# Gravitational entropy and the large scale geometry of spacetime

### APA

Turok, N. (2022). Gravitational entropy and the large scale geometry of spacetime. Perimeter Institute. https://pirsa.org/22040112

### MLA

Turok, Neil. Gravitational entropy and the large scale geometry of spacetime. Perimeter Institute, Apr. 14, 2022, https://pirsa.org/22040112

### BibTex

@misc{ pirsa_PIRSA:22040112, doi = {10.48660/22040112}, url = {https://pirsa.org/22040112}, author = {Turok, Neil}, keywords = {Quantum Gravity}, language = {en}, title = {Gravitational entropy and the large scale geometry of spacetime}, publisher = {Perimeter Institute}, year = {2022}, month = {apr}, note = {PIRSA:22040112 see, \url{https://pirsa.org}} }

**Collection**

**Subject**

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 symmetry and analyticity at the bang with finite Weyl curvature. These boundary conditions allow gravitational instantons for universes with positive Lambda, massless (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 de Sitter entropy. As the total entropy in radiation is increased further, the most probable large-scale geometry for the universe becomes increasingly flat, homogeneous and isotropic. I’ll also briefly summarize recent progress towards elaborating this picture into a fully predictive cosmological theory.

Zoom Link: https://pitp.zoom.us/j/95474226765?pwd=clN5UHBnSTFiSVAzM0p3dEdDMzV2Zz09