Frozen-in fermionic singlet dark matter in non-standard cosmology with a decaying fluid

TL;DR

This paper investigates how a decaying fluid in the early Universe affects fermionic singlet dark matter production via freeze-in, revealing that entropy injection alters dark matter density and mass bounds from structure formation.

Contribution

It introduces a detailed analysis of freeze-in dark matter production in a non-standard cosmology with entropy injection, highlighting the impact on relic density and mass constraints.

Findings

Entropy injection reduces dark matter density after freeze-in.

Enhanced production rate needed to match observed dark matter density.

Modified cosmology can relax or tighten dark matter mass bounds.

Abstract

We perform a detailed study of dark matter production via freeze-in under the assumption that some fluid dominates the early Universe before depositing its energy to the plasma causing entropy injection. As a dark matter candidate we consider a fermionic singlet that is produced through its interactions with a scalar particle in the thermal plasma. The fluid alters the expansion rate of the Universe, as well as the scaling of the temperature, which significantly affects the evolution of both the number density and the mean momentum of the dark matter particle. We identify and discuss in detail the effects of the evolution of these quantities by considering several examples representing dark matter production at different stages of expansion and entropy injection. We find that, since the dark matter density is reduced when the entropy injection to the plasma continues after freeze-in, in order to reproduce its observational value an enhanced rate of dark matter production is required relative to standard cosmology. Furthermore, the impact of the assumed non-standard cosmological history on the dark matter mean momentum can result in either a relaxed or a tightened bound on the dark matter mass from large structure formation data.