Black-hole spectroscopy: quasinormal modes, ringdown stability and the pseudospectrum

TL;DR

Black-hole spectroscopy uses gravitational wave ringdown signals to study black hole properties, but spectral instabilities and environmental effects pose challenges for accurate mode detection and interpretation.

Contribution

This paper introduces the pseudospectrum as a new tool to analyze the spectral stability of black-hole quasinormal modes and explores its implications for gravitational-wave spectroscopy.

Findings

Quasinormal modes often exhibit spectral instabilities.

Spectral instabilities can impact gravitational-wave signal interpretation.

Pseudospectrum analysis provides insights into mode stability and waveform robustness.

Abstract

Black-hole spectroscopy is a powerful tool to probe the Kerr nature of astrophysical compact objects and their environment. The observation of multiple ringdown modes in gravitational waveforms could soon lead to high-precision gravitational-wave spectroscopy, thus it is critical to understand if the quasinormal mode spectrum itself is affected by astrophysical environments, quantum corrections, and other generic modifications. In this chapter, we will review the black-hole spectroscopy program and its challenges regarding quasinormal mode detection, the overtone status and the recent evidence that supports the existence of nonlinearities in the spectrum of black holes. We will then discuss a newly introduced non-modal tool in black-hole physics, namely the pseudospectrum; a mathematical notion that can shed light on the spectral stability of quasinormal modes, and discuss its novel applications in black holes and exotic horizonless compact objects. We will show that quasinormal modes generically suffer from spectral instabilities, explore how such phenomena can further affect black-hole spectroscopy, and discuss potential ringdown imprints and waveform stability issues in current and future gravitational-wave detectors.