Monday, June 26, 2017

Perimeter Institute Conference on 'Bounce Scenarios in Cosmology'

With the rising prominence of alternative theories of gravity within mainstream physics, and the increasing realization of the need to evolve a viable alternative to inflation as a possible solution to traditional cosmological conundrums, it was inevitable that the implications of one for the other would come to be discussed at a conference. It used to be thought, well, naively at any rate (and thirty years ago) that quantum gravity is such an esoteric subject so far removed from anything observable, that the very idea might be too extreme. But with the advent of new and advanced observational techniques; indeed, with gravitational waves an entirely new window into the universe, and with a large corpus of work by quantum gravitationalists (running into several thousand papers over the past 30 years, as Prof. Abhay Ashtekar, one of the main speakers here, tells us in his conference Abstract), the idea of an alternative not only to inflation but the Big Bang itself has come into its own. And, observational consequences of quantum gravity and loop quantum cosmology have been worked out. This is the central theme of this conference at the Perimeter Institute, that begins today, 6/26/17, and goes till Wednesday 6/28. Neil Turok, Director of the Perimeter Institute, and himself an expert on the subject matter of the conference, in his opening remarks, set out eight questions on the blackboard (a very useful device, one reminds oneself, even with all the advanced imaging technology at one's disposal these days!), that he hoped would get answered over the three days of the conference, and which I am transcribing here with the benefit of a tweetpic by Dr Francesca Vidotto. (emphasis in the specific words and phrases in the transcript below is reproduced as it was in the original board-script.)
1. What is the new physics responsible for the bounce? 
2. Might this new physics resolve the singularity for the perturbations as well as the background? 
3. Does this new physics have an observational signature? (my own favorite, a question that might be answered in Dr Ue-Li Pen's talk -SB) 
4. What general principles underlie the theory, beyond wanting to resolve the singularity? 
5. Does a consistent picture for cosmology require that both the perturbations and the background are quantised? 
6. Does the bounce or pre-bounce phase help in setting initial conditions? 
7. What are the common features of different cosmological bounce scenarios? (close runner up in my fav list :-) SB) 
8. Can they be classified in a useful way? 
Of course, one question that did not make it into the list, probably because its scope is slightly wider than merely being about Bounce Scenarios - is this one, whether inflation could occur despite (or even, because of; or even more disjunctively and non-causatively, separately of!) the quantum bounce. Some would argue that the quantum bounce makes inflation 'unnecessary', but as a theoretical physicist, one would need to also establish whether inflation is indeed consistent (or not) with a quantum bounce - beyond just saying whether inflation becomes moot or superfluous 'since the bounce takes care of cosmological conundrums'! And in any case, there are more than one kind of possible scenarios for the quantum bounce, which is the point of the conference clearly expressed in the title -, so that some may be consistent with inflation, and some may not; thus making both points of view - the one that sees the quantum bounce as an alternative to inflation, and the one that sees the two co-existing, describing different phases of the very early universe - right; or right within some set of assumptions, or at least, not necessarily in contradiction with each other!

Thursday, June 22, 2017

QCD Phase Diagram - NuPECC Long Range Plan