Monitoring a de Sitter universe through an anti-de Sitter window

Abstract

We propose that AdS${}_3$ gravity coupled to dS${}_2$ end-of-the-world branes is dual to a unitary holographic CFT${}_2$ with non-unitary conformal boundary conditions. These boundary conditions have complex $g$-functions and appear in conjugate pairs. The associated gravitational path integral admits no real saddles, but does admit complex saddles. We show that an AdS${}_3$ black hole microstate with a dS${}_2$ brane behind the horizon corresponds to the unitary time evolution of a pure CFT state prepared by a Euclidean path integral on a cylinder with such boundary conditions. The construction predicts a boundary-condition-changing primary with $h=-c/8$, which resides in the boundary sector rather than the bulk spectrum and is therefore compatible with unitarity of the underlying CFT. This realizes dS holography as state preparation in a unitary AdS/CFT Hilbert space.

Revtex, 8 pages, 3 figures