Linear stability of black holes in shift-symmetric Horndeski theories with a time-independent scalar field

TL;DR

This paper analyzes the linear stability of static, spherically symmetric black holes with scalar hair in shift-symmetric Horndeski theories, identifying conditions for stability and exploring specific solutions' stability properties.

Contribution

It provides a comprehensive stability analysis of black holes in shift-symmetric Horndeski theories, including new stability conditions and examples of stable solutions.

Findings

Nontrivial scalar hair black holes are generally unstable near the horizon in reflection-symmetric theories.

Certain nonasymptotically flat hairy black holes in cubic Galileon theories are free of instabilities.

Perturbatively constructed asymptotically flat black holes with scalar-Gauss-Bonnet coupling are stable.

Abstract

We study linear perturbations about static and spherically symmetric black holes with a time-independent background scalar field in shift-symmetric Horndeski theories, whose Lagrangian is characterized by coupling functions depending only on the kinetic term of the scalar field $X$. We clarify conditions for the absence of ghosts and Laplacian instabilities along the radial and angular directions in both odd- and even-parity perturbations. For reflection-symmetric theories described by a k-essence Lagrangian and a nonminimal derivative coupling with the Ricci scalar, we show that black holes endowed with nontrivial scalar hair are unstable around the horizon in general. This includes nonasymptotically flat black holes known to exist when the nonminimal derivative coupling to the Ricci scalar is a linear function of $X$. We also investigate several black hole solutions in nonreflection-symmetric theories. For cubic Galileons with the Einstein-Hilbert term, there exists a nonasymptotically flat hairy black hole with no ghosts/Laplacian instabilities. Also, for the scalar field linearly coupled to the Gauss-Bonnet term, asymptotically flat black hole solutions constructed perturbatively with respect to a small coupling are free of ghosts/Laplacian instabilities.