Severely Constraining Dark Matter Interpretations of the 21-cm Anomaly

Abstract: 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 ~ 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 study, 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 range of models that remains viable is that in which a small fraction, ~ 0.3-2%, of the dark matter consists of particles with a mass of ~ 10-80 MeV and which couple to the photon through a small electric charge, epsilon ~ 10^{-6}-10^{-4}. 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}-L_{\tau} gauge boson or annihilations to a pair of rapidly decaying hidden sector scalars.

• The paper rigorously tests dark matter cooling models using CMB, BBN, and supernova data to sharply limit possible parameter spaces.
• It finds that only a minor component (0.3–2%) with masses between 10–80 MeV and small electric charge can feasibly account for the anomaly.
• The study highlights the need for additional depletion mechanisms and further exploration of millicharged interactions to reconcile observations with cosmological data.

Severely Constraining Dark Matter Interpretations of the 21-cm Anomaly

This paper critically examines the implications of a finding by the EDGES Collaboration, which reported a stronger-than-expected absorption feature in the global 21-cm spectrum, centered at a redshift $z \sim 17$. This anomalous signal has been hypothesized to indicate cooling of baryons through interactions with dark matter. The authors rigorously scrutinize this hypothesis by leveraging constraints from the cosmic microwave background (CMB), light element abundances, Supernova 1987A, and laboratory experiments.

The investigation reveals that most potential dark matter parameter spaces that could produce such a 21-cm signal are constrained out. Specifically, the only plausible region left is for models where a minor fraction, approximately $0.3-2\%$, of the dark matter comprises particles with masses around 10-80 MeV, accompanied by a small electric charge ($\epsilon \sim 10^{-6}-10^{-4}$). These charged particles are referred to as millicharged dark matter and are shown to be severely restricted by existing astrophysical and experimental data.

The paper emphasizes that for dark matter to efficiently cool the gas at the observed level, it must be relatively light and decoupled primarily by velocity-dependent interactions. However, stringent limits on fifth-force particle interactions and additional energy contributions to the radiation density during recombination further confine viable models. Consequently, the study identifies millicharged dark matter as the only feasible candidate under existing constraints.

To maintain the consistency of the dark matter density with observational limits, particularly given that the ideal parameter space predicts an overabundance, additional depletion mechanisms are required. The authors explore the necessity of pairing dark matter annihilations with alternate processes to manage abundance levels. Potential solutions involve annihilations through specific gauge bosons or rapidly decaying hidden sector interactions. Models such as annihilations via the $U(1)_{L_{\mu}-L_{\tau}}$ gauge boson are suggested, which are significantly less impacted by typical electromagnetic constraints as they avoid coupling directly to electrons.

In exploring scenarios where dark matter annihilates to hidden sector particles or Standard Model fermions, the analysis considers forbidden and impeded final states as additional avenues to sidestep the constraints imposed by the CMB and BBN. Such innovative model builds and interactions reveal the interplay between particle mass, charge, annihilation cross-sections, and depleting interactions for compliance with cosmological observations.

For future research, the findings suggest concentrated foci on millicharge interactions, along with further scrutiny into non-trivial model constructions that incorporate rapid depletion of millicharged dark matter in the early universe. Monitoring upcoming constraints from laboratory experiments and astrophysical data will be critical in validating or excluding these theoretical models.

In conclusion, this paper presents a comprehensive evaluation of the plausibility of millicharged dark matter as a source of the 21-cm anomaly, offering crucial insights while identifying stringent astrophysical and experimental constraints. It sets a precise framework for future theoretical and observational pursuits in refining and testing dark matter interaction models.