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In big bounce models, what drives the contracting phase?

  1. Sep 9, 2015 #1
    We know that the universe is expanding at an accelerating rate. In big bounce (or oscillatory) models of the universe, why does the expansion of the universe end? And what drives the following, contracting phase?

    Is it possible that dark energy could inverse its action at some point in the future, and effectively start pulling the universe together instead of stretching it apart? Or is some entirely different, "contracting" force involved?
     
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  3. Sep 9, 2015 #2

    phinds

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    In the discredited big bounce model, either there is no dark energy or dark energy somehow magically changes state and gravity takes over again.

    It is possible that a horses will evolve into unicorns, but I wouldn't bet money on it.
     
  4. Sep 9, 2015 #3
    Gravity, the big bounce model was popular before scientists learned that the universe is accelerating.
     
  5. Sep 9, 2015 #4

    bapowell

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    There is no single "big bounce model". The discovery of accelerated expansion does not, in itself, have anything to say about bouncing universes. For example, the cyclic model, advanced by Steinhardt and Turok based on their "ekpyrotic" brane collision, incorporates accelerated expansion.
     
  6. Sep 9, 2015 #5

    wabbit

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    There are many big bounce models in quantum gravity, and there are models where the universe recollapses, but the two features are unrelated - a bounce is an alternative to a bang, it describes what may have happened ~14bn years ago and says nothing about what may happen some time in the distant future.
     
  7. Sep 9, 2015 #6

    marcus

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    Wabbit has this absolutely right. Let's not get bounce cosmology as a category confused with "cyclic". There is a growing research interest in bounce cosmology as a way both to resolve the classical singularity at start of expansion and also to arrive at predictions which might be tested by observation.

    Not sure what you could be talking about, NJrunner. Research in bounce cosmology e.g. LQC, has grown enormously since around year 2000. Discovery of the positive cosmological constant Lambda (leading to late-time acceleration) was in 1998.
    LQC is compatible with positive Lambda, and the standard cosmic model most people use called LambdaCDM (lambda cold dark matter).
    You might be interested in looking over the December 2014 paper by Ed Wilson-Ewing and Yi-fu Cai called
    LambdCDM Bounce Scenario

    As Wabbit and BrianPowell already indicated, please don't confuse BOUNCE cosmology with the more speculative "oscillatory" ideas. It is really really speculative to think that expansion could ever stop, and to imagine mechanisms that might cause this. There is no observational evidence to support it. There are comparatively few papers being written exploring the "oscillatory" idea. I recall there was one this year by Jerzy Kowalski-Glikman and a couple of other people, that kind of toyed with the idea. But papers like that are rare. I even hesitate to give the link because it is so "far out in left field". I'll get it though
     
  8. Sep 9, 2015 #7

    marcus

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    http://arxiv.org/abs/1507.00226
    Cyclic universe from Loop Quantum Gravity
    Francesco Cianfrani, Jerzy Kowalski-Glikman, Giacomo Rosati
    (Submitted on 1 Jul 2015)
    We discuss how a cyclic model for the flat universe can be constructively derived from Loop Quantum Gravity. This model has a lower bounce, at small values of the scale factor, which shares many similarities with that of Loop Quantum Cosmology. We find that quantum gravity corrections can be also relevant at energy densities much smaller than the Planckian one and that they can induce an upper bounce at large values of the scale factor.
    4 pages
     
  9. Sep 9, 2015 #8
    Forgive me for confusing bounce and cyclic cosmologies - my bad!

    With that out of the way: if you grant that the big bounce model is correct, what could hypothetically have caused the contraction of the universe preceding ours?
     
  10. Sep 9, 2015 #9

    wabbit

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    Presumably the same thing that causes the universe to collapse from now towards the big bang/bounce in reverse time - gravity. Unless I'm mistaken, models such as the LCDM bounce mentionned by Marcus are time-symmetric.
     
  11. Sep 9, 2015 #10

    marcus

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    Who knows? It is not science' job to answer the question "why does existence exist?"

    Cosmology progresses by asking and investigating questions that are TIMELY in the sense of being RIPE to be answered based on what we currently know and are able to observe. Now these particular researchers (like Ed Wilson Ewing) are focusing on OUR PARTICULAR START OF EXPANSION.
    The idea of a "singularity" is kind of absurd, a symptom of the breakdown of a 1915 theory that doesn't allow for quantum effects. So what really
    happened at the start of expansion.

    So you make a model involving some quantum effects in basic GR and you run it back in time and instead of a blowup, failure, "singularity" glitch you see it rebound and expand out again going back in time. So you see a prior collapse phase that rebounds.
    you don't ask where that came from because that is not a ripe question to be addressed. the aim is to understand a particular event, the start of expansion, and to derive predictions. Can you model it so that it fits the observational data? Can you get a SIMPLER best fit model?

    Maybe with this new model you can get the observed EFFECTS OF INFLATION without having to postulate an exotic "inflaton" field that conveniently dies away when it has smoothed things out just the right amount. maybe the smoothness is inherited from a prior contracting phase aided by the bounce mechanism itself. Interesting possibility. Let's study it.

    Seriously MyKK, how about giving this 14 page paper a try. It has some non-technical parts and I think it exemplifies the best in bounce cosmology (the most empirical/observational) at the moment. I'm a Wilson-Ewing fan.

    http://arxiv.org/abs/1412.2914
    A ΛCDM bounce scenario
    Yi-Fu Cai, Edward Wilson-Ewing
    (Submitted on 9 Dec 2014 (v1), last revised 28 Jan 2015 (this version, v2))
    We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Importantly, as this scenario predicts a positive running of the scalar index, observations can potentially differentiate between it and inflationary models. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
    14 pages, 8 figures.
     
    Last edited: Sep 9, 2015
  12. Sep 10, 2015 #11
    This is really terrific.

    Another idea I came across in reading about cyclic models (the one suggested by Steinhardt and Turok, if memory serves) is that dark energy is going to decay and become attractive rather than repulsive, which would lead to a contracting phase. By your standards, is this idea unripe?
     
  13. Sep 12, 2015 #12
    We don't really know why the universe is expanding right ? Yes it's because of dark energy but we don't know what dark energy is right ? If we ever find out why the universe is expanding we might be able to tell if it can ever contract and why.
     
  14. Sep 12, 2015 #13

    bapowell

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    Dark energy is one proposal to explain *accelerated* expansion. Non-accelerated expansion is not fueled by anything -- it was an initial condition at the big bang.
     
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