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

Understand

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.