Dissecting the ensemble in JT gravity

Abstract: We calculate bulk and boundary correlators in JT gravity by summing over geometries. The answers are reproduced by computing suitable ensemble averages of correlators of chaotic quantum systems. We then consider bulk correlators at large spatial separations and find that semiclassical decay eventually makes way for erratic oscillations around some nonzero answer. There is no cluster decomposition because of wormholes connecting distant regions. We construct more microscopic versions of JT gravity which are dual to a single quantum system by including a set of branes in the gravitational theory the data of which describes the Hamiltonian of the dual system. We focus on the bulk description of eigenstates which involves end of the world branes and we explain how observables factorize due to geometries connecting to these branes.