From regular black holes to horizonless objects: quasi-normal modes, instabilities and spectroscopy

TL;DR

This paper investigates the quasi-normal modes and stability of regular black hole mimickers, revealing long-lived modes and potential instabilities, and assesses their detectability via gravitational-wave observations.

Contribution

It provides a detailed analysis of QNM spectra for Bardeen-like and Simpson-Visser metrics, clarifies previous misunderstandings, and explores observational prospects.

Findings

Long-lived modes in ultracompact, horizonless configurations

Deviations from standard GR black hole spectra

Potential detectability with future gravitational-wave detectors

Abstract

We study gravitational and test-field perturbations for the two possible families of spherically symmetric black-hole mimickers that smoothly interpolate between regular black holes and horizonless compact objects accordingly to the value of a regularization parameter. One family can be described by the Bardeen-like metrics, and the other by the Simpson-Visser metric. We compute the spectrum of quasi-normal modes (QNMs) of these spacetimes enlightening a common misunderstanding regarding this computation present in the recent literature. In both families, we observe long-living modes for values of the regularization parameter corresponding to ultracompact, horizonless configurations. Such modes appear to be associated with the presence of a stable photon sphere and are indicative of potential non-linear instabilities. In general, the QNM spectra of both families display deviations from the standard spectrum of GR singular BHs. In order to address the future detectability of such deviations in the gravitational-wave ringdown signal, we perform a preliminary study, finding that third generation ground-based detectors might be sensible to macroscopic values of the regularization parameter.