Dark Matter Freeze-in Production in Fast-Expanding Universes

TL;DR

This paper investigates how a faster early-universe expansion, caused by an additional dominant species, suppresses freeze-in dark matter production, requiring stronger couplings for observed relic abundance, thus impacting detection prospects.

Contribution

It provides a detailed analytical and numerical study of freeze-in dark matter production in non-standard cosmologies with faster expansion rates.

Findings

Dark matter production via freeze-in is significantly suppressed in fast-expanding universes.

Couplings between dark matter and visible particles must be increased by orders of magnitude to match observed relic abundance.

Enhanced couplings improve the prospects for detecting freeze-in dark matter.

Abstract

If the dark matter is produced in the early universe prior to Big Bang nucleosynthesis, a modified cosmological history can drastically affect the abundance of relic dark matter particles. Here, we assume that an additional species to radiation dominates at early times, causing the expansion rate at a given temperature to be larger than in the standard radiation-dominated case. We demonstrate that, if this is the case, dark matter production via freeze-in (a scenario when dark matter interacts very weakly, and is dumped in the early universe out of equilibrium by decay or scattering processes involving particles in the thermal bath) is dramatically suppressed. We illustrate and quantitatively and analytically study this phenomenon for three different paradigmatic classes of freeze-in scenarios. For the frozen-in dark matter abundance to be as large as observations, couplings between the dark matter and visible-sector particles must be enhanced by several orders of magnitude. This sheds some optimistic prospects for the otherwise dire experimental and observational outlook of detecting dark matter produced by freeze-in.