Signs of Dark Matter at 21-cm?

TL;DR

This paper investigates whether dark matter interactions could explain the anomalous 21-cm absorption signal observed by EDGES, concluding that most scenarios are strongly constrained, but a small millicharged component remains plausible.

Contribution

The study analyzes velocity-dependent dark matter interactions as a cooling mechanism for hydrogen, highlighting constraints and identifying a viable subcomponent of millicharged dark matter.

Findings

Most dark matter interaction scenarios are ruled out by experimental constraints.

Neutral hydrogen plays a minor role in cooling; residual electrons and protons are key.

A subcomponent (~1%) of millicharged dark matter could explain the signal.

Abstract

Recently the EDGES collaboration reported an anomalous absorption signal in the sky-averaged 21-cm spectrum around $z=17$. Such a signal may be understood as an indication for an unexpected cooling of the hydrogen gas during or prior to the so called Cosmic Dawn era. Here we explore the possibility that dark matter cooled the gas through velocity-dependent, Rutherford-like interactions. We argue that such interactions require a light mediator that is highly constrained by 5th force experiments and limits from stellar cooling. Consequently, only a hidden or the visible photon can in principle mediate such a force. Neutral hydrogen thus plays a sub-leading role and the cooling occurs via the residual free electrons and protons. We find that these two scenarios are strongly constrained by the predicted dark matter self-interactions and by limits on millicharged dark matter respectively. We conclude that the 21-cm absorption line is unlikely to be the result of gas cooling via the scattering with a dominant component of the dark matter. An order 1\% subcomponent of millicharged dark matter remains a viable explanation.