Gravity from entanglement for boundary subregions

TL;DR

This paper demonstrates that the RT formula for boundary subregions in AdS/CFT fully determines bulk metric perturbations satisfying linearized Einstein equations, extending previous results and supporting subregion duality.

Contribution

It generalizes the connection between RT formula validity on boundary segments and bulk Einstein equations to subregions, including black hole states.

Findings

RT formula on boundary segments determines bulk metric perturbations.

Bulk Einstein equations follow from RT formula validity on subregions.

Supports subregion-subregion duality in holography.

Abstract

We explore several aspects of the relation between gravity and entanglement in the context of AdS/CFT, in the simple setting of 3 bulk dimensions. Specifically, we consider small perturbations of the AdS metric and the CFT vacuum state and study what can be learnt about the metric perturbation from the Ryu-Takayanagi (RT) formula alone. It is well-known that, if the RT formula holds for all boundary spacelike segments, then the metric perturbation satisfies the linearized Einstein equations throughout the bulk. We generalize this result by showing that, if the RT formula holds for all spacelike segments contained in a certain boundary region, then the metric perturbation satisfies the linearized Einstein equations in a corresponding bulk region (in fact, it is completely determined in that region). We also argue that the same is true for small perturbations of the planar BTZ black hole and the CFT thermal state. We discuss the relation between our results and the ideas of subregion-subregion duality, and we point out that our argument also serves as a holographic proof of the linearized RT formula for boundary segments.