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