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Penrose's argument that q.g. can't remove the Big Bang singularity 
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#1
Nov512, 01:54 PM

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PF Gold
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I came across this argument in the book The Nature of Space and Time, which is based on a series of lectures given by Hawking and Penrose. Although it relates to Penrose's Weyl curvature hypothesis (WCH), it does not depend on it, and that, to me, makes it a lot more interesting, since I wouldn't bet a sixpack on the validity of the WCH.
As a preliminary, Penrose observes that (in my possibly inaccurate paraphrase): (1) The Big Bang was not a generic state. A generic Big Bang state would have had a large Weyl curvature, but the universe we see looks nothing like the one that would have resulted from such an initial state. Our Big Bang appears to have had a small or even vanishing Weyl curvature. (2) The evolution of our universe has led to a state with nonvanishing Weyl curvature. (At black hole singularities, we even have diverging Weyl curvature.) At the end of his first lecture, someone in the audience asks whether he thinks quantum gravity removes singularities. He says: Presumably he has his cosmic cyclic cosmology (CCC) model in mind here (this was in 1996). Although CCC no longer looks viable, that doesn't resolve the issue he raises, which seems pretty modelindependent. The possibility that occurs to me is that the big bang singularity is removed by quantum effects, the entropy of the universe was minimized at the big bang, and there is timereversal symmetry, so that the thermodynamic arrow of time was reversed in the universe before the big bang. Thermodynamically, the big bang would then look like an extremely unlikely thermal fluctuation, but presumably whoever set the boundary conditions of the universe got to choose to make it that way. 


#2
Nov512, 03:07 PM

P: 343

You seem to suggest the existence of a creator?
Thermodynamics isn't time symmetric so you can't have the universe as a thermal state the decreases in entropy before the big bang started. The ccc models I guess are some kind of loop hole in the 2nd law like mapping states of high entropy to states of low entropy. I'm not sure how much sense that makes though. 


#3
Nov512, 04:46 PM

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But if that isn't the option one picks, then what counterargument is there to Penrose's? 


#4
Nov512, 06:10 PM

P: 343

Penrose's argument that q.g. can't remove the Big Bang singularity
The second law of thermodynamics has an explanation from statistical physics. I can't understand how you could explain this flip? To create some state before the big bang that created the special low entropy state at the big bang would require some fine tuned prebig bang state. It can't begin from a generic 'crunch'.
I don't think there is a couter argument to Penrose's argument. Something drastic has to happen to spacetime at the big bang. 


#5
Nov512, 07:16 PM

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#6
Nov512, 09:49 PM

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Maybe complete thermal equilibrium of the entire universe is acutally equivalent to everything being in one state that can degenerate, and that's how you can go from one cycle of the universe to the next. 


#7
Nov512, 10:11 PM

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#8
Nov612, 06:08 AM

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Nothing is trivial here. LQC works with much symmetry, it's just a toy model.Toy models can be useful but they're not reality. 


#9
Nov612, 09:36 AM

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#10
Nov612, 03:01 PM

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#11
Nov612, 07:08 PM

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No one has solved the reason for the low entropy initial conditions of cosmology. The problem exists for almost every single proposal. Loop or other.
Taken at face value, it rules out almost all of cosmology. 


#12
Nov712, 05:07 AM

P: 3,009

Don't wanna sound sarcastic, but if GR had no problem not following strictly the previously "sacred" first law of thermodynamics, what prevents it from not strictly following the second too? I always considered both laws of thermodynamics in the same pack, but that seemed to be just me, last time I argued this here I was told that they are independent of each other and the second one was more important than the first if one was to choose wich one should be disobeyed by a theory like GR. I can't say I'm totally convinced of that, though. 


#13
Nov712, 12:22 PM

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If gravity becomes repellent, what happens to BH entropy? If the collapsing universe, prior to bounce, consists mainly of black holes, and its entropy is predominantly BH entropy, then how does one define the global entropy as gravity becomes increasingly repellent going into the bounce? There seem to be problems with the definition of entropy underlying the 2nd law, when one tries to apply it in this context. 


