Domain Wall formation from $Z_2$ spontaneous symmetry breaking/restoration in Scalar-Einstein-Gauss-Bonnet theory

TL;DR

This paper investigates the formation and evolution of domain walls in a scalar-Einstein-Gauss-Bonnet theory with spontaneous $Z_2$ symmetry breaking, analyzing their cosmological implications and observational signatures.

Contribution

It provides a detailed numerical analysis of domain wall formation in Scalar-EGB gravity and assesses their potential observational signatures, including gravitational waves and primordial black holes.

Findings

Static domain walls can form in de Sitter backgrounds.

Cosmic expansion causes domain walls to 'melt' during radiation era.

Detection of such domain walls and associated signals is unlikely with foreseeable experiments.

Abstract

This study offers a detailed analysis of domain wall formation and its cosmological consequences in Einstein-Gauss-Bonnet gravity coupled to a scalar field. A central aspect of the model is the scalar field Lagrangian's ability to spontaneously break and restore its $Z_2$ discrete symmetry. This spontaneous symmetry breaking is a fundamental prerequisite for topological defect formation. In this context, domain walls arise as kink-like, solitonic solutions that interpolate between the distinct vacuum states of the theory. We perform a detailed numerical analysis of the dynamics of a neutral scalar field non-minimally coupled to the Gauss-Bonnet invariant, exploring its behavior across different cosmological backgrounds. Our results show that coupling to the Gauss-Bonnet term enables the formation of static domain walls with a fixed proper distance within a de Sitter (inflationary) background. Furthermore, we extend our analysis to a radiation-dominated epoch, where we identify that the cosmic expansion causes the "melting" of these domain walls. To assess the potential observational signatures of this scenario, we calculate the predicted spectrum of stochastic gravitational waves generated by the network dynamics using {\it CosmoLattice} package. We also examine the possible generation of Primordial Black Holes (PBHs) associated with collapsing domain walls. Regrettably, our calculations indicate that the direct observational detection of such domain walls from this model lies beyond the reach of foreseeable experiments. Our results constitute a No-Go argument against the generation of PBHs as well as of large amplitude GW signals from domain walls in a Scalar-EGB spontaneous symmetry breaking mechanism.