Severely Constraining Dark Matter Interpretations of the 21-cm Anomaly

TL;DR

This paper critically examines dark matter models proposed to explain the 21-cm absorption anomaly, finding most are ruled out by existing constraints, leaving only a narrow viable parameter space involving millicharged particles.

Contribution

It provides a comprehensive analysis of dark matter scenarios for the 21-cm anomaly, applying multiple astrophysical and laboratory constraints to identify the limited viable models.

Findings

Most dark matter models are ruled out by constraints.

A small fraction (~0.3-2%) of dark matter with specific properties remains viable.

Viable models require additional depletion mechanisms to avoid overproduction.

Abstract

The EDGES Collaboration has recently reported the detection of a stronger-than-expected absorption feature in the global 21-cm spectrum, centered at a frequency corresponding to a redshift of z ~ 17. This observation has been interpreted as evidence that the gas was cooled during this era as a result of scattering with dark matter. In this study, we explore this possibility, applying constraints from the cosmic microwave background, light element abundances, Supernova 1987A, and a variety of laboratory experiments. After taking these constraints into account, we find that the vast majority of the parameter space capable of generating the observed 21-cm signal is ruled out. The only range of models that remains viable is that in which a small fraction, ~ 0.3-2%, of the dark matter consists of particles with a mass of ~ 10-80 MeV and which couple to the photon through a small electric charge, epsilon ~ 10^{-6}-10^{-4}. Furthermore, in order to avoid being overproduced in the early universe, such models must be supplemented with an additional depletion mechanism, such as annihilations through a L_{\mu}-L_{\tau} gauge boson or annihilations to a pair of rapidly decaying hidden sector scalars.