paper

Quasi-local energy and microcanonical entropy in two-dimensional nearly de Sitter gravity

arXiv:2203.00700 · doi:10.1007/JHEP08(2022)075

Abstract

We study the semi-classical thermodynamics of two-dimensional de Sitter space () in Jackiw-Teitelboim (JT) gravity coupled to conformal matter. We extend the quasi-local formalism of Brown and York to , where a timelike boundary is introduced in the static patch to uniquely define conserved charges, including quasi-local energy. The boundary divides the static patch into two systems, a cosmological system and a black hole system, the former being unstable under thermal fluctuations while the latter is stable. A semi-classical quasi-local first law is derived, where the Gibbons--Hawking entropy is replaced by the generalized entropy. In the microcanonical ensemble the generalized entropy is stationary. Further, we show the on-shell Euclidean microcanonical action of a causal diamond in semi-classical JT gravity equals minus the generalized entropy of the diamond, hence extremization of the entropy follows from minimizing the action. Thus, we provide a first principles derivation of the island rule for symmetric backgrounds, without invoking the replica trick. We discuss the implications of our findings for static patch de Sitter holography.

arXiv comment48 pages + 4 appendices, 13 figures; minor comments added v2, JHEP version; v3 corrected formulae and plot for heat capacity and fixed typo in Appendix A (original conclusions unaltered)

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