Latest Penrose video on Conformal Cyclic Cosmology

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  • #1
marcus
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http://pirsa.org/11040063/

Big Perimeter audience, appreciative interest, long question period after.
He undercuts his critics (who said the concentric circles could have appeared by chance) by criticising their methods and makes at least one valid point. I would say that the critics still may win the argument but they may have to do some more work to accomplish this.

Meanwhile the CCC scenario still has some life in it. And the talk itself is lively. He is an entertaining speaker. He has also done a lot more work, worked out equations the universe satisfies during "crossover" between aeons. Thought the whole process through more carefully.

(For me) still not especially convincing, but intriguing nevertheless.
 

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  • #2
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Bojowald proposes something similar in Loop Quantum Gravity. The Big Bounce and Cyclic Cosmology requires a time when there is a decreasing entropy in one cycle. How it agrees with a second law of thermodynamic and holographic principle ?
 
  • #3
marcus
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We are used to associating "time" with the formation of structures in the gravitational field---the process by which geometric homogeneity/uniformity develops into lumpiness/inhomogeneity.

If gravity suddenly became universally repellent (instead of attractive) and all that structure unraveled and blew away---returning the universe to uniformity---and then gravity resumed its normal attractive role, what would you say had happened regarding "time" and "entropy"?

Bojowald proposes something similar in Loop Quantum Gravity.
Loop cosmology has changed since the time it was centered around the work of Bojowald.

It could be misleading to say "Bojowad proposes"

The best short non-technical summary of Loop cosmology at present, I think, is pages 15 and 16 of Rovelli's December 2010 review of the Loop program overall. BTW this acknowledges the initial importance of Bojowald's work. Please have a look at those two pages and see whether you agree. http://arxiv.org/abs/1012.4707 .

For people with more specialized knowledge there is the longer more technical review by Ashtekar http://arxiv.org/abs/1005.5491 and the very interesting March 2011 paper by Ashtekar et al on inflation in Loop cosmology 1103.2475 .

I would not say that this is "something similar" to Penrose CCC idea. Penrose involves much speculative new physics spread over trillions of years of time and has many unresolved issues. Penrose has no bounce.

By contrast the Loop bounce (as summarized by Ashtekar) is pretty simple. Basically one just quantizes Gen Rel/Friedmann model, and notes that at high density gravity is repellent.

Therefore during a brief high-density episode of repellent gravity (the "bounce") the evolution is from structure to uniformity.
 
  • #4
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I wonder how are the coupling constants readjusted after an eon. I see no explanation for that.
 
  • #5
marcus
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If you watched the video you saw that there are many questions he doesn't have answers for and his main goal is to persuade others to work on the the program, to fill in various parts of the picture.

He described going to experts in this or that specialty and asking them "what if" questions.
To construct a more exact detailed picture he needs lots of help. Who can say whether he will get adequate help?

My intuition is that it is not a good idea. But I would be glad to hear that some good people had gotten interested and were helping him. It is interesting enough that I think it should be developed, instead of remaining as it is today: a vague half-baked conjecture (mostly in one person's mind).

I think one of the main reasons Penrose is hooked on this idea is that he has misunderstood how the Second Law applies to the bounce. He is good friends with Ashtekar (Penrose is part-time at Abhay's Institute at Penn State) and they have a long-standing friendly disagreement about this. In Abhay's view the Second Law is no problem for the bounce, and Penrose does not accept this, so starting in 2005 he has been promoting this bounceless "Aeon" alternative.

I see him deeply entrenched in a mistake. But I still want to pay close attention to what he says---he's a brilliant creative guy. I feel I can learn more from some Penrose mistakes than from ordinary folks' validities.
 
  • #6
atyy
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Worth watching for the transparencies alone. His previous wild guesses were spin networks and twistors. There's no way this is wrong.
 
  • #7
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Marcus:
thank you once again for bringing an interesting paper to our attention.
I've been watching the Penrose talk and the first 40 minutes plus is generally understandable to us laymen...around the 45 minute mark (of 87 minutes total ) he starts whizzing thru mathematics and that's been rather a blur for me so far.

I did find a helpful (for me) overview discussion in Wikipedia about Penrose's ideas on this subject:

http://en.wikipedia.org/wiki/Conformal_Cyclic_Cosmology

Others looking for a brief description might find it useful.
 
