Constraining Dark Matter Annihilation in Early Matter Dominated Era Cosmologies Using the Global 21 cm Absorption Signal

Although standard cosmology assumes that relativistic particles dominate the energy density of the Universe quickly after inflation, a variety of well-motivated scenarios predict an early matter dominated era (EMDE) before the onset of the radiation dominated era (RDE). Subhorizon dark matter (DM) density perturbations grow faster during an EMDE than during an RDE, leading to an increased abundance of microhalos that form far earlier than in standard models of strucuture formation. This enhancement of small-scale structure boosts the DM annihilation rate, and we discuss how to compute boost factors for cosmologies with EMDEs. Since the first halos form by direct collapse, we assign a halo to each local maxima that collapses in the dark matter density field following an EMDE, and we predict the halo's central density profile from the properties of the associated maximum. The boost factor due to EMDE cosmologies can be several orders of magnitude greater than in standard cosmology, and the additional energy injected from annihilating DM increases the temperature of the intergalactic medium, TK. During the Dark Ages the sky- averaged (global) 21 cm signal is highly sensitive to TK, and we use forecasts of the global 21 cm signal to present bounds on the DM annihilation cross section in cosmologies with EMDEs.