abstract: The Rayleigh-Jeans tail of the CMB is poorly constrained at low energies, and therefore could be highly distorted from a blackbody consistent with current measurements. I will describe how dark
radiation, composed of dark photons, can inject soft photons from resonant oscillations during the dark ages. A small fraction of the
Universe’s energy in dark photons can produce large distortions in the Rayleigh-Jeans tail. Measurements of cosmological 21cm absorption test the CMB spectrum at low energies, because injected 21cm photons produce deeper absorption than the standard cosmology. A smoking gun of dark photon oscillations is the presence of sharp edges, as a function of redshift, in the 21cm signal.