Quasinormal modes and quantum black hole interiors

Abstract

Quasinormal modes (QNMs) of black hole perturbations in the large overtone limit, namely, asymptotic QNMs, are highly sensitive to a black hole's interior geometry. We probe the singularity structure of a class of exact (2+1)-dimensional quantum black hole solutions to semi-classical gravity by computing their asymptotic QNM spectra. We do this using methods of complex analysis in which the radial coordinate is analytically continued to the complex plane. The leading order QNM frequencies all fit the generic form $ω=(\text{offset})+n(\text{gap})$, for overtone number $n$, which we also confirm numerically. Under a general assumption about the global Stokes topology of the complexified radial coordinate, we show how to reconstruct the scaling exponent of more general (spacelike or timelike) black hole singularities from the offset. We then compute subleading corrections to the asymptotic QNMs, from which we provide a robust method for extracting the scaling of the metric function near the singularity. Our findings exemplify the transition between "Kasner eons", successive regimes encountered on approach to a spacelike singularity, as non-perturbative quantum effects become dominant. For charged quantum black holes, the asymptotic QNMs exhibit a crossover between two regimes in which the neutral and charge contributions to the metric function dominate, respectively. Within the crossover region, the QNMs develop an oscillatory behavior that may be a signature of the inner horizon.