Convergence of the Fefferman-Graham expansion and complex black hole anatomy

TL;DR

This paper investigates the convergence properties of the Fefferman-Graham expansion in holographic spacetimes, linking singularities and horizons to the expansion's radius of convergence and exploring how much of the interior can be reconstructed.

Contribution

It establishes a connection between metric singularities, black hole features, and the convergence radius of the Fefferman-Graham expansion, providing new insights into holographic spacetime reconstruction.

Findings

Black holes with spacelike singularities have a convergence radius equal to the horizon radius.

Black holes with timelike singularities have a smaller convergence radius.

Finite convergence radius can occur without horizons or singularities, as shown by explicit examples.

Abstract

Given a set of sources and one-point function data for a Lorentzian holographic QFT, does the Fefferman-Graham expansion converge? If it does, what sets the radius of convergence, and how much of the interior of the spacetime can be reconstructed using this expansion? As a step towards answering these questions we consider real analytic CFT data, where in the absence of logarithms, the radius is set by singularities of the complex metric reached by analytically continuing the Fefferman-Graham radial coordinate. With the conformal boundary at the origin of the complex radial plane, real Lorentzian submanifolds appear as piecewise paths built from radial rays and arcs of circles centred on the origin. This allows singularities of Fefferman-Graham metric functions to be identified with gauge-invariant singularities of maximally extended black hole spacetimes, thereby clarifying the physical cause of the limited radius of convergence in such cases. We find black holes with spacelike singularities can give a radius of convergence equal to the horizon radius, however for black holes with timelike singularities the radius is smaller. We prove that a finite radius of convergence does not necessarily follow from the existence of an event horizon, a spacetime singularity, nor from caustics of the Fefferman-Graham gauge, by providing explicit examples of spacetimes with an infinite radius of convergence which contain such features.