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.

Figure 1

The ALP $(m_a,{\mathrm{\Omega }}_a{h}^2)$ space satisfying Eq. (7), color-coded with the resulting 21 cm brightness temperature at $z=17$. Comparison with the best-fit EDGES result then suggests a $m_a\in (10,450)\mathrm{meV}$ range of compatibility. Since the QCD axion fixes the relationship between these quantities in terms of the dark matter density parameter $X$ appearing in Eq. (12), we overlay lines of fixed $X$ to show the dependence on this quantity.

Figure 2

The portion of the axion parameter space relevant for our purposes, reproduced from Ref. [32], with the 21 cm brightness temperature at $z\sim 17$ overlaid from axion-induced cooling processes in the benchmark case of $X=1$. The yellow band denotes QCD axion models with varying electromagnetic or color anomaly coefficients, while the black curves indicate possible sensitivities for the proposed IAXO experiment. The best fit axion mass value preferred by the EDGES observations in this case is 150 meV, while beyond $\sim 300\mathrm{meV}$ the ordinary $\mathrm{\Lambda }\mathrm{CDM}$ result for $T_{21}$ prevails. Variations in $X$ enlarge the preferred range to $m_a\in (100,450)\mathrm{meV}$ in the absence of fine-tuning, effectively shifting the color-coded region within the QCD axion band.