Einstein-Gauss-Bonnet Cosmological Theories at Reheating and at the End of the Inflationary Era

In this work we study the GW170817-compatible Einstein-Gauss-Bonnet theories during the reheating and the end of inflationary era. Given the scalar field potential $V(\phi)$ which can have some intrinsic importance for the theory, determining the scalar coupling function $\xi(\phi)$ can be cumbersome due to lack of analyticity. The GW170817 observation constrains the scalar coupling function and the scalar field potential to have some interdependence, thus during the slow-roll era one can calculate the scalar coupling function. However, when the slow-roll era ends, it is expected that the scalar coupling function should have a different form and the same applies for the reheating era, assuming that the scalar potential of the theory does not change. In this work we exactly aim to highlight this feature of Einstein-Gauss-Bonnet theories, as the Universe evolves through distinct sequential evolution eras, and we focus on how to determine the scalar coupling function during the various evolutionary eras, from inflation to the reheating era. Regarding both the end of the inflationary era and the reheating era, it is found that the Hubble rate obeys a constant-roll-like condition of the form $\dot{H}=\delta H^2$, thus the determination of the scalar Gauss-Bonnet function $\xi(\phi)$ is reduced to solving a differential equation. A mentionable feature of the era exactly at the end of inflation is that the Klein-Gordon equation is decoupled from the field equations, because the Gauss-Bonnet invariant is zero. We provide several examples of interest to support our arguments.