Bouncing geodesics, black hole singularities, and singularities of thermal correlators

TL;DR

This paper explores how bouncing geodesics relate to black hole singularities and their signatures in boundary correlation functions within a holographic framework, providing new insights into diagnosing spacetime singularities.

Contribution

It establishes a rigorous connection between bouncing geodesics, propagator divergences, and singularities in holographic spacetimes, extending the analysis beyond geodesic regimes.

Findings

Retarded propagators diverge when connected by null geodesics.

Bouncing geodesics encode singularity information in correlation functions.

Examples show limitations of bouncing geodesics in detecting certain singularities.

Abstract

Bouncing geodesics have been used as valuable probes of black hole singularities. In the dual boundary theory, the presence of bouncing geodesics is encoded in the analytic structure of correlation functions. Thus, when their existence is related to the presence of a black hole singularity, this presents a practical holographic framework to analyse, diagnose, and classify spacetimes with curvature singularities. To make this intuition precise, we use the Hadamard theory of hyperbolic differential equations to prove that both bulk and boundary retarded propagators diverge whenever two points can be connected by a null geodesic. We clarify why this statement remains valid beyond the geodesic regime (for operators of any dimension) and examine how holographic renormalisation modifies the structure of the dual propagator. We also present a general characterisation of bouncing geodesics and the associated singularities in correlation functions for arbitrary spacetimes. Furthermore, we compare the analytic structure of the correlators in position and momentum space and discuss explicit examples. Finally, we demonstrate the validity and concrete limitations of the bouncing geodesic approach to the study of black hole singularities. In particular, we show an explicit example of a black hole in the self-dual linear axion model, which has a curvature singularity despite the absence of bouncing geodesics.