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Does the big rip happen in LQC?

  1. Nov 16, 2009 #1

    MTd2

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    Does anyone know?
     
  2. jcsd
  3. Nov 17, 2009 #2

    tom.stoer

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    As far as i know it doesn't.

    LQC produces gravity-induces corrections to the matter interaction at high densities which may explain inflation rather generically w/o any inflation field, customized potential or special interaction term. At a few 10 - 100 Planck-length these effects should become invisible.

    The big rip is due to a special feature of a specific dark energy model - which applies at low densities / in the vacuum / at large scales. Neither LQG nor LGC make predictions regarding DE.

    There are some speculations from Smolin that LQG-like models could lead to an effect which looks like accelerated expansion and which could replace DE; the idea is that there is a mismatch between micro-causality (according to e.g. spin network links) and macro-causality (according to the derived smooth manifold). As far as I know this is really speculation as there are no detailed calculations available.

    There are ideas that LQG could be based on a q-deformed SU(2) rather than standard SU(2). In these models there is a relation between the deformation parameter q and a cosmological constant (but keep in mind: the big rip cannot be generated by a standard cosmological constant!)
     
  4. Nov 17, 2009 #3
    Yes, it does!

    As far as I know it does :biggrin:

    This result is well explained in a paper by Param Singh:
    Are loop quantum cosmos never singular? http://arxiv.org/abs/0901.2750
    The behavior of loopy universe is studied for an equation of state that has been used in literature to study quintessence and phantom dark energy.

    Big Rip is a future singularity that emerge for a phantom-like equation of states, namely a scalar field with negative kinetic energy. At finite time, the scale factor go to infinity, but because of this wired EoS the energy density increase with the scale factor and go to infinity too. So, also Big Rip deals with high energy density and it's cured by LQG since the energy density is bounded by a certain critical density next to the Planck scale.

    More puzzling is the situation with other kind of future singularity arising in this context: Big Rip is the number I, then you have the number II Sudden Singularity, the number III and the number VI, that differ depending on which/how they diverge scale factor, density, pressure, curvature and so on... One should notice that not all these singularities are so meaningful, since there could be a divergence even if the system is not "destroyed" and the equation of motion can be continued across the singularity.

    Cheers, F
     
  5. Nov 17, 2009 #4

    MTd2

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    Thank you for your answers. The purpose of my question is to make sense out of the initial bounce that is the big bang in LQC. It seems a crazy coincidence that our universe is the last in cycle of infinite cyclic universes.

    Last night I remembered of the big rip. And I thought why not bouncing back? Extreme regimes means that quantum corrections become important just like in the very low energy limit. For example, String Theory does not have black holes, in principle, but fuzzy spheres. Like, you don't cross a black hole, as you enter it, you are dissolved and your strings are incorporated to a dense string gas.

    String theory is not the only theory where that happens. See Matt Visser for a discussion:

    http://xxx.lanl.gov/abs/0902.0346
     
  6. Nov 17, 2009 #5

    tom.stoer

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    I have completely missed this paper!

    Seems that singularity resolution is a rather generic feature of LQC. It is interesting that the big bang singularity and the big crunch, big rip, etc. are cured by the same mechanism. This comes somehow as a surpsise because the origin of the singularities is different:
    - big bang is a generic feature of GR (Hawking / Penrose singularity theorems)
    - big crunch, big rip etc. are not generic features of GR but are due to specific matter / energy content

    So it seems that LQC can cure more than GR can be blamed for ...
     
  7. Nov 17, 2009 #6

    MTd2

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    Just one thing. After the bounce in the big rip and bounce in the big bang several times, shouldn't the inhomogeneities become bigger and bigger?
     
  8. Nov 17, 2009 #7
    Dont forget that before the Big Rip cosmological horizons become closer and closer, and hence they emit more and more high-temperature radiation. So just before the Big Rip vacuum becomes filled with very dense radiation, which can prevent the Big Rip because of the positive energy density.

    If LQG is consistent with Hawking radiation on the big scales, then it MUST happen.
     
  9. Nov 17, 2009 #8

    marcus

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    I'm curious why you say "several times". One of the most common LQC models has only one bounce.

    One cannot automatically assume there are several bounces, or that there are an infinite number.

    In a given LQC model there might, for instance, be a contraction of infinite duration, followed by a bounce, then followed by an expansion of infinite duration. That's a common scenario in fact.

    But the question of "what happens to the anisotropy?" is a good one! Likewise the question "what happens to the entropy?" At least I think they are good questions, and I think not fully answered yet. People are still studying very simple cases of non-singular cosmology.
     
  10. Nov 17, 2009 #9

    MTd2

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    ...> 1 bounce for the big rip -> 1 bounce for the big bang -> 1 bounce for the big rip ->...

    cyclic

    BTW, it is in the paper posted by Francesca. A bounce in the big rip.
     
    Last edited: Nov 17, 2009
  11. Nov 17, 2009 #10

    MTd2

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    Hmm, nice heuristic thought! :D Never thought of that. Thank you.
     
  12. Nov 17, 2009 #11

    marcus

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    MTd2, this is what I am wondering about. Do you think that the LQC "initial bounce" occurs only in that infinite cyclic context?

    BTW I think the answer to your main question is NO. Typically a big rip does not happen in the LQC model.
    I think Francesca said that a big rip CAN happen (if you put in something to cause it) but I don't think she was saying that it must happen or that it always does happen.

    If Francesca is still here, maybe she will clarify what she said.

    You would have to put something special in by hand to make it happen.

    If you really want a big rip to happen, you can probably set things up so that it will occur, but it is not a generic feature.
     
