Impact of H0 prior on the evidence for dark radiation

Recent analyses that include cosmic microwave background (CMB) anisotropy measurements from the Atacama Cosmology Telescope and the South Pole Telescope have hinted at the presence of a dark radiation component at more than two standard deviations. However, this result depends sensitively on the assumption of an Hubble Space Telescope prior on the Hubble constant, where H-0 = 73.8 +/- 2.4 km/s/Mpc at 68% c.l.. Here we repeat this kind of analysis assuming a prior of H-0 = 68 +/- 2.8 km/s/Mpc at 68% c.l., derived from a median statistics (MS) analysis of 537 non-CMB H-0 measurements from Huchra's compilation. This prior is fully consistent with the value of H-0 = 69.7 +/- 2.5 km/s/Mpc at 68% c.l. obtained from CMB measurements under assumption of the standard ACDM model. We show that with the MS H-0 prior the evidence for dark radiation is weakened to similar to 1.2 standard deviations. Parametrizing the dark radiation component through the effective number of relativistic degrees of freedom N-eff, we find N-eff 3.98 +/- 0.37 at 68% c.l. with the Hubble Space Telescope prior and N-eff 3.52 +/- 0.39 at 68% c.l. with the MS prior. We also discuss the implications for current limits on neutrino masses and on primordial Helium abundances.