Quadratic gravity corrections to scalar QNMs of rapidly rotating black holes

TL;DR

This paper computes leading-order deviations in scalar quasinormal modes of rapidly rotating black holes within quadratic gravity theories, extending previous work to near-extremal spins using advanced numerical solutions.

Contribution

It introduces a method to calculate quasinormal mode corrections for high-spin black holes in scalar Gauss-Bonnet and Chern-Simons gravity, surpassing previous spin limitations.

Findings

Accurate quasinormal mode deviations for spins up to 0.99

Corrections can increase significantly for spins above 0.9

Pseudo-spectral methods enable precise calculations for high spins

Abstract

In an effective-field-theory framework for gravity, black-hole quasinormal mode spectra acquire corrections in quadratic-curvature, scalar-tensor extensions of general relativity. Previous calculations of such corrections were limited to moderate spins, since the corresponding background solutions relied on expansions in the spin parameter. Using recently constructed numerical black-hole solutions valid for large spin, we compute the leading-order deviations from general relativity in the scalar quasinormal mode spectrum of rotating black holes in scalar Gauss-Bonnet and dynamical Chern-Simons gravity. We solve the resulting perturbation equations with pseudo-spectral collocation methods, allowing us to determine the quasinormal-mode corrections for dimensionless spins up to $a/M=0.99$, with accuracy better than $\lesssim 10^{-3}$ for the $l=m=0$ mode and $\lesssim 10^{-6}$ for higher multipoles. For spins $a/M>0.9$, the corrections to certain modes can increase by orders of magnitude.