Early-time cosmological solutions in scalar-Gauss-Bonnet theory

TL;DR

This paper explores early-universe solutions in scalar-Gauss-Bonnet gravity, revealing inflationary and singularity-free phases depending on the coupling parameter, with Ricci scalar effects negligible at high curvature.

Contribution

It introduces specific early-time cosmological solutions in scalar-Gauss-Bonnet theory with quadratic coupling, highlighting the role of coupling sign in inflationary and non-singular behaviors.

Findings

Negative coupling yields inflationary de Sitter solutions.

Positive coupling results in expanding singularity-free solutions.

Ricci scalar effects are negligible during early high-curvature phases.

Abstract

We consider a gravitational theory that contains the Einstein term, a scalar field and the quadratic Gauss-Bonnet term. We focus on the early-universe dynamics, and demonstrate that the Ricci scalar does not affect the cosmological solutions at early times, when the curvature is strong. We then consider a pure scalar-GB theory with a quadratic coupling function: for a negative coupling parameter, we obtain solutions that contain always an inflationary, de Sitter phase, while for a positive coupling function, we find instead expanding singularity-free solutions.