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21cm from Dark Radiation

The cosmological absorption of 21cm photons probes the spectrum of Cosmic Microwave Background (CMB) photons at energies far below any direct measurements.  The EDGES experiment has presented a first tentative detection of the absorption of 21cm photons from the reionization epoch, when the first stars formed in our Universe.  EDGES sees deeper absorption than predicted by the standard cosmology, which can be explained if there are more 21cm photons than predicted by the blackbody spectrum of the CMB.  After reviewing the basics of 21cm cosmology, I will describe how dark radiation, composed of dark photons, may have injected photons with wavelength 21cm into the primordial plasma.  Therefore, 21cm cosmology can be exploited as a novel probe of dark radiation.  The unexpected EDGES result may be the first evidence for a new dark radiation component.  I will also comment on how dark radiation may provide a common explanation of the EDGES result and the ARCADE radio excess.