Tighter Limits on Dark Matter Explanations of the Anomalous EDGES 21cm Signal

TL;DR

This paper refines constraints on millicharged dark matter as an explanation for the EDGES 21cm anomaly, showing only a very small fraction of DM with specific masses could marginally account for it without conflicting with existing limits.

Contribution

It provides a detailed analysis of the parameter space for millicharged dark matter, incorporating new CMB constraints and astrophysical considerations, to assess its viability as an explanation for the EDGES signal.

Findings

Only a tiny fraction (~0.0115% to 0.4%) of dark matter can be millicharged within the viable mass range.

Most parameter space is excluded by CMB, BBN, and astrophysical constraints.

Moderate fine tuning is necessary to reconcile the model with observations.

Abstract

We investigate the hypothesis that Coulomb-type interactions between dark matter (DM) and baryons explain the anomalously low 21cm brightness-temperature minimum at redshift z ~ 17 that was recently measured by the EDGES experiment. In particular, we reassess the validity of the scenario where a small fraction of the total DM is millicharged, focusing on newly derived constraints from Planck 2015 cosmic microwave background (CMB) data. Crucially, the CMB power spectrum is sensitive to DM-baryon scattering if the fraction of interacting DM is larger than (or comparable to) the fractional uncertainty in the baryon energy density. Meanwhile, there is a mass-dependent lower limit on the fraction for which the required interaction to cool the baryons sufficiently is so strong that it drives the interacting-DM temperature to the baryon temperature prior to their decoupling from the CMB. If this occurs as early as recombination, the cooling saturates. We precisely determine the viable parameter space for millicharged DM, and find that only a fraction (m_chi/MeV) 0.0115% <~ f <~ 0.4% of the entire DM content, and only for DM-particle masses between 0.5 MeV - 35 MeV, can be charged at the level needed to marginally explain the anomaly, without violating limits from SLAC, CMB, Big-Bang nucleosynthesis (BBN), or stellar and SN1987A cooling. In reality, though, we demonstrate that at least moderate fine tuning is required to both agree with the measured absorption profile and overcome various astrophysical sources of heating. Finally, we point out that a ~0.4% millicharged DM component which is tightly coupled to the baryons at recombination may resolve the current 2-sigma tension between the BBN and CMB determinations of the baryon energy density. Future CMB-S4 measurements will be able to probe this scenario directly.