Severely Constraining Dark-Matter Interpretations of the 21-cm Anomaly

The EDGES Collaboration has recently reported the detection of a stronger-than-expected absorption feature in the global 21-cm spectrum, centered at a frequency corresponding to a redshift of $z\simeq 17$. This observation has been interpreted as evidence that the gas was cooled during this era as a result of scattering with dark matter. In this Letter, we explore this possibility, applying constraints from the cosmic microwave background, light element abundances, Supernova 1987A, and a variety of laboratory experiments. After taking these constraints into account, we find that the vast majority of the parameter space capable of generating the observed 21-cm signal is ruled out. The only viable models are those in which a small fraction, $\sim 0.3\%–2\%$, of the dark matter consists of particles with a mass of $\sim 10–80\mathrm{MeV}$ and which couple to the photon through a small electric charge, roughly ${10}^{-6}–{10}^{-4}$ as large as the electron charge. Furthermore, in order to avoid being overproduced in the early Universe, such models must be supplemented with an additional depletion mechanism, such as annihilations through a $L_{\mu }\text{−}{L}_{\tau }$ gauge boson or annihilations to a pair of rapidly decaying hidden sector scalars.

Figure 1

Constraints on Dirac fermion millicharged dark matter from Supernova 1987A (gray) [23], the SLAC millicharge experiment (blue) [24], the light element abundances produced during big bang nucleosynthesis (red, labeled $\mathrm{\Delta }{N}_{\mathrm{eff}}$) [25], and on the impact on the cosmic microwave background of dark matter scattering with baryons (pink, labeled CMB, KD) [14] and dark matter annihilations (purple, labeled CMB ann.) [28]. In each panel, the solid black region can explain the amplitude of the observed 21-cm absorption feature as reported by the EDGES Collaboration [2]. The dashed black line denotes where the thermal relic abundance corresponds to the indicated value of $f_{\mathrm{DM}}$, assuming only millicharge interactions.

Figure 2

Constraints on millicharged dark matter, $\chi$, in the viable parameter space of Fig. 1 (bottom left panel) with a new mediator $V$ of mass $m_V=3m_{\chi }$ and gauge coupling $g_{\chi }=1$. (For other choices of this coupling and mass ratio, the constraints are generically stronger.) Left: We assume $V$ couples equally to all three generations of charged standard model leptons. The blue band approximately shows the parameter space which can explain the amplitude of the observed 21-cm absorption feature. This is ruled out by constraints from the BABAR [36, 37, 38] and E137 [37, 39] experiments over the entire range of parameter space that is capable of generating the reported signal. Shown as dashed lines are projected constraints from the Belle II [40], BDX [37, 41], and LDMX [40, 42] experiments. Right: Parameter space in which $V$ couples to $L_{\mu }\text{−}{L}_{\tau }$. Future measurements by the NA64 [43] and ${\mathrm{M}}^3$ experiments [44] are expected to be sensitive to this scenario. In the green band, $V$ resolves the $(g-2{)}_{\mu }$ anomaly [45, 46, 47, 48]. The shaded gray region is constrained by the CCFR experiment [49, 50].