Gravitational wave physics and astronomy: Genesis(重力波物理学・天文学:創世記)
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“INTERNATIONAL CONFERENCE on GRAVITY and COSMOLOGY 2019” and “30TH TEXAS SYMPOSIUM ON RELATIVISTIC ASTROPHYSICS”

“INTERNATIONAL CONFERENCE on GRAVITY and COSMOLOGY 2019”
Takahiro Tanaka (Kyoto University)

ICGC2019 in Mohari, Dec. 10-13, 2019

  ICGC is a series of workshop held in India every four years. About 200 people are participating and I could see most of familiar faces in India.
  On the first day, the talk by David Blair was about “Einstein-First”, which is a kind of program to teach the concepts of modern physics including relativity and quantum mechanics. He quoted the Einstein’s words in 1948 “Common sense is the collections of prejudices gathered by the age 18”. The idea is to give a series of experiences to possess the modern physics concepts as common sense to small children from the elementary school level to the high school level. I’m not sure if it is correct, but I got an impression that the program looks a kind of expansion of the exhibition at the science museum. Many people are participating in this program from various countries, but there is no participant from Japan. The course is composed of 4year lower level and upper level courses. And they say there are 25 chapters. I wonder if it is possible to teach the more fundamental things appropriately, if we take so many house for this kind of advanced contents. Also, I had my personal doubt if it is really good thing to teach such kind of concepts isolated from the ordinary life as an authorized knowledge to young children who cannot logically judge whether the statement is correct or not. But as an experiment, this action looks a little interesting.
Group Photograph
  There were talks about Event Horizon Telescope by Lia Medeiros and about the Planck results by Tuhin Ghosh. Chris van Den Broeck gave a talk about modified gravity test using gravitational waves, which overlaps with my own talk, scheduled on the same day in the earlier session, but he covered really wider range of topics. He mentioned the braneworld model as an example of the decay of gravitational wave amplitude. To my knowledge, I do not know any viable braneworld model that predicts detectable decay of gravitational wave amplitude. I asked this point later, and it turned out that his comment on the braneworld was not so serious.
Teruaki Suyama was also an invited speaker and gave a review about the primordial black holes. I did not recognize the explicit form for how the threshold amplitude of density fluctuation for the primordial black hole formation is modified due to the angular momentum. The expression for the threshold that he has shown is independent of the angular momentum for the matter dominant case, which does not agree with the intuition. I thought this might be a little worth of further investigation.
   On the 2nd day, Tirthankar Roy Choudhury talked about cosmology with the first stars. Basically, the interrelation between the first star formation and the reorganization history of the universe and the related observations such as 21cm line was discussed. This can be used to test the standard cosmology and also the extended models of gravity. It might be necessary to think about this direction of research more seriously. There are also the talks about future observation of the large scale structure of the universe, mainly focused on Euclid and SKA by Stefano Camera, about dark matter by Basudeb Dasgupta. Li Ju gave a talk about the third generation gravitational wave detectors. She also introduced the Australian plan, OzGravHF, which is oriented to the high frequency band, with 2-4km arm length. She also showed the estimate for the accuracy of the determination of H0 by the future network by ET + double CE, which can be down to 0.004% just by using the binary black hole observations without counterpart.
  In the evening there was a public lecture on the Eddington’s 1919 expedition by Kailash C. Sahu. There was a public rumor that Eddington discarded the data that contradicts with Einstein’s prediction, but the truth is that it was the Dyson who discarded the data, and the detailed explanation why they should do so is given in their paper in detail. In fact, the focus of the discarded plates was not good enough, but later technology revealed that the measured shifts of the stellar positions were consistent with the GR prediction. The speaker, Kailash, C. Sahu, performed the measurement of the deflection of light by a star other than the Sun, STEIN 2051B, using Hubble telescope.
  On the third day,there were talks about Information loss paradox by Sumanta Chakraborty and about LIGO by Jess Mclver and Summaya Nissanke. In Sammaya’s talk her work with Hotokezaka-kun was introduced, which improved the measurement of Hubble constant from GW170817 by factor two, combining the data of radio observation. The point is that the inclination angle is constrained by the radio observation, which reduce the error in the estimate of the luminosity distance. There is also an interesting talk by Tejinder P. Singh, who talked about the spontaneous localization and its extension to include gravity. The idea is to start with matrix variables. Assuming that a kind of thermal equilibrium is realized in a short time, commutation relation pops up as a statistical average. This idea seems to be introduced by Adler many years ago. At first grans, such a scenario is in conflict with Bell’s inequality, but in this particular scheme, the inequality seems to be satisfied somehow. There is also a YouTube program of this lecture by T.P. Singh. Vincent Vennim gave a talk about stochastic N, and at the end of his talk he introduced his new paper, in which they point out an exponential tail in the probability distribution for large amplitude of curvature perturbation is realized in the model in which the stochastic motion becomes important. If this is indeed the case, it may affect the primordial black hole formation scenario a lot. However, not the over-density but the curvature perturbation is used to set the PBH formation threshold. If we use the over-density threshold, which is pointed out to be more appropriate in recent works, the result may change.
  On the last day, there were talks by Bharat Ratra, Alexei Starobinsky, Sommath Bharadwaj, and Suneeta Varadarajan, about cosmological parameters and dark energy model, the simplest inflationary universe model, 21cm observation and AdS instability, respectively. First three talks were quite pedagogical ones. Suneeta’s talk was also a nice review about AdS instability, making connection with the Hamiltonian dynamics at around resonance. When we consider a massless scalar field in a fixed AdS spacetime, the eigen frequencies keep simple rational ratio. So resonance can explain the cascade of the power into higher frequency modes. However, if we add mass to the scalar field, the resonance condition is violated in general. Nevertheless the instability occurs if the initial amplitude of perturbation exceeds some small threshold value. She said this can be understood by using Benetin/Gallavotti theorem, although I could not understand the detail of the theorem.
  There were three parallel sessions and many poster presentations. The conference site is a university and we do not have to move around for meals. So, the conference was very dense in contents.


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