Natural Explanation for 21 cm Absorption Signals via Axion-Induced Cooling.

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 while in the condensed phase, a small amount of axion dark matter can explain these observations within the context of standard models of axions and axionlike particles. The EDGES best-fit result favors an axionlike particle mass in the (10, 450) meV range, which can be compressed for the QCD axion to (100, 450) meV in the absence of fine tuning. Future experiments and large scale surveys, particularly the International Axion Observatory (IAXO) and EUCLID, should have the capability to directly test this scenario.