    Last edited: Nov 17, 2009
  13. Nov 17, 2009 #12

    MTd2

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    No, I don't think it happens only that context of the cyclic model. But only 1 bounce is something as ugly as supersymmetry. I don't know how to explain right now, but it is disgusting. Fortunately, there is a big contracting bounce.

    Alright Marcus, let me say that by big rip I meant that, instead of a big rip happening a big contracting bouncing happens, as it is shown in the paper pointed out by Francesca.

    http://arxiv.org/abs/0901.2750
    section 4.1, p.10

    "The universe instead of ripping apart in finite time, recollapses and the evolution continues."
     
  14. Nov 17, 2009 #13

    marcus

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    No it is not in the paper (by Param Singh) that Francesca posted. The paper does not say that there is a bounce in a big rip.

    Or did I miss something? If you think it says that, please find some discussion in the paper and paste it here so we can have a look.

    Singh gives a unified treatment of singularities in LQC, he discusses the various possible kinds. He does not say that they are all the same :biggrin:

    Let's look at sections 4.1:
    4.1. Type I Singularities
    If the value of α is chosen between 3/4 < α < 1 and A is positive, then the model gives a big rip (type I) singularity in GR. The scale factor, energy density and pressure diverge at a finite time and the DEC is violated (for all times). There is no big bang in the classical theory (since DEC is violated). The model is devoid of an initial singularity.
    In LQC, the big rip singularity is avoided. The energy density initially grows as in the classical theory, however when it becomes comparable to ρcrit , departures from classical trajectories become significant. Eventually, ρ becomes equal to ρcrit and the Hubble rate vanishes with ä taking negative value. The universe instead of ripping apart in finite time, recollapses and the evolution continues. The Ricci scalar, its derivatives and higher curvature invariants are bounded in the entire evolution.​

    This behavior is not generic. It can happen, according to Singh, if you make it happen. He does not say that it always automatically does happen.
     
    Last edited: Nov 17, 2009
  15. Nov 17, 2009 #14

    MTd2

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    Meh, I posted before seeing your edition to the post. But the universe expands faster and faster, so I guess we live in a kind of universe that has a type I singularity in the future, since we classically perceive a big rip in the future.
     
  16. Nov 17, 2009 #15

    marcus

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    No we do not classically perceive that. The standard cosmo model that virtually all professional cosmologists use is the LambdaCDM. It has slow steady acceleration without a big rip.

    Observations do not favor the big rip prediction. Sure "maybe". But maybe a lot of things :biggrin:

    You should get clear on some basic cosmology.
    So far observational data is consistent with a constant Lambda. That kind of acceleration does not imply rip.
    It just approximates a deSitter universe----nothing remarkable, no future singularity.
    Just endless, slowly accelerating, expansion. Very smooth picture.

    There is a key number w, called the "dark energy equation of state". Observations continue to narrow down the confidence interval for w around the value of -1. If w = -1 as an exact constant, then we are in the LambdaCDM case.
    Back in 2003-2004 there was a lot of excited talk about "big rip", which appeals to popular imagination, but in the professional community you don't hear much about it any more. They were imagining that the dark energy density changes over time and/or the equation of state w changes over time---causing weird stuff to happen.
    The observational cosmologists keep checking for signs of change and so far what they see favors exact constant dark energy parameters. If tomorrow they get new data and see something different, well, then we can start talking about big rip again :rolleyes:
     
    Last edited: Nov 17, 2009
  17. Nov 17, 2009 #16

    MTd2

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    What about the cosmological constant? As the universe expands, the matter content, and thus, the positive pressure, becomes proportionally null compared to the negative, leading to a big rip.
     
  18. Nov 17, 2009 #17
    Don't forget to write me a small cheque when you get your future Nobel prize :)

    I just wanted to say that for the same very reason Kerr singularity does not form: freely falling observers experience increasing tidal forces and become surrounded (in their frames) by the horizons. Singularity for them is in the future, as you know, exactly like in the Big Rip case. So Big Rip spacetime and kerr solution near singularity are very similar.

    So Kerr singularity is not a 'point' but rather a 'cloud', constantly absorbing it's own Hawking radiation, smearing its mass around small, but not infinitely small space region.
     
  19. Nov 17, 2009 #18

    marcus

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    As far as I can see, the section 4 of Singh that you point to does not go very deep or have much general significance.

    Notice that it is using a 2005 classical model by Odintsov Nojiri and Tsujikawa (ONT).
    http://arxiv.org/abs/hep-th/0501025

    All the parameters like alpha, A, B, etc that he is talking about are not LQC parameters. They are from the ONT paper.
    Singh says clearly at the beginning that he is using the ONT scheme which describes a "general dark energy scenario". This was developed in the classical FRW cosmology context.

    So ONT supplies him with a schema or a format ( his equation #22) which has parameters alpha, A, B etc. that you can vary so as to get all kinds of weird dark energy that people used to talk about---phantom energy, quintessence, etc etc.

    This is not LQC, it is the 2005 model of Odintsov, Nojiri and Tsujikawa. A convenient way to schematize a lot of different dark energy cases.

    So then he plays around with the parameters in the ONT model and sees how the energy density changes. And he just uses isolated facts about LQC to see how they would affect the ONT model.

    Like in LQC there is rhocrit energy density at which point gravity repels instead of attracts. He slips that fact in, and looks to see how things change.

    All the time he is not solving any LQC equation or running any LQC model, he is using the ONT model. And seeing how an isolated fact about LQC might affect.

    He doesn't talk about what these weird kinds of dark energy are, he doesn't put any of them in to an actual LQC model, and run it. All those cases are just hidden under the rug of the ONT equation #22 scheme.
     
  20. Nov 17, 2009 #19

    MTd2

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    So, at least, it is possible that LQC may allow a rip bounce.
     
  21. Nov 17, 2009 #20

    atyy

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    Cantonese?
     
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