On Thermal Gravitational Contribution to Particle Production and Dark Matter

TL;DR

This paper analyzes how gravitational interactions in the early universe could produce dark matter particles across a wide mass range, highlighting the significance of inflaton contributions and spin effects.

Contribution

It provides analytic formulas for gravitational particle production with full mass dependence and explores the impact of inflaton-induced production on dark matter relic abundance.

Findings

Dark matter with TeV to 10^{16} GeV mass can have correct relic abundance.

Inflaton contributions can dominate particle production, matching inflationary Hubble parameter dependence.

Fermion production from inflatons is suppressed by mass compared to bosons.

Abstract

We investigate the particle production from thermal gravitational annihilation in the very early universe, which is an important contribution for particles that might not be in thermal equilibrium or/and only have gravitational interaction, such as dark matter (DM). For particles with spin 0, 1/2 and 1 we calculate the relevant cross sections through gravitational annihilation and give the analytic formulas with full mass-dependent terms. We find that DM with mass between TeV and $10^{16}$GeV could have the relic abundance that fits the observation, with small dependence on its spin. We also discuss the effects of gravitational annihilation from inflatons. Interestingly, contributions from inflatons could be dominant and have the same power dependence on Hubble parameter of inflation as that from vacuum fluctuation. Also, fermion production from inflatons, in comparison to boson, is suppressed by its mass due to helicity selection.