#14
Nov712, 05:35 PM

P: 5,632

..does quantum gravity remove singularities..
We are, alas, unlikely to resolve the question here. Well, I know I won't be!!! Marcus: [QUOTE][Penrose argues against LQC bounce, but the essence of the bounce is that gravity becomes repellent due to quantum corrections at nearPlanck densitythat's why there is a bounce./QUOTE] I agree that's conventional wisdom, but when a 'correction' starts a universe, color me 'suspicious'. Here are some related references I saved from other discussions in these forums.....no, they don't give a definitive answer. BEFORE THE BIG BANG: AN OUTRAGEOUS NEW PERSPECTIVE AND ITS IMPLICATIONS FOR PARTICLE PHYSICS Roger Penrose Mathematical Institute, 2429 St Giles’, Oxford OX1 3LB, U.K. http://arxiv.org/PS_cache/arxiv/pdf/...009.1136v1.pdf and a 2009 perspective from Steve Carlip that may offer interesting possibilities: The Small Scale Structure of Spacetime 


#15
Nov712, 06:14 PM

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The big difference is that Penrose uses entropy in his reasoning whereas LQC doesn't. Both approaches are incomplete: Penrose has no detailed model at all, LQG is a detailed model but with too many simplifications.



#16
Nov712, 08:06 PM

PF Gold
P: 316

"The nature of space and time" is an old book. I think he put similar views forward in "The Emperor's new mind". You know that Penrose has made a bit of a Uturn and now argues that in the thermal death of one universe (in which he presumes there are no nonzero rest mass particles) there is no way of building clocks or reference systems to provide a notion of time intervals or length intervals and so the universe is induistinguishable from zero volume big bang situation (this is the view he puts forward in his new book "Cycles of time"). I have been wondering how he reconciles these seemingly contradictory views. Glad bcrowell brought it up. Would like to understand better.



#17
Nov712, 08:50 PM

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It is really interesting to consider how the entropy of a BH could be defined and could evolve when gravity becomes repellent! On the face of it, considered naively, the entropy should change sign: Suppose we take the BekensteinHawking effective description at face value: S = A/(4G_{Newton}) and the effective value of G_{Newton} goes temporarily negative. Then unless the black hole has dissipated by then it would seem to have negative entropy This is not how one would argue in reality, just meant to be suggestive. In LHC gravity becomes repellent at extreme density. that is what causes the bounce. So all I can say is that this makes the definition of entropy itself an extremely interesting problem (in the context of LQC models). In the talk by Penrose I attended he did not address this at all, just waved his hands. So he actually did not make logical contact with LQG. But it was otherwise a delightful and stimulating talk about his new (Conformal Cyclic) Cosmology idea. 


#18
Nov712, 10:45 PM

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Negative entropy does not make sense. It is defined (for a microcanonical ensemble) as the logarithm of the number of microstates. You cannot, by definition, have a negative value.
Now, whether entropy is or is not defined in the quantum gravity regime is one question. However if you believe in unitary physics, you do run into a contradiction at some stage from the global point of view. So it is true that there is a problem of principle. If you take a state in the far past pre bounce (where slices are nice, well behaved and semiclassical), and a state in the far future post bounce (likewise), and derive that the former has higher entropy than the latter, that does violate the second law (and unitarity) regardless of what tricks you want to pull in the middle. Amongst other catastrophes, it implies that you do not have reversible physics. Now, as I said, these types of stat mech arguments are essentially a problem with all proposals really (eg an infinite finetuning in a boundary conditions or alternatively a discontinuity in the laws of physics). Interestingly there might be a way out if you believe in observer complementarity in which case inflation might potentially resolve some of the finetuning (b/c crucially the all important volume factor enters (and dissappears) from the picture). See recent papers by Banks et al 


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