Instability of spherically-symmetric black holes in Quadratic Gravity

TL;DR

This paper studies the linear stability of two types of spherically-symmetric black holes in Quadratic Gravity, revealing a long-wavelength instability that imposes a lower bound on their horizon radius.

Contribution

It extends previous instability analysis to the non-Schwarzschild branch, showing both branches are unstable below a critical horizon radius.

Findings

Both black hole branches exhibit long-wavelength instability.

Instability occurs below a critical horizon radius where branches intersect.

Classical perturbations may set a lower bound on black hole size in Quadratic Gravity.

Abstract

We investigate the linear stability of the two known branches of spherically-symmetric black holes in Quadratic Gravity. We extend previous work on the long-wavelength (Gregory-Laflamme) instability of the Schwarzschild branch to a corresponding long-wavelength instability in the non-Schwarzschild branch. In both cases, the instability sets in below a critical horizon radius at which the two black-hole branches intersect. This suggests that classical perturbations enforce a lower bound on the horizon radius of spherically-symmetric black holes in Quadratic Gravity.