Entanglement Entropy in Jackiw-Teitelboim de Sitter gravity with Timelike Boundaries

Abstract: The consideration of timelike boundaries in de Sitter static patches has a broad motivation, such as the formulation of a well-defined canonical ensemble and the realization of a natural framework for static patch holography. In this work we study Jackiw-Teitelboim de Sitter gravity with symmetric timelike reflecting boundaries, which, in the presence of both cosmological and "black hole" horizons, naturally separate the spacetime into a "black hole system" and a "cosmological system". We apply the island formula to compute the entanglement entropy of conformal matter in both systems. In the "black hole system" an island appears, causing the entanglement entropy to saturate at the horizon value and preventing late-time growth. In the "cosmological system" no island appears, and the entanglement entropy can become arbitrarily large depending on the position of the boundaries, indicating a tension with unitarity.