combining cosmological hints into some complete picture of universe could be essential piece of these puzzle.
    Which is exactly how we ended up with the model we have now.

    As for the rest, it is beyond my level of understanding really.
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  10. #9  
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    I don't think "bounce" has necessarily exclude inflation, but indeed provides explanation without it.
    My intension was much simpler - just provide standard alternative hypothesis to discuss between two possibilities.

    And I'm not asking about the ultimate answer, but only the 'gut feeling'/ intuition and arguments behind it for discussion.

    ps. About scenario 3), I see I've simplified Penrose picture - the article says entropy doesn't only evaporate with black holes, but that black holes gather information from around and destroys it ... but entropy is about ordering of energy and hypothetical Hawking radiation ultimately transforms all gathered energy into kind of thermal radiation - maximally disordered ... I don't see how it could increase ordering?
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