Charged black holes in string-inspired gravity: II. Mass inflation and dependence on parameters and potentials

TL;DR

This paper explores how the parameters of string-inspired gravity models influence mass inflation inside charged black holes, finding that bias in the Brans-Dicke field prevents mass inflation unless it is well trapped by a potential.

Contribution

It demonstrates the conditions under which mass inflation occurs in string-inspired gravity models, highlighting the role of the Brans-Dicke field bias and potential trapping.

Findings

Mass inflation is absent when the Brans-Dicke field is biased toward strong or weak coupling.

A Cauchy horizon and mass inflation require no bias or trapping of the Brans-Dicke field.

Bias in the Brans-Dicke field prevents the formation of a Cauchy horizon.

Abstract

We investigate the relation between the existence of mass inflation and model parameters of string-inspired gravity models. In order to cover various models, we investigate a Brans-Dicke theory that is coupled to a U(1) gauge field. By tuning a model parameter that decides the coupling between the Brans-Dicke field and the electromagnetic field, we can make both of models such that the Brans-Dicke field is biased toward strong or weak coupling directions after gravitational collapses. We observe that as long as the Brans-Dicke field is biased toward any (strong or weak) directions, there is no Cauchy horizon and no mass inflation. Therefore, we conclude that to induce a Cauchy horizon and mass inflation inside a charged black hole, either there is no bias of the Brans-Dicke field as well as no Brans-Dicke hair outside the horizon or such a biased Brans-Dicke field should be well trapped and controlled by a potential.