A de Sitter Farey Tail

TL;DR

This paper explores quantum gravity in three-dimensional de Sitter space, summing over geometries including instantons, revealing divergences in the vacuum partition function that challenge standard regularization methods.

Contribution

It introduces a sum over geometries analogous to the Farey tail in AdS, incorporating non-trivial topologies and instantons into the de Sitter quantum gravity path integral.

Findings

Sum over geometries is formulated as a modular coset sum.

Perturbative corrections are computed to all orders using Chern-Simons theory.

The vacuum partition function diverges, resisting standard regularization.

Abstract

We consider quantum Einstein gravity in three dimensional de Sitter space. The Euclidean path integral is formulated as a sum over geometries, including both perturbative loop corrections and non-perturbative instanton corrections coming from geometries with non-trivial topology. These non-trivial geometries have a natural physical interpretation. Conventional wisdom states that the sphere is the unique Euclidean continuation of de Sitter space. However, when considering physics only in the causal patch of a single observer other Euclidean geometries, in this case lens spaces, contribute to physical observables. This induces quantum gravitational effects which lead to deviations from the standard thermal behaviour obtained by analytic continuation from the three sphere. The sum over these geometries can be formulated as a sum over cosets of the modular group; this is the de Sitter analog of the celebrated "black hole Farey tail." We compute the vacuum partition function including the sum over these geometries. Perturbative quantum corrections are computed to all orders in perturbation theory using the relationship between Einstein gravity and Chern-Simons theory, which is checked explicitly at tree and one-loop level using heat kernel techniques. The vacuum partition function, including all instanton and perturbative corrections, is shown to diverge in a way which can not be regulated using standard field theory techniques.