A natural explanation for 21 cm absorption signals via the QCD axion

The EDGES Collaboration has reported an anomalously strong 21 cm absorption feature corresponding to the era of first star formation, which may indirectly betray the influence of dark matter during this epoch. We demonstrate that, by virtue of the ability to mediate cooling processes whilst in the condensed phase, a small amount of axion dark matter can explain these observations within the context of standard models of the QCD axion. Notably, this effect only requires adjustment of a single parameter to match to the EDGES data. The results favour an axion mass in the vicinity of 0.15 eV, and thus future experiments and large scale surveys, particularly the International Axion Observatory (IAXO) and EUCLID, should have the capability to directly test this scenario.