Triangulating quantum gravity in AdS$_3$

Abstract: The path integral of pure 3D gravity with negative cosmological constant is formulated on a finite region of spacetime $M$, with boundary conditions that fix geodesic lengths or dihedral angles on $\partial M$. In the dual CFT, this quasi-local amplitude calculates corrections to the Gaussian ensemble of OPE coefficients for black hole states. By triangulating $M$ with generalized tetrahedra, we develop a general method to construct semiclassical geometries and to calculate the exact gravitational path integral on a fixed hyperbolic topology. The path integral with fixed-length boundary conditions is a Virasoro TQFT amplitude-squared, and with fixed-angle boundary conditions it is a partition function of Conformal Turaev-Viro theory. The two are related by a modular S-transform. In addition, we show how to translate the calculation of OPE statistics from Virasoro TQFT to the metric formalism, on general topologies. These results are derived exactly, and some examples are also checked semiclassically, including the geometries dual to the Virasoro 6j-symbol and the modular S-matrix. The classical saddlepoint geometries are finite-volume hyperbolic 3-manifolds ending on pleated Riemann surfaces, which have vanishing extrinsic curvature except on geodesics where they can bend into corners. The hyperbolic volumes of these geometries match the predictions of Conformal Turaev-Viro theory and the dual CFT.