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Roger Penrose and Conformal Cyclic Cosmology

Posted July 23, 2020 6:00 PM by Jorrie

I have mentioned cosmological work from Sir Roger Penrose, a British mathematical physicist, in-passing a few times in the past, but have never zoomed in a bit. Despite the fact that his work in general relativity is more highly regarded than his cosmology work, the latter has recently evoked some renewed interest among the Cosmo-community. He came up with an interesting variant of his earlier (discredited?) cyclic cosmological model proposals, now called Conformal Cyclic Cosmology (CCC).

So what has changed to evoke the new interest? CCC is technically tough to follow, but in a nutshell, he performs a conformal rescaling at time > 10100 yr (one Googol year) that equates the then cold empty space to the hot Big-bang, without the inflationary period. It does two neat tricks: Firstly, it resets the universe's entropy to a low value again without needing inflation; Secondly, it allows signals from our cycle (aeon) through to the next aeon. Likewise, signals from the preceding aeon have been allowed through to our aeon. He claims that their signatures have already been observed in the CMB radiation - by both the WMAP and Planck missions. Interesting!

There are many talks and interviews with sir Roger recorded in the last few years. Instead of doing an injustice to his work, I recommend this particular talk of mid-2019 at the Paris Astrophysics Institute: "Are we Seeing Hawking Points in the Microwave Sky?". Penrose's 'Hawking points' are Hawking radiation from evaporation of mega-massive black holes near the end of the previous aeon, that are posited to have left a defined imprint on the CMB.

The video sports clear hand-drawn graphics with the typical Penrose clarity of explanation, which is amazing for a (then) 88 year old. It is a tad long (1h10m), but well worth the time. After the initial introductions, he starts talking about 5 minutes into the YouTube video. But be aware: once the talk starts, it is difficult to stop watching - so pick your time.

I noticed that he has lately been invited by many science popularizers to their podcasts and public discussions. I get the feeling that some inflation- and string-threorists are trying to get him off their backs. I can imagine that their students are asking difficult questions about sir Roger's ideas, so they can't ignore him anymore...

I did a wee-bit of self-study into CCC, so you are welcome to ask my opinion, once you have watched the video (I can't compete!).

J-

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#1

Re: Roger Penrose and Conformal Cyclic Cosmology

07/24/2020 9:38 PM

Cosmic reincarnation... I thought it sounded crazy at first, but not any crazier than the universe inflating from smaller than a proton. Maybe he's on the right track.

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/25/2020 4:54 AM

Yes, maybe. I liked his stacked expanding visual, but not the stacked funnel part, because while matter is the dominating energy in the universe, one cannot conformally rescale space without having some weird form of conformal time. I adapted my Relativity-4-Engineers Fig. 15.4 to the following log-log Plank scale depiction:

This shows more vividly that the singularity and inflation are eliminated, without any weird scaling.

It also made me realize that the actual size of the universe could stay infinite throughout all the cycles - it is only the observable universe that scales up and down. And that conformal down-scaling resets the particle horizon and entropy - and hence time...

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/25/2020 11:23 AM

Since we are allowed to choose any consistent coordinate system in physics, another interesting way of looking at it is through the "telescope" of stacked conformal time against comoving distance. It "portrays" the Penrose infinities beautifully, because the right-hand vertical scale shows the cosmic scale factor from essentially zero to infinity (equivalence of zero and infinity in this scheme).

Adapted from an original diagram by cosmologist Tamara Davis.

Comoving distance is proper distance divided by the scale factor, taking the cosmic expansion out of the distance to a specific comoving object, e.g. a galactic cluster. The horizontal scale near the top of each panel shows the present redshift of such objects. The timescale is chosen so that the particle horizon (green dashes, giving the farthest a mass-less particles could have traveled since tau=0) grows at unity slope. This gives the conformal time tau, i.e. conformal time equals the particle horizon radius at all times.

The blue line tau = now should have been only in the top panel and obviously represents our present. The panels shows that galactic clusters will all move out of our Hubble sphere and future event horizon, but eventually our own cluster's matter will also decay and black holes will evaporate, leaving only radiation and a few, what Penrose called ultra-relativistic particles, around.

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/27/2020 10:27 AM

An interesting Blog post by Sabine Hossenfelder on Sir Roger (some 2 years ago):

Roger Penrose still looks for evidence of universal rebirth

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/27/2020 7:06 PM

This guy, I think, does a pretty good job of explaining Conformal Cyclic Cosmology...

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/28/2020 12:59 AM

I could not find any reference to CCC in the video - it's more or less the anti-pole of CCC, i.e. inflation! Did you accidentally post the wrong one?

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/28/2020 5:55 AM

You're right, Jorrie. Here is the one I meant to post.

https://www.youtube.com/watch?v=PC2JOQ7z5L0

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/28/2020 9:41 AM

Thanks Rixter, I have missed this one.

Yes, this guy Matt O'Dowd and his team is excellent.

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Re: Roger Penrose and Conformal Cyclic Cosmology

07/29/2020 3:06 AM

There are other viable bounce models, with to me the most convincing one coming from Paul Steinhardt, the Albert Einstein Professor in Science at Princeton and his Simons Foundation group.

As far as I can understand their 2019 A New Kind of Cyclic Universe, it differs from Penrose's CCC in the sense that it substitutes the dark energy parameter Lambda (a constant over time) to a decreasing parameter, called quintessence in physics.[a]

According to this scheme, not only does the dark energy density parameter decay to zero, but it goes negative and stops the expansion and then let the universe contract under strict limiting conditions. These conditions prevent the universe to collapse into a singularity and start a new expansion while the energies are still compatible with general relativity. This means no quantum gravity and/or string theory concepts are required during the bounce.

Now this is something that I can live with comfortably, but only if I can understand how that bounce works! The explanation given by one of the collaborators, Anna Ijjas in Recent Advances in Constructing Smooth Cosmological Bounces I find rather tough to follow.

I like the way in which they use the Hubble radius in their scheme. During accelerating expansion due to the cosmological constant, the Hubble radius grows to a constant value and then stays the same forever. With a reversing cosmological constant, it will first increase and then later decrease to eventually approach zero. This is cleverly used in the bounce mathematics, which is however a little above my present level.

Since it is modern work from a respected source, I think it warrants more study...

[a] Wikipedia: "Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic; that is, it changes over time, unlike the cosmological constant which, by definition, does not change. Quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy."

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