The EDGES 21 cm Anomaly and Properties of Dark Matter

TL;DR

The paper investigates the EDGES 21 cm anomaly, proposing that light dark matter emitting soft photons, especially axion mini-clusters, can naturally explain the signal, contrasting with other dark matter interaction models.

Contribution

It introduces a novel solution involving soft photon emission from light dark matter, specifically axion mini-clusters, to explain the 21 cm anomaly.

Findings

Dark matter-hydrogen interaction models are in tension with cosmological probes.

Soft photon emission from photophilic dark matter can account for the anomaly.

Axion mini-clusters are a promising candidate for the observed signal.

Abstract

The recently claimed anomaly in the measurement of the 21 cm hydrogen absorption signal by EDGES at $z\sim 17$, if cosmological, requires the existence of new physics. The possible attempts to resolve the anomaly rely on either (i) cooling the hydrogen gas via new dark matter-hydrogen interactions or (ii) modifying the soft photon background beyond the standard CMB one, as possibly suggested also by the ARCADE~2 excess. We argue that solutions belonging to the first class are generally in tension with cosmological dark matter probes once simple dark sector models are considered. Therefore, we propose soft photon emission by light dark matter as a natural solution to the 21 cm anomaly, studying a few realizations of this scenario. We find that the signal singles out a photophilic dark matter candidate characterised by an enhanced collective decay mechanism, such as axion mini-clusters.