Roger Penrose still looks for evidence of universal rebirth

Roger Penrose really should have won a Nobel Prize for something. Though I’m not sure for what. Maybe Penrose-tilings. Or Penrose diagrams. Or the Penrose process. Or the Penrose-Hawking singularity theorems. Or maybe just because there are so many things named after Penrose.

And at 87 years he’s still at it. Penrose has a reputation for saying rude things about string theory, has his own interpretation of quantum mechanics, and he doesn’t like inflation, the idea that the early universe underwent a rapid phase of exponential expansion. Instead, he has his own theory called “conformal cyclic cosmology” (CCC).

According to Penrose’s conformal cyclic cosmology, the universe goes through an infinite series of “aeons,” each of which starts with a phase resembling a big bang, then forming galactic structures as usual, then cooling down as stars die. In the end the only thing that’s left are evaporating black holes and thinly dispersed radiation. Penrose then conjectures a slight change to particle physics that allows him to attach the end of one aeon to the beginning of another, and everything starts anew with the next bang.

This match between one aeon’s end and another’s beginning necessitates the introduction of a new field – the “erebon” – that makes up dark matter, and that decays throughout the coming aeon. We previously met the erobons because Penrose argued their decay should create noise in gravitational wave interferometers. (Not sure what happened to that.)

If Penrose’s CCC hypothesis is correct, we should also be able to see some left-over information from the previous aeon in the cosmic microwave background around us. To that end, Penrose has previously looked for low-variance rings in the CMB, that he argued should be caused by collisions between supermassive black holes in the aeon prior to ours. The search for that, however, turned out to be inconclusive. In a recent paper with Daniel An and Krzysztof Meissner he has now suggested to look instead for a different signal.

The new signal that Penrose et al are looking for are points in the CMB at the places where in the previous aeon supermassive black holes evaporated. He and collaborators called these “Hawking Points” in memory of the late Stephen Hawking. The idea is that when you glue together the end of the previous aeon with the beginning of ours, you squeeze together the radiation emitted by those black holes and that makes a blurry point at which the CMB temperature is slightly increased.

Penrose estimates the total number of such Hawking Points which should be in the total cosmic microwave background is about a million. The analysis in the paper, covering about 1/3 of the sky, finds tentative evidence for about 20. What’s with the rest remains somewhat unclear, presumably too weak to be observed.

They look for these features by generating fake “normal” CMBs, following standard procedure, and then trying to find Hawking Points in these simulations. They have now done about 5000 of such simulations, but none of them, they claim, has features similar to the actually observed CMB. This makes their detection highly statistically significant, with a chance of less than 1/5000 that the Hawking Points which they find in the CMB are due to random chance.

In the paper, the authors also address an issue that I am guessing was raised by someone else somewhere, which is that in CCC there shouldn’t be a CMB polarization signal like the one BICEP was looking for. This signal still hasn’t been confirmed, but Penrose et al claim pre-emptively that in CCC there should also be a polarization, and it should go with the Hawking Peaks because:

“primordial magnetic fields might arise in CCC as coming [...] from galactic clusters in the previous aeon […] and such primordial magnetic fields could certainly produce B-modes […] On the basis that such a galactic cluster ought to have contained a supermassive black hole which could well have swallowed several others, we might expect concentric rings centred on that location”

Quite a collection of mights and coulds and oughts.

Like Penrose, I am not a big fan of inflation, but I don’t find conformal cyclic cosmology well-motivated either. Penrose simply postulates that the known particles have a so-far unobserved property (so the physics becomes asymptotically conformally invariant) because he wants to get rid of all gravitational degrees of freedom. I don’t see what’s wrong with that, but I also can’t see any good reason for why that should be correct. Furthermore, I can’t figure out what happens with the initial conditions or the past hypothesis, which leaves me feeling somewhat uneasy.

But really I’m just a cranky ex-particle-physicist with an identity crisis, so I’ll leave the last words to Penrose himself:

“Of course, the theory is “crazy”, but I strongly believe (in view of observational facts that seem to be coming to light) that we have to take it seriously.”