Ali and Das, "Cosmology from quantum potential"

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  • #1
bcrowell
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Main Question or Discussion Point

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?
 

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  • #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!
 
  • #3
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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.
 
  • #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 ?
 
  • #5
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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.
 
  • #6
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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...
 
  • #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?
 
  • #8
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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'.
 
  • #9
Perhaps t = 0 is asymptotically approached, but never actually reached.
This was my thought as well. Who knows what the "correct" interpretation is...
 
  • #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
atyy
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  • #13
wabbit
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Great link, thanks.
 
  • #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.
 
  • #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 .
 
  • #16
atyy
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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 .
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.
 
  • #17
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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.
 
  • #18
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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:

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/ .
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?
 
  • #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 ....
 
  • #20
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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.
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.
 
  • #21
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The Big Bang theory/model/result is remarkably successful at explaining current observations.
Obviously there are some problems but this can be expected in my opinion as GR still can't probe certain regimes.

I don't know if anybody read the StackExchange posts. I have to agree with the top answer there.
Paraphrasing:

They start from one idea, replacing geodesics by Bohmian trajectories.
Then a lot of similar work is quoted, after which they posit the results which essentially ends the calculations.
After that they try interpreting the results using various substitutions. Culminating in a nice cosmological constant with the right order of magnitude.

The main problem I myself have, is the referring to other work and using it in a "similar" way.
They don't state what similar means, did they use additional steps? Did they abandon the similar method half-way through using a new approach?

Another is that they use large scale homogeneity and isotropy, but dismiss this assumption to a footnote.
Granted it is not wrong to do so but such assumptions usually deserve a spot in the main text.

I hope some clarifications and/or comments come along soon.
 
  • #22
wabbit
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Another is that they use large scale homogeneity and isotropy, but dismiss this assumption to a footnote.
Granted it is not wrong to do so but such assumptions usually deserve a spot in the main text.
I don't think this is completely fair, they do acknowledge it in the text at least - quoting from the conclusion: "While inhomogeneous or anisotropic perturbations are not expected to significantly affect these results, it would be useful to redo the current study with such small perturbations to rigorously confirm that this is indeed the case. Also, as noted in the introduction, we assume it to follow general relativity, whereas the Einstein equations may themselves undergo quantum corrections, especially at early epochs, further affecting predictions. Given the robust set of starting assumptions, we expect our main results to continue to hold even if and when a fully satisfactory theory of quantum gravity is formulated. For the cosmological constant problem at late times on the other hand, quantum gravity effects are practically absent and can be safely ignored. We hope to report on these and related issues elsewhere."

Besides it seems quite natural when trying out an idea to use a simplified highly symmetric setup where it's easier to "solve". Also, they're not claiming to revolutionize anything, at least what I got from it is more something like "it's interesting how using Bohmian trajectories they seem to be able to model some aspects in the intermediate/ semi-classical regime of QG approaching a BB singularity, which might yield a better undertsanding of some aspects of the cosmological constant." I don't think their approach can, structurally, go beyond that, but I may well be wrong.

The fascinating paper http://arxiv.org/abs/1407.8262 linked to by atyy above is in a completely different class IMHO.
 
  • #23
wabbit
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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.
Very interesting. Might deserve it's own thread.
 
  • #24
Ken G
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A problem I have with all this is, how do we know what time means when we have quantum corrections like that? In the simple nonrelativistic quantum mechanics I am familiar with, time is simply a parameter of the theory, it is strictly a rule of thumb that this parameter will correspond to something we can physically measure. This "rule of thumb" is reflected in the formal structure in the fact that time is not an observable, does not have an operator associated with it, and even though it has an uncertainty relation with energy, it is not a conjugate observable to energy because all conjugate pairs are formally equivalent to momentum and distance. Maybe the situation changes in relativistic quantum mechanics or quantum field theory, but my impression was that these fixes don't completely resolve the issue.

My understanding of this is that you can formulate operators that function locally like our macroscopic time measurements, and they will be canonically conjugate to the energy operator, but there is no guarantee they will function globally like the time parameter. So this is the general problem I have, it seems to me the status of time in quantum mechanics is quite iffy-- we know how time works in our own experience, and we can make formal operators that behave like we measure time to behave, but we have no way of knowing if any of that would still hold true on the Planck scale. In short, we have no idea that anything we call the time parameter in quantum mechanics, and apply quantum corrections to and Bohmian trajectories ruled by, will actually function the way we imagine time should function in the early universe.

Put differently, it may not matter a whit if some t parameter can go to negative infinity or not, what we really want to know is if, in some sense, an "infinite amount of stuff can happen" looking backward toward the beginning. I'm not sure the quantum corrections resolve that basic issue, it may be just another essentially philosophically imposed assumption of the Bohmian approach that no formal theory can provide justification for. In other words, what if the t parameter in the Bohmian approach is nothing but a mathematical coordinate, that is locally tangent to what we regard as time, but does not retain that property on the Planck scale-- would that not make all this "the Big Bang never happened" stuff a tempest in a teacup?

Also, in regard to getting the scale of the cosmological constant, it seems to me it is circular reasoning. They embed the deBroglie wavelength corresponding to the length scale of the universe today, which is essentially the length scale when dark energy takes over the dynamics, and then pretend that this is some kind of "natural" parameter to place in their theory. If it is "natural" for dark energy to rule the dynamics starting now, then of course we are going to get a "natural" scale for dark energy, but the real problem is, it is not "natural" for dark energy to just start ruling the dynamics now! The current length scale of the universe is not a natural parameter to embed in any theory that claims to "explain" something.

Finally, let me add that this whole business reminds me of the expert debate on what happens inside the event horizon of a black hole. Without observational constraints on any of this, should we pay any attention to these guessing games?
 
  • #25
wabbit
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Finally, let me add that this whole business reminds me of the expert debate on what happens inside the event horizon of a black hole. Without observational constraints on any of this, should we pay any attention to these guessing games?
Well the Rovelli analysis of what happens inside a Black hole ends up with some pretty vivid testable predictions. If we happen to be blessed with a population of primordial BHs of just the right mass, the result could be stunning; even if in practice we might need to wait a couple more billion years for confirmation, who said patience isn't a virtue?
 

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