Reheating era in Gauss-Bonnet theories of gravity compatible with the GW170817 event

TL;DR

This paper explores how the reheating era can be modeled within Einstein-Gauss-Bonnet gravity, ensuring compatibility with gravitational wave observations, and derives conditions for viable inflationary models with specific scalar couplings.

Contribution

It introduces a framework for describing reheating in Gauss-Bonnet gravity, including new equations and constraints, and analyzes two inflationary models consistent with observational bounds.

Findings

Reheating equations resemble those of scalar field models with modified scalar potential.

Both inflationary models can satisfy theoretical and observational constraints.

Reheating can be characterized by either stiff matter or radiation-like equations of state.

Abstract

In the present article we showcase how the reheating era can be described properly in the context of Einstein-Gauss-Bonnet gravity assuming that the primordial gravitational waves propagate with the velocity of light. The equations of the duration of reheating along with the reheating temperature are derived and as demonstrated, their expressions are quite similar to the case of a canonical scalar field where now the second derivative of the Gauss-Bonnet scalar coupling function appears and effectively alters the numerical value of the scalar potential. The appearance of such term is reminiscing of a $\lambda R$ model of gravity where $\lambda$ is now dynamical. We consider two viable inflationary models of interest, the former involves an error function as scalar Gauss-Bonnet coupling function and the latter a Woods-Saxon scalar potential. It is shown that for both models the aforementioned quantities can be in agreement with theoretical expectations. The only constraint that is needed is the assumption that the second time derivative of the Gauss-Bonnet scalar coupling function is actually lesser than the Planck mass squared, that is $\ddot\xi<\frac{M_{Pl}^2}{8}$ in order to obtain a viable description. We find that a free parameter of the theory and specifically the potential amplitude for the Woods-Saxon model during the inflationary era, dictates the effective equations of state and therefore the reheating epoch can be described either by a type of stiff matter with EoS parameter equal to unity or by an EoS parameter close to that of the radiation.