Gravitational portals in the early Universe

TL;DR

This paper investigates gravitational interactions during reheating after inflation, focusing on how they produce matter, radiation, and dark matter, and establishing bounds on the maximum reheating temperature in early Universe scenarios.

Contribution

It provides a detailed analysis of gravitational particle production mechanisms during reheating, including bounds on the maximal temperature, which are minimal and broadly applicable.

Findings

Gravitational production of dark matter and radiation is significant during reheating.

A lower bound of about 10^{12} GeV on the maximal temperature is derived.

Reheating temperatures are effectively limited to below 10^9 GeV due to gravitational effects.

Abstract

We consider the production of matter and radiation during reheating after inflation, restricting our attention solely to gravitational interactions. Processes considered are the exchange of a graviton, $h_{\mu \nu}$, involved in the scattering of the inflaton or particles in the newly created radiation bath. In particular, we consider the gravitational production of dark matter (scalar or fermionic) from the thermal bath as well as from scattering of the inflaton condensate. We also consider the gravitational production of radiation from inflaton scattering. In the latter case, we also derive a lower bound on the maximal temperature of order of $10^{12}$ GeV for a typical $\alpha-$attractor scenario from $\phi \phi \rightarrow h_{\mu \nu} \rightarrow$ Standard Model fields (dominated by the production of Higgs bosons). This lower gravitational bound becomes the effective maximal temperature for reheating temperatures, $T_{\rm{RH}} \lesssim 10^9$ GeV. The processes we consider are all minimal in the sense that they are present in any non-minimal extension of the Standard Model theory based on Einstein gravity and can not be neglected. We compare each of these processes to determine their relative importance in the production of both radiation and dark matter.