Monday, 12 October 2015

CosPA 2015 Liveblog: Day One, Session Four

I missed the first parallel session because I as deep in conversation.  With only two parallel sessions, I've gone for one on gravitational physics.

4:00 pm: Gravity on Equation of State, Hyeong-Chan Kim

Fix the surface-singularity problem in Born-Infeld gravity by allowing strong gravitational effects to modify the equation of state.  Exploit the local equivalence of freely-falling and flat spacetimes.  Use simple model of ideal adiabatic gas in constant gravity field.  Find that only effect is simple change in number density: gains a position dependence, localised towards bottom of container.  Local properties same as for ideal gas for local density.

Weak-field limit changes the equation of state only at second order in the local field strength.  As such, for most astrophysical systems this effect can be ignored.  However, for macroscopic fields even in the non-relativistic Newtonian regim, there are noticeable differences.  There are several cases that this corresponds to
  • A system in total thermal equilibrium, so all system must be considered
  • Near an event horizon (need GR)
  • Large de Broglie wavelength (need QM)
  • Self-gravitational effects vary more rapidly than inverse size of system (impossible in GR)
Some discussion over that last point.  I don't think it's clear to me what exactly is being meant.

Some work with a toy example (self-gravitating sphere in thermal equilibrium).  Not quite following the intending meaning.

4:25 pm: Interferometric Probes of Planckian Quantum Gravity, Ohkyung Kwon

Good opening joke: "if you heard this talk a month and a half ago, I hope you're in the other room"!

Quantum gravity is non-renormalisable, which is problematic.  From the field theory framework, much fruitful work but still limitations.  From gravity side, what if NR due to quantisation of spacetime?  This is part of loop quantum gravity.  Solve ths problem by coupling quantum fields to dynamical metric and spacetime.  Obviously, still an open problem.

Reason to consider macroscopic scales: a QFT in a box has a volume-independent energy density proportional to the UV cutoff.  A black hole, in contrast, has volume-dependent energy density.  There is an upper bound on the volume for which these two results can be consistent.  See also the cosmological constant problem.  Entropic models for gravity are dominated by spacetime degrees of freedom.

What phenomenological regions are not yet excluded, but potentially accessible?  Can these be related to actual models?

Interferometers have now reached Planckian spectral densities: the uncertainties in time fluctations can be pushed below Planck lengths.

I'm interested in this subject, but pretty badly lost.

Key question: comparison with LIGO.  Sensitivity of experiment here is higher, but at much higher frequency.  Hence not a suitable tool to measure gravity waves outside of certain very exotic sources.  Hence look to measure some kind of correlation of coherent noise.

4:50 pm: A Particle Probing Thermodynamics in Rotating AdS Black Hole, Bogeun Gwak

Addresses the cosmic censorship hypothesis.

Change black hole properties by throwing particles into it.

A lot of technical details.  "Redefining" energy and rotation is probably legit but phrased awkwardly.

Problem: can black hole be sup up to remove horizon by throwing things into it?  Cosmic censorship hypothesis says this is not possible.  Find that in this case, black hole mass always increases by enough to (over)-compensate for increased rotation.  Can not remove horizon this way.

5: 15 pm: Transition Probability of Neutrino-Electron Elastic Scattering for Dirac and Majorana Neutrino, Asan Damanik 

Turns out this speaker is not around, so we finish early. 

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