Gauss-Bonnet Cosmology: large-temperature behaviour and bounds from Gravitational Waves

TL;DR

This paper analyzes the high-temperature behavior of Gauss-Bonnet cosmology, revealing attractor states, evolution paths, and how the model enhances gravitational wave backgrounds, leading to new bounds on the reheating temperature from BBN constraints.

Contribution

It provides a detailed phase space analysis of dilaton-Einstein-Gauss-Bonnet cosmology at high temperatures, identifying attractors and evolution paths, and derives bounds on reheating temperature from gravitational wave and BBN data.

Findings

Identification of three main attractors in the high-temperature regime.

Eight possible evolution paths in the autonomous system.

Enhanced gravitational wave background in certain parameter regions.

Abstract

We provide a transparent discussion of the high temperature asymptotic behaviour of Cosmology in a dilaton-Einstein-Gauss-Bonnet (dEGB) scenario of modified gravity with vanishing scalar potential. In particular, we show that it has a clear interpretation in terms of only three attractors (stable critical points) of a set of autonomous differential equations: $w=-\frac{1}{3}$, $w=1$ and $1<w<\frac{7}{3}$, where $w\equiv p/\rho$ is the equation of state, defined as the ratio of the total pressure and the total energy density. All the possible different high-temperature evolution histories of the model are exhausted by only eight paths in the flow of the set of the autonomous differential equations. Our discussion clearly explains why five out of them are characterized by a swift transition of the system toward the attractor, while the remaining three show a more convoluted evolution, where the system follows a meta-stable equation of state at intermediate temperatures before eventually jumping to the real attractor at higher temperatures. Compared to standard Cosmology, the regions of the dEGB parameter space with $w=-\frac{1}{3}$ show a strong enhancement of the expected Gravitational Wave stochastic background produced by the primordial plasma of relativistic particles of the Standard Model. This is due to the very peculiar fact that dEGB allows to have an epoch when the energy density $\rho_{\rm rad}$ of the relativistic plasma dominates the energy of the Universe while at the same time the rate of dilution with $T$ of the total energy density is slower than what usually expected during radiation dominance. This allows to use the bound from BBN to put in dEGB a constraint $T_{\rm RH}\lesssim 10^8 - 10^9$ GeV on the reheating temperature of the Universe $T_{\rm RH}$. Such BBN bound is complementary to late-time constraints from compact binary mergers.