# Spontaneous scalarization in generalised scalar-tensor theory

## TL;DR

This paper investigates the mechanisms behind spontaneous scalarization in relativistic stars and black holes within generalized scalar-tensor theories, focusing on the role of specific terms in the Horndeski action that induce tachyonic instabilities.

## Contribution

It identifies all terms in the Horndeski action responsible for the effective mass term leading to scalarization, clarifying the conditions for tachyonic instability.

## Key findings

- All contributing terms to the effective mass in the Horndeski action are characterized.
- The conditions for tachyonic instability triggering scalarization are explicitly identified.
- The analysis enhances understanding of the nonlinear dynamics in scalar-tensor theories.

## Abstract

Spontaneous scalarization is a mechanism that endows relativistic stars and black holes with a nontrivial configuration only when their spacetime curvature exceeds some threshold. The standard way to trigger spontaneous scalarization is via a tachyonic instability at the linear level, which is eventually quenched due to the effect of non-linear terms. In this paper, we identify all of the terms in the Horndeski action that contribute to the (effective) mass term in the linearized equations and, hence, can cause or contribute to the tachyonic instability that triggers scalarization.