Superradiant Instabilities in a Class of Scalar-Tensor Horndeski Theory

TL;DR

This paper investigates superradiant instabilities in a scalar-tensor Horndeski theory, revealing how kinetic coupling to curvature can lead to wave amplification and black hole instability.

Contribution

It introduces the analysis of superradiance in Horndeski black holes with kinetic curvature coupling, extending previous studies to new scalar-tensor models.

Findings

Kinetic coupling creates a trapping potential outside the black hole horizon.

Superradiant amplification increases with coupling strength.

Reissner-Nordström black holes become unstable due to this effect.

Abstract

We study the superradiance effect in a class of scalar-tensor Horndeski theory. We first study the dynamics of a massive charged scalar wave scattered off the horizon of a Reissner-Nordstr\"om black hole which except its canonical coupling to gravity it is also coupled kinetically to curvature. We find that a trapping potential is formed outside the horizon of a Reissner-Nordstr\"om black hole, due to this coupling, and as the strength of the new coupling is increased, the scattered wave is superradiantly amplified, resulting to the instability of the Reissner-Nordstr\"om spacetime. We then considered the backreacting effect of the scalar field coupled to curvature interacting with the background metric and we study the superradiant effect and find the superradiance conditions of a massive charged wave scattered off the horizon of a Horndeski black hole.