Resonance of black hole quasinormal modes in coupled systems

TL;DR

This paper explores how coupled systems of black hole quasinormal modes exhibit resonance phenomena at avoided crossings, providing potential new signatures for detecting physics beyond standard models.

Contribution

It introduces a new definition of excitation factors for coupled systems and demonstrates resonance effects in a black hole within Einstein-Maxwell-axion theory.

Findings

Resonance occurs at avoided crossings in coupled QNM systems.

Amplification of excitation factors indicates resonance.

Resonance can appear between modes from different degrees of freedom.

Abstract

Black hole quasinormal modes (QNMs) can exhibit resonant excitations associated with avoided crossings in their complex frequency spectrum. Such resonance phenomena can serve as novel signatures for probing new physics, where additional degrees of freedom are commonly introduced. Motivated by this possibility, we investigate QNMs in systems where multiple degrees of freedom are coupled with each other, and introduce a definition of excitation factors suitable for such systems. To demonstrate our formulation, we apply it to a black hole in the Einstein-Maxwell-axion theory, where we find that avoided crossings can appear even between longest-lived modes originating from the fundamental modes of different degrees of freedom, in contrast to the Kerr case in General Relativity. We show that the excitation factors are indeed amplified as a manifestation of resonance at parameter values corresponding to the avoided crossings.