# Cosmological instability of scalar-Gauss-Bonnet theories exhibiting scalarization

## TL;DR

This paper investigates the instability caused by scalar-Gauss-Bonnet coupling in scalar-tensor theories, revealing that such theories can lead to rapid cosmological instabilities during inflation, challenging their viability.

## Contribution

It demonstrates that scalar-Gauss-Bonnet coupling induces tachyonic instabilities in cosmological solutions, extending the concept of scalarization beyond compact objects to the early universe.

## Key findings

- Instability occurs during inflation, disrupting standard cosmological evolution.
- Scalar-Gauss-Bonnet coupling acts as a tachyonic mass for scalar modes.
- Rapid instability challenges the viability of these theories.

## Abstract

In a subclass of scalar-tensor theories, it has been shown that standard general relativity solutions of neutron stars and black holes with trivial scalar field profiles are unstable. Such an instability leads to solutions which are different from those of general relativity and have non-trivial scalar field profiles, in a process called scalarization. In the present work we focus on scalarization due to a non-minimal coupling of the scalar field to the Gauss-Bonnet curvature invariant. The coupling acts as a tachyonic mass for the scalar mode, thus leading to the instability of general relativity solutions. We point out that a similar effect may occur for the scalar modes in a cosmological background, resulting in the instability of cosmological solutions. In particular, we show that a catastrophic instability develops during inflation within a period of time much shorter than the minimum required duration of inflation. As a result, the standard cosmological dynamics is not recovered. This raises the question of the viability of scalar-Gauss-Bonnet theories exhibiting scalarization.