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

TL;DR

This work investigates domain-wall formation from spontaneous $Z_2$ symmetry breaking and restoration in a scalar field coupled to the Gauss-Bonnet invariant within Einstein-Gauss-Bonnet gravity. Using both analytic and numerical methods, including one-loop Coleman–Weinberg corrections, the authors show that static domain walls can arise in a de Sitter (inflationary) background, but these walls rapidly melt during a radiation-dominated epoch as the GB term decays. Gravitational waves and primordial black holes from the wall network are found to be undetectable with foreseeable experiments, establishing a No-Go result for such signatures in this model. The study also demonstrates a pronounced dependence of the GW spectrum on the initial Hubble parameter and highlights the limitations of scalar-field simulations for predicting plasma-mediated effects, suggesting future work on scalar-fluid interactions to fully assess observational prospects.

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.

Figures (8)

• Figure 1: Numerical solution of Eq.\ref{['FullEqAfterSubC']} for different values of $C$. We see that walls are smeared by the expansion as $C$ approaches $2$, whereas bigger $C$ corresponds to thin wall. It is also noticeable that transition region is always at the de Sitter horizon.
• Figure 2: Comparison of domain wall's static configurations with and without loop correction.
• Figure 3: Spectrum of GWs during RD stage. Duration of simulations $\tau_f = 10\tau_i$ and $m=10^{-3}H_0$.
• Figure 4: Spectrum of GWs during RD stage. Time of simulations $\tau_f = 10\tau_i$ and $m=0$.
• Figure 5: Spectrum of GWs during RD stage. Here we compare spectra obtained with different initial Hubble parameters. One can easily see strong dependence on its value. Although even for unrealisticly high values of $H_0$ does not produce detectable spectrum.