Holography of 3d Flat Cosmological Horizons

TL;DR

This paper derives the entropy of 3d flat cosmological horizons using a dual field theory with BMS3 symmetry, connecting flat holography to the well-understood AdS/CFT correspondence.

Contribution

It provides the first derivation of horizon entropy in 3d flat space via dual field theory state counting, extending holography beyond AdS geometries.

Findings

Reproduces bulk entropy from dual theory in large charge limit

Shows flat horizons obey a thermodynamic first law

Establishes a connection between BMS3 symmetry and Galilean Conformal Algebra

Abstract

We provide a first derivation of the Bekenstein-Hawking entropy of 3d flat cosmological horizons in terms of the counting of states in a dual field theory. These horizons appear in the shifted-boost orbifold of R^{1,2}, the flat limit of non-extremal rotating BTZ black holes. These 3d geometries carry non-zero charges under the asymptotic symmetry algebra of R^{1,2}, the 3d Bondi-Metzner-Sachs (BMS3) algebra. The dual theory has the symmetries of the 2d Galilean Conformal Algebra, a contraction of two copies of the Virasoro algebra, which is isomorphic to BMS3. We study flat holography as a limit of AdS3/CFT2 to semi-classically compute the density of states in the dual, exactly reproducing the bulk entropy in the limit of large charges. Our flat horizons, remnants of the BTZ inner horizons also satisfy a first law of thermodynamics. We comment on how the dual theory reproduces the bulk first law and how cosmological bulk excitations are matched with boundary quantum numbers.