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!), that he hoped would get answered over the three days of the conference, and which I am transcribing here with the benefit of a tweet by Dr Francesca Vidotto.
(emphasis in the specific words and phrases is 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 Bounce Scenarios is this one, whether inflation could occur despite (or even because of, or even more, separately of) the quantum bounce. Some would argue that the quantum bounce makes inflation 'unnecessary', but as theoretical physicists, one would need to also establish whether inflation is indeed consistent (or not) with a quantum bounce beyond just 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, the point of the conference, 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.


Thursday, June 22, 2017

Eastern Gravity Meeting June 9-10 2017, State College PA

The 20th Eastern Gravity Meeting (EGM20) was held at the Institute for Gravitation and the Cosmos, Penn State University earlier this month, June 9-10, 2017. Of the 46 contributed papers, as many as 19 were presented by students. The program was divided into eight sessions: 2 each on: Gravitational Waves (one chaired by Mike Eracleous & the other by Chad Hanna, both of Penn State); Numerical Relativity (one chaired by KG Arun of ICTS Bengaluru and the other by Jeffrey Winicour of the University of Pittsburgh) and Quantum Aspects (one chaired by Abhay Ashtekar and the other by Nelson Yokomizo, both of Penn State); and one each on: Mathematical General Relativity (chaired by BS Sathyaprakash of Penn State) and Astrophysics, Cosmology & Observations (chaired by Shara Shandera of Penn State).

While session chairs were thus drawn almost entirely from the host university, the participants were from up and down the East Coast, from UMass Dartmouth to the Florida Atlantic University - with large contingents from Case Western Reserve University, the Rochester Institute of Technology, the University of Pittsburgh, Temple University, Cornell University, and even the National Autonomous University of Mexico (UNAM)!

I read through the program in detail, and while all the sessions had interesting papers,  papers from the two sessions on Quantum Aspects piqued my interest the most, followed by those in the session on Cosmology, Astrophysics and Observations (this just reflects my own interests and should not be taken in any way whatsoever as a comment on the quality of papers in the other sessions). 

The APS Student Prize was shared by two papers - one by Rahul Kashyap of UMass Dartmouth on Type Ia Supernovae through Spiral Instability in Binary White Dwarf Mergers (from the Cosmology, Astrophysics and Observations session) and the other On the Conceptual Confusion in the Notion of Transverse-Traceless Modes by Beatrice Bonga of Penn State from the first session on Gravitational Waves


Wednesday, June 21, 2017

The Nuclear Physics European Collaboration Committee NuPECC Long Range Plan




The Neutron Star Equation of State and Gravitational Waves



Monday, June 19, 2017

Bernard Schutz on Binary Black Hole Formation in Globular Clusters




Saturday, June 17, 2017

Can Inflation exist in a Quantum Bouncing Cosmology?





Thursday, June 15, 2017

The Alternative Gravity Landscape

Dr Tessa Baker of Oxford University has come up with a brilliant idea for a taxonomy of modified theories of gravity: simply, or as it turns out, not quite so simply, parse the constituent clauses of Lovelock's Theorem, and then 'break' them, (i.e., violate them) one (or possibly more than one) at a time, and voila, you get the Alternative Gravity Landscape! She has presented it in other fora before, but I came across it in twitter reportage of the Conference on Cosmology and the Future of Spacetime held at Western University's Rotman School of Philosophy in June 2017.
Roughly paraphrased, Lovelock's theorem is the statement that the only local, second-order gravitational field equations that can be derived from an action that consists solely of the four-dimensional metric tensor of spacetime are the Einstein field equations, with a possible cosmological constant term.


