Gravitational odd-parity perturbation of a Horndeski hairy black hole: quasinormal mode and parameter constraint

TL;DR

This paper refines the calculation of quasinormal modes for a specific Horndeski hairy black hole, demonstrating stability, identifying unique decay patterns, and exploring implications for gravitational wave tests of black hole hair.

Contribution

It introduces a new auxiliary field and coordinate to improve QNM calculations in Horndeski gravity, confirming stability and analyzing the impact of hair on gravitational wave signals.

Findings

The proposed method cures superluminal propagation issues.

The black hole is stable under odd perturbations.

Mode decay rates differ from GR, with potential observational signatures.

Abstract

During the binary black hole coalescence, gravitational waves emitted at the ringdown stage can be well described by black hole perturbation theory, where the quasinormal modes (QNMs) become the important ingredient in modeling the pattern waveform. In general relativity (GR), the QNMs can be obtained from solving the Regge-Wheeler (RW) equation of a non-rotating black hole. While in Horndeski gravity, the isospectrality between the odd and even parity perturbations is broken due to the scalar field, but the odd perturbation equation can be simplified into a modified RW equation from the perturbed action. In this paper, we propose a new auxiliary field and tortoise coordinate to refine the modified RW equation in Horndeski gravity, and calculate the QNMs frequencies of the odd perturbation of a specific hairy black hole. It is found that this proposal not only cures the superluminal propagation addressed in the previous literature, but also hold the original QNM spectrum of the odd perturbation. Moreover, our results indicate that such a Horndeski hairy black hole is stable under the odd perturbation, which is also verified by the time evolution of the perturbation. In particular, in contrary to GR, the modes with $\ell=2$ can decay faster than modes with $\ell>2$ for a certain range of the Horndeski hair, and the link between the null geodesics and QNM for the odd perturbation in the current theory is violated. Then, we use the ringdown QNMs to preliminarily investigate the signal-to-noise-ratio (SNR) rescaled measurement error of the Horndeski hair. We obtain significant effects of the angular momentum and overtone on the error bound of the hair parameter. We hope that our findings could inspire more theoretical and phenomenological work on the test of the no-hair theorem of black hole using gravitational wave physics.