Impact of H_{0} 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 H0=73.8±2.4  km/s/Mpc at 68% c.l.. Here we repeat this kind of analysis assuming a prior of H0=68±2.8  km/s/Mpc at 68% c.l., derived from a median statistics (MS) analysis of 537 non-CMB H0 measurements from Huchra’s compilation. This prior is fully consistent with the value of H0=69.7±2.5  km/s/Mpc at 68% c.l. obtained from CMB measurements under assumption of the standard ΛCDM model. We show that with the MS H0 prior the evidence for dark radiation is