Tuesday, June 13, 2017

Possible Lepton Universality Violations in B-meson decays


Cosmology and the Future of Spacetime - London ON June 12-14 2017


Sunday, June 11, 2017

HIRAX - The Hydrogen Intensity & Real-time Analysis Experiment



Square Kilometre Array Engineering Meeting 2017




Friday, June 9, 2017

PRIZM: The Probing Radio Intensity at High-Z from Manion Experiment


Thursday, June 8, 2017

NASA's NICER: The Neutron Star Interior Composition ExploreR





Wednesday, June 7, 2017

The Gravitational Wave Event #GW170104, #ATLAS17aeu and GRB170105A





Monday, June 5, 2017

GW1701014, GW151226 & GW170104 Likely Formed BBH via Different Mechanisms





Wednesday, May 31, 2017

An Axion Star Stability Diagram - DJE Marsh et al


Tuesday, May 30, 2017

Thanu Padmanabhan 60th Birthday Festschrift: Gravity and the Quantum





I first met up with Prof Thanu 'Paddy' Padmanabhan in 1982/83 in the pages of Science Today (a general-interest Indian science magazine at the level of New Scientist or Scientific American, now sadly defunct, which I had been reading or at least leafing through, since 1972 or earlier). Clearly, Paddy believed in writing popular articles for a broad scientific audience, even when he was just in his mid-twenties!  

Science Today would regularly rope in active scientists, usually from TIFR, to write articles on frontier research topics in particle physics and astrophysics - I remember articles by many famous names - Narlikar, Chitre, Sreekantan, Udgaonkar, MGK Menon, to name only a few, written at a level that would challenge all but the actual expert. Casual readers would thus come away with a bit of appreciation for the research, but the introspective ones among them would realize also that there was a lot more to the subject that lay beyond their own comprehension. Paddy's article, in similar style, perhaps at a slightly higher level even than the average prevalent in Science Today at the time, was on the Early Universe (and as I remember it, discussed mainly the inflationary scenario). It was part of a series of articles he wrote for Science Today at that time. Thankfully, by the time my General Relativity teacher at Delhi University, Prof. N. Panchapakesan, referred it to me around July 1983, I had already seen and read it, even if not entirely digested it !

Although I have never had the good fortune of meeting Paddy personally, I have certainly come to know him very well indeed through his numerous books, and popular as well as scholarly articles. Not that I've read all of them. (but I certainly hope I eventually will!) So here's wishing him a very happy 60th (in the Indian tradition, known as the shashtiabdipoorti - the achievement of the 60th anniversary) and a very active and continuing professional life in quantum gravity and related subjects in the years to come. 

Paddy's first doctoral student, (now) Professor Tejinder Pal Singh of TIFR - happens to be an exact contemporary of mine, who I have tracked since 1977, the year we both passed our Class X exams from schools in the same geographic area (he doing so, of course, with greater distinction!). I have also followed Paddy's own doctoral adviser, Prof Jayant Vishnu Narlikar, for even longer, both through the pages of Science Today and his scientific writings - so much so that a paper from 1979 I presented while still a first-year undergraduate at St Stephen's College, was based on Prof Narlikar's work on white holes and particle astrophysics. I thus feel really fortunate to have known Paddy not only through his own work, but also through that of his mentor Narlikar and his student TP Singh, a veritable scientific dynasty among Indian gravitational physicists!!

Australian National University Centre for Gravitational Physics


Friday, May 26, 2017

Testing Gravity with the Square Kilometre Array (SKA)





3 Interesting Slides From the 'Physics with the Square Kilometre Array' Conference


Wednesday, May 24, 2017

'Gravity Waves' and 'Gravitational Waves'




Tuesday, May 23, 2017

Gravity Research Foundation Awards 2017






Monday, May 22, 2017

New Book 'Loop Quantum Gravity: The First Thirty Years'







Friday, May 12, 2017

Augmented and Numerical Kludge Gravitational Waveforms



Monday, May 1, 2017

The Gravitational Wave Spectrum




Saturday, January 14, 2017

Some Recent Tweets on the Laser Interferometric Gravitational Observatory - LIGO


























Sunday, January 8, 2017

Prof Abhay Ashtekar Speaks at Ben Gurion University