21-cm Constraints on Dark Matter Annihilation after an Early Matter-Dominated Era

TL;DR

This paper explores how an early matter-dominated era affects small-scale structure formation and dark matter annihilation, and assesses the potential of 21-cm observations to detect these effects and distinguish heating sources.

Contribution

It models the impact of an early matter-dominated era on dark matter annihilation signals and evaluates the sensitivity of 21-cm measurements to these effects.

Findings

21-cm power spectrum can potentially detect dark matter annihilation after an EMDE

Distinct heating signatures in 21-cm power spectrum can differentiate between dark matter and astrophysical X-ray heating

Future 21-cm measurements at z~17 may not surpass gamma-ray bounds in constraining dark matter annihilation

Abstract

Although it is commonly assumed 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 Big Bang nucleosynthesis. Subhorizon dark matter density perturbations grow faster during an EMDE than during a radiation-dominated era, leading to the formation of "microhalos" far earlier than in standard models of structure formation. This enhancement of small-scale structure boosts the dark-matter annihilation rate, which contributes to the heating of the intergalactic medium (IGM). We compute how the dark matter annihilation rate evolves after an EMDE and forecast how well measurements of the 21-cm background can detect dark matter annihilation in cosmologies with EMDEs. We find that future measurements of the global 21-cm signal at a redshift of $z\sim 17$ are unlikely to improve on bounds derived from observations of the isotropic gamma-ray background, but measurements of the 21-cm power spectrum have the potential to detect dark matter annihilation following an EMDE. Moreover, dark matter annihilation and astrophysical X-rays produce distinct heating signatures in the 21-cm power spectrum at redshifts around 14, potentially allowing differentiation between these two IGM heating mechanisms.