A Holographic Map from AdS$_3$ to CFT$_2$
We propose a holographic map from the semiclassical Hilbert space of pure general relativity in $\text{AdS}_{3}$ to that of $\text{CFT}_{2}$. We define the bulk Hilbert space by semiclassically quantising the phase space in the basis of a fixed-area network on a Cauchy slice $\Sigma$. A fixed-area network is a maximal non-intersecting set of geodesics on $\Sigma$ whose lengths and angular momenta have been fixed. Our holographic map differentiates between `external' geodesics that are homotopic to a connected component of $\partial \Sigma$, and `internal' geodesics which are not. The lengths and angular momenta of external geodesics become conformal weights of primaries in the Hilbert space of the CFT living on the corresponding component of $\partial \Sigma$. The fixed-area network determines the wave function, which is given by a network of OPE coefficients of primaries whose weights are determined by the corresponding lengths. For sufficiently semiclassical states, there is an agreement between bulk and boundary inner products. We apply this proposal to various physics questions. The boundary dual of a bulk gauge turns out to be an emergent basis for sufficiently semiclassical states. We also define boundary operators that measure the lengths of geodesics behind the horizon, again in semiclassical states. Finally, we apply our map to closed universes and find a failure of semiclassicality in simple cases, which can be partially alleviated by the addition of a massive probe.