Constraining dark photons and their connection to 21 cm cosmology with
  CMB data
  In the inhomogeneous Universe, the cosmological conversion of dark photons
into ordinary photons (and vice versa) may happen at a great number of
resonance redshifts. This alters the CMB observed energy spectrum and degree of
small-scale anisotropies. We utilize results from the EAGLE simulation to
obtain the conversion probability along random line-of-sights to quantify these
effects. We then apply our results to the case where dark photons are sourced
by dark matter decay and their high-redshift conversion into ordinary photons
modify the global 21 cm signal expected from the cosmic dawn era. Concretely,
we show that a significant portion of the parameter space for which a converted
population of photons in the Rayleigh-Jeans tail of the CMB explains the
absorption strength observed by EDGES, is ruled out from the brightness
temperature measurements of COBE/FIRAS and the CMB anisotropy measurements of
Planck and SPT.
