The Dark Matter Annihilation Boost from Low-Temperature Reheating

TL;DR

This paper explores how early Universe conditions, especially low-temperature reheating, can significantly boost dark matter annihilation signals through enhanced small-scale structure, offering new ways to probe cosmic history via gamma-ray observations.

Contribution

It introduces the concept that early matter-dominated eras can increase dark matter annihilation signals, providing a novel method to constrain reheating scenarios using gamma-ray data.

Findings

Enhanced substructure boosts annihilation rates by orders of magnitude.

Dark matter masses below 1 TeV may be constrained by gamma-ray observations.

Reheating history impacts dark matter detection prospects.

Abstract

The evolution of the Universe between inflation and the onset of big bang nucleosynthesis is difficult to probe and largely unconstrained. This ignorance profoundly limits our understanding of dark matter: we cannot calculate its thermal relic abundance without knowing when the Universe became radiation dominated. Fortunately, small-scale density perturbations provide a probe of the early Universe that could break this degeneracy. If dark matter is a thermal relic, density perturbations that enter the horizon during an early matter-dominated era grow linearly with the scale factor prior to reheating. The resulting abundance of substructure boosts the annihilation rate by several orders of magnitude, which can compensate for the smaller annihilation cross sections that are required to generate the observed dark matter density in these scenarios. In particular, thermal relics with masses less than a TeV that thermally and kinetically decouple prior to reheating may already be ruled out by Fermi-LAT observations of dwarf spheroidal galaxies. Although these constraints are subject to uncertainties regarding the internal structure of the microhalos that form from the enhanced perturbations, they open up the possibility of using gamma-ray observations to learn about the reheating of the Universe.