Timelike boundaries in de Sitter JT gravity and the Gao-Wald theorem

The Gao-Wald theorem says that de Sitter spacetime reacts to a positive energy perturbation by getting "taller." How does this change in the presence of timelike boundaries? We study this question in two-dimensional de Sitter JT gravity coupled to conformal matter. The effect of the boundaries has its roots in quantum corrections to the vacuum energy of the CFT due to Casimir-like effects. We consider two different kinds of timelike boundaries, either at locations where the coordinate φ is constant or at locations where the dilaton is constant. For each kind we compute the vacuum expectation value of the matter CFT stress tensor. The stress tensor violates the null energy condition in the first case and saturates it in the second case, with each case exhibiting a negative energy density in the vacuum state. We then compute the semiclassical backreaction of this energy density and show how it can make the spacetime "fatter" or "taller," depending on the regime. The spacetime getting fatter corresponds to an increase in the dilaton everywhere and hence also in the static patch horizon area in the higher-dimensional solution, and the spacetime getting taller corresponds to a decrease. We comment on the implications of our results for the quantum theories living on the timelike boundaries.