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I found this comment by Penrose interesting:

"Eterntiy is no time at all for a photon"
 
  • #9
bcrowell
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He undercuts his critics (who said the concentric circles could have appeared by chance) by criticising their methods and makes at least one valid point.
Has he written up this response anywhere?
 
  • #10
marcus
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I don't watch this closely since I don't think it's likely enough to make it very interesting, so I could have missed something.

The latest I know of (from P&G about CCC) is this:
http://arxiv.org/abs/1012.1486

This paper purports to reply to the critics, and there has been no rejoinder that I could find.
One of the critics, Zibin, put out an April Fools paper, but there was no serious response to Penrose's rebuttal that I could see so far.

The critics would need to have cited 1012.1486, and nobody has so far (except on April 1.)

Some of what Penrose said in his Perimeter talk was a repeat of 1012.1486 (pointing to an alleged procedural deficiency on the part of the two critical studies). You have to judge for yourself how much weight to give this defensive point made in the December paper and in the Perimeter talk. It looks like a valid objection to me (not expert in these matters) but does not mean that Penrose is right. Again not speaking as an expert, I think it simply means that to lay the matter to rest the critics need to redo their analysis.

"Both groups simulate maps using the CMB power spectrum for LCDM, while we simulate a pure Gaussian sky plus the WMAP's noise..."

The critics, he says, used the observed CMB power spectrum which could already have been affected by CCC, if the hypothesis is true. They used that observed spectrum to generate random data and, I guess, found circles. He wants them to use a "clinically sterile" theoretical power spectrum, not the observed one, and do the same analysis. To me that sounds like a fair request.

In other words, eliminate all possibility that "CCC-ness" has crept in through the power spectrum, and redo their simulation. Is that a mere quibble, or not?
 
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  • #11
bcrowell
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Thanks for the reference, Marcus. Personally, I was interested enough in CCC to buy Penrose's book and give a talk on it at my school, but the observational situation seems pretty negative at this point.
 
  • #12
marcus
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It is an interesting possibility that signatures of pre-bang events could be discerned in the CMB!
However these might not indicate Penrose CCC. These people Nelson and Wilson-Ewing, think that it has not been ruled out yet that there could after all be some circles!
They consider different ways these could arise (not only from CCC).
http://arxiv.org/abs/1104.3688
Pre-Big-Bang Cosmology and Circles in the Cosmic Microwave Background
William Nelson, Edward Wilson-Ewing
21 pages, 3 figures
(Submitted on 19 Apr 2011)
"We examine the possibility that circles in the cosmic microwave background could be formed by the interaction of a gravitational wave pulse emitted in some pre-big-bang phase of the universe with the last scattering surface. We derive the expected size distribution of such circles, as well as their typical width and (for concentric circles) angular separation. We apply these results in particular to conformal cyclic cosmology, ekpyrotic cosmology as well as loop quantum cosmology with and without inflation in order to determine how the predicted geometric properties of these circles would vary from one model to the other, and thus, if detected, could allow us to differentiate between various pre-big-bang cosmological models. We also show that the angular width and the sine of the angular radius of such circles are inversely proportional. This relation can be used in order to determine whether or not circles observed in the cosmic microwave background are due to energetic pre-big-bang events."

So it is still possible that Penrose and Gurzadyan actually did see some indication of circles, whether or not they came about by CCC. Story may not be over.
 
  • #13
marcus
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The authors are new generation QG, Wilson-Ewing is one of Ashtekar's recent PhD students and co-authors, who has just this year joined Rovelli's group in Marseille as postdoc. William Nelson is a recent PhD from King's College London who is now posdoc with Ashtekar at Penn State. I believe his advisor was Sakellariadou, with whom he has co-authored quite a number of papers.

For me, the possibility that there might be relic gravitational waves (from before bang) discernible in the ancient light of the Background is given a bit more credibility by the fact that these two young QG researchers, both with excellent track record, have been willing to spend some time thinking about it.
 
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  • #14
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Thanks for sharing .. this was interesting.

Two things that popped into my mind while watching this

(1) No computer, power point?

(2) Is Roger Penrose a Hindu?
 

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