Insights on Dark Matter from Hydrogen during Cosmic Dawn

TL;DR

Upcoming 21-cm measurements during cosmic dawn can reveal properties of dark matter, especially if a subpercent fraction has a minicharge of about 10^{-6} and masses between 1-60 MeV, potentially explaining EDGES results.

Contribution

This paper proposes that a small fraction of dark matter with a minicharge can significantly influence baryonic temperature during cosmic dawn, offering a new way to detect dark matter properties.

Findings

Subpercent minicharged dark matter can cool baryons enough to match EDGES observations.

Minicharged dark matter with specific charge and mass ranges is consistent with current constraints.

21-cm experiments can potentially discover or constrain minicharged dark matter.

Abstract

The origin and composition of the cosmological dark matter remain a mystery. However, upcoming 21-cm measurements during cosmic dawn, the period of the first stellar formation, can provide new clues on the nature of dark matter. During this era, the baryon-dark matter fluid is the slowest it will ever be, making it ideal to search for dark matter elastically scattering with baryons through massless mediators, such as the photon. Here we explore whether dark-matter particles with an electric "minicharge" can significantly alter the baryonic temperature and, thus, affect 21-cm observations. We find that the entirety of the dark matter cannot be minicharged at a significant level, lest it interferes with Galactic and extragalactic magnetic fields. However, if minicharged particles comprise a subpercent fraction of the dark matter, and have charges $\epsilon \sim 10^{-6}$---in units of the electron charge---and masses $m_\chi \sim 1-60$ MeV, they can significantly cool down the baryonic fluid, and be discovered in 21-cm experiments. We show how this scenario can explain the recent result by the EDGES collaboration, which requires a lower baryonic temperature than possible within the standard model, while remaining consistent with all current observations.