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Ali and Das, "Cosmology from quantum potential"

  1. Feb 11, 2015 #1

    bcrowell

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    There was a paper last year by Ali and Das, "Cosmology from quantum potential," http://arxiv.org/abs/1404.3093 . Some elements of the popular media seem to be picking up on it and describing it as a paper that says that the big bang didn't exist, e.g., http://www.glennbeck.com/2015/02/10/watch-the-big-bang-never-happened/ . Some of my colleagues have been hearing about this from their students.

    I'm not a quantum gravity specialist, but from a brief inspection it looks like the paper is speculative and simply suggests what most physicists have suspected for 50 years, which is that the big bang singularity in classical GR is probably a feature that we should not believe in when we get beyond the Planck scale.

    Is my impression right?
     
  2. jcsd
  3. Feb 11, 2015 #2
    I've spent a few hours reading and considering this paper and the original paper by Das that proposed the quantum-corrected Raychaudhuri equation (QRE). You are right that it suggests that singularities cannot actually exist in nature, and that a quantum perspective on the whole thing would probably be beneficial. That's exactly what Das claims the QRE can do.

    The classical Raychaudhuri equation assumes that particles follow continuous paths as one would draw on paper. Since we live in a quantum world, we know that is not true. From the paper, "the quantum corrected Raychaudhuri equation (QRE) ... was obtained by replacing geodesics with quantal (Bohmian) trajectories." So the QRE arises from the fact that we live in a quantum universe; this was not considered in the original Raychaudhuri equation. Importantly, the QRE resolves the original Raychaudhuri equation in the ℏ → 0 limit. Every other statement in the paper follows from analysis of the QRE, and in my opinion, the implications seem quite natural.

    There are three major things to take away from the QRE and the recent paper by Ali and Das.

    1. Singularities are not an inevitability according to the QRE; it works out in such a way that spacetime can be severely warped, but never converging.
    2. Ali and Das assert that there are two correction terms in the QRE. The first one corresponds to a cosmological constant and the small mass of a graviton. The second term predicts that the age of the universe is infinite. (I interpret this as saying that the Big Bang was not the beginning of time, but merely an event in an infinite time span.)
    3. The QRE explains the smallness problem (the 10^-123 magnitude of the cosmological constant) and the coincidence problem (why did the Universe expand in the particular way that it did?).

    I leave a portion of the authors' summary here for your convenience:

    "In summary, we have shown here that as for the QRE, the second order Friedmann equation derived from the QRE also contains two quantum correction terms. These terms are generic and unavoidable and follow naturally in a quantum mechanical description of our universe. Of these, the first can be interpreted as cosmological constant or dark energy of the correct (observed) magnitude and a small mass of the graviton (or axion). The second quantum correction term pushes back the time singularity indefinitely, and predicts an everlasting universe."

    I'm really interested to hear what anyone else has to say about this paper — I found it quite intriguing!
     
  4. Feb 11, 2015 #3

    Demystifier

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    I would just like to say that acceptability of the conclusions in this paper depend on whether one accepts that the Bohmian interpretaton of quantum theory is the correct interpretation, despite the fact that Bohmian trajectories cannot be directly measured.
     
  5. Feb 11, 2015 #4

    wabbit

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    The bounce results seem to be in line with other QG model, interesting because of the approach but not groundbreaking. What I find intriguing are the results about the cosmological constant problem and coincidence problem. How robust do these seem to specialists ?
     
  6. Feb 11, 2015 #5

    Chronos

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    I'm unconvinced a bounce is implied by QRE corrections. Perhaps t = 0 is asymptotically approached, but never actually reached. This would appear to be consistent with the Planck star hypothesis proposed by Rovelli last year. It is curious the value for the cosmological constant can be approximated in this manner.
     
  7. Feb 11, 2015 #6

    wabbit

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    You're right their model may not imply an actual bounce. However if not it would mean we are an infinite time away from the bigbang ? Or rather, since at some point (going pastward) the universe will reach its minimum size, continuing either it bounces or there is some sort of static state lasting forever before it goes bang, since the universe has an infinite past in their model. Weird but it could be what their model says...
     
  8. Feb 11, 2015 #7

    martinbn

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    I am sure I don't understand the idea, but it seems that they are talking about families of world lines of actual particles. But then how does it work for singularities in vacuum solutions?
     
  9. Feb 11, 2015 #8

    Chronos

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    The passage of time is a very tricky issue. In the Planck star paper Rovelli noted that according to the black hole's clock, scarcely any time would pass before it would re-emerge as a white hole. To an external observer, however, it seems to take 'forever'.
     
  10. Feb 11, 2015 #9
    This was my thought as well. Who knows what the "correct" interpretation is...
     
  11. Feb 11, 2015 #10

    wabbit

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    In their model there is no t=0 - the universe's age is infinite (in the proper time of a comoving observer or something - the same meaning we asign to "the universe is 14by old".)
     
  12. Feb 11, 2015 #11

    bcrowell

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  13. Feb 11, 2015 #12

    atyy

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  14. Feb 11, 2015 #13

    wabbit

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    Great link, thanks.
     
  15. Feb 11, 2015 #14

    atyy

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    Is that really a correct way to get a Bohmian model of quantum gravity? For comparison, another Bohmian model of early cosmology is http://arxiv.org/abs/1407.8262 Primordial quantum nonequilibrium and large-scale cosmic anomalies by Samuel Colin and Antony Valentini.
     
  16. Feb 11, 2015 #15

    wabbit

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    I did not think they were building a model of quantm gravity, seemed more like quantum tractectories in GR, kinda semiclassical .
     
  17. Feb 11, 2015 #16

    atyy

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    But if they do that, then the background will still be governed by the classical Einstein field equations, and there will be a singularity. So I think the only hope is that it is fully quantum, and then quantum gravity effects remove the singularity, like in some loop quantum cosmology models.
     
  18. Feb 11, 2015 #17

    wabbit

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    Yes, that's not quite what they do, they derive correction terms to GR from the behaviour of quantum tracjectories. Still, quantum effects are correction terms as far as the gravitational field is concermed and they cannot touch the high energy regime.
     
  19. Feb 11, 2015 #18
    I came across this topic last night while watching a lecture by Roger Penrose:



    At one point in his talk I ran a search on standard big bang cosmology to try to consolidate some connections I was trying to resolve in his talk. Low and behold, once I ran the search, what popped up on the google search engine was something like "the big bang never happened" and a link the Phys.org article and the Glenn Beck link:

    I'm not a cosmologist but I have interest in it and pretty much thought we had a pretty good handle on what happened back to 10^-43 seconds through the "first 3 minutes." The news of this topic and model's like Penrose's CCC model now make me question to what measure can I trust the reliability of these ostensibly pseudo-exact measurements of the features of the birth of the universe I read in the big bang and chronology of the universe wiki's, say

    http://en.wikipedia.org/wiki/Big_Bang
    http://en.wikipedia.org/wiki/Chronology_of_the_universe

    I only say this because it is unsettling to see Glenn Beck and his panel of 3 chorusing an "I told you so" complete with their resident science expert who thinks Piltdown man was made of plastic laughing about this.

    My question is, why am I studying this standard big bang cosmology that the universe was such and such a size at such and such a time, and baryongenesis happened at this microsecond, and the the quark gluon plasma happened at that microsecond, when everyone goes running for the hills when someone comes along and gets an article published in Physics Letters B which says that the big bang never happened and the universe is eternal. Are we THAT unsure of what happened in the first 3 minutes? Or if there even was a first 3 minutes?
     
  20. Feb 11, 2015 #19

    wabbit

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    Sorry , way over my head here, but I don't see how this article ischanging anything in that area. Whether the big bang was an instant beginning, a transisition from a prior semistable state, or a bounce isn't going to change the picture that much... And this particular paper seems rather tentative, not worth this buzz which is just an effect of the "The Big Bang never happened" sensationalization. Seems rather silly to me but I'm no expert so ....
     
  21. Feb 11, 2015 #20
    That's a good point, that's what I want to know, what is it about the standard big bang cosmology we can rely on, and what is speculative. When I read things that say, this happened at 10^-43 seconds, that happened at 10^-32 seconds, this other thing happened at 10^-6 seconds, etc., I tend to think that they've done their research and I can rely on that. The standard view as far as I can remember is that these times are relative to a point in time, a singularity. If we're cavalier about throwing out this singularity, then what do we reference these exact times against? Penrose suggests that there was no reconvergence to a singularity at each transition from aeon epoch to aeon epoch. So how do we reconcile that with inflationary models? Again, as a non-cosmologist, what do we know to a good degree of confidence, and what specifically is on the frontiers of speculation.
     
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