Neil Turok - Gravitational entropy and the large scale geometry of spacetime

Thursday 5 May 2022, 14:00 → 15:00 Europe/London

OC218

Abstract: I’ll review a new, simpler explanation for the large scale geometry of spacetime, presented recently by Latham Boyle and me in arXiv:2201.07279. The basic ingredients are elementary and well-known, namely Einstein’s theory of gravity and Hawking’s method of computing gravitational entropy. The new twist is provided by the boundary conditions we proposed for big bang-type singularities, respecting CPT and conformal symmetry (traceless matter stress energy) as well as analyticity at the bang. These boundary conditions allow gravitational instantons for universes with positive Lambda, massless (exactly conformal) radiation and positive or negative space curvature. Using these new instantons, we are able to infer the gravitational entropy for a complete set of quasi-realistic, four-dimensional cosmologies. If the total entropy in radiation exceeds that of Einstein’s static universe, the gravitational entropy exceeds the famous de Sitter entropy. As it increases further, the most probable large-scale geometry becomes increasingly flat, homogeneous and isotropic. I’ll summarize recent progress towards elaborating this picture into a fully predictive cosmological theory.