The dark matter annihilation boost from low-temperature reheating

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.

Concentrated dark matter: Enhanced small-scale structure from codecaying dark matter

“…We note that the above expressions agree with those of Ref. [3]. The scale of reheating, given by Eq.…”

“…Following Refs. [2,3], which use the Press-Schechter formalism [22], we now comment on the qualitative features of the predicted substructure.…”

“…Recently, however, it has been proposed that an early period of matter domination (before BBN) would have observable implications [1][2][3]. The idea is that a period of early matter domination would lead to structure formation in DM prior to freeze-out.…”

“…References [2,3] established the fraction of the dark matter abundance found in microhalos today resulting from an early matter dominated phase. There, it was found that the effect of the enhanced growth can be captured by altering the transfer function on the relevant scales, namely between the beginning of matter domination and the time of reheating.…”

“…These early seeds of structure can result in concentrated dark matter, where the bulk of dark matter is found in dark compact objects today. This intriguing possibility has led to a surge of studies of early structure formation [2][3][4][5][6][7][8][9]. However, while these models do predict significant matter domination and pre-BBN structure formation, one often finds that the perturbations do not survive to today.…”