Subregion algebras in classical and quantum gravity

Abstract: We study the kinematics and dynamics of subregion algebras in classical and perturbative quantum gravity associated with portions of null surfaces such as event horizons and finite causal diamonds. We construct half-sided supertranslation generators by extending subregion phase spaces of the event horizon to include doubled pairs of corner edge modes obtained from splitting the horizon, namely relative boosts and null translations of the respective corners. These edge modes carry a corner symplectic form and give rise to canonical charges generating half-sided boosts and translations. We show that the null translation generator is necessarily two-sided in the complementary translation edge modes. The charges act nontrivially on gravitationally dressed local observables on the horizon, such that the horizon subalgebra naturally takes the form of a crossed product by the associated automorphism group. Quantizing the extended phase space after linearizing around a black hole background, we obtain for each horizon cut a Type II${\infty}$ von Neumann algebra equipped with a trace, whose von Neumann entropy coincides with the generalized entropy of that cut. The integrability of the half-sided null translation generator lifts to the existence of a self-adjoint operator that implements null time evolution on the Type II$\infty$ horizon subalgebras. The area operator is identified as the bulk implementation of the Connes cocycle flow for one-sided observables in excited states. The nesting property of the resulting one-parameter family of horizon subalgebras implies a generalized second law for non-stationary linearized perturbations of Killing horizons. Lastly, we use gravitational half-sided modular inclusion algebras to prove the quantum focusing conjecture in the perturbative quantum gravity regime.