Multi-scalar Gauss-Bonnet gravity: scalarized black holes beyond spontaneous scalarization

TL;DR

This paper explores a nonlinear scalarization mechanism in multi-scalar Gauss-Bonnet gravity, revealing new scalarized black hole solutions that evade previous observational constraints and analyzing their stability and thermodynamics.

Contribution

It extends the concept of nonlinear black hole scalarization to multi-scalar Gauss-Bonnet theories with exponential couplings, demonstrating new solutions with zero scalar charge.

Findings

Existence of scalarized black holes with zero scalar charge.

Numerical demonstration of solutions for various coupling functions.

Analysis of thermodynamics and stability of solution branches.

Abstract

Recently, a new nonlinear mechanism for black hole scalarization, different from the standard spontaneous scalarization, was demonstrated to exist for scalar Gauss-Bonnet theories in which no tachyonic instabilities can occur. Thus Schwarzschild black hole is linearly stable but instead nonlinear instability can kick-in. In the present paper we extend on this idea in the case of multi-scalar Gauss-Bonnet gravity with exponential coupling functions of third and fourth leading order in the scalar field. The main motivation comes from the fact that these theories admit hairy compact objects with zero scalar charge, thus zero scalar-dipole radiation, that automatically evades the binary pulsar constraints on the theory parameters. We demonstrate numerically the existence of scalarized black holes for both coupling functions and for all possible maximally symmetric scalar field target spaces. The thermodynamics and the stability of the obtained solution branches is also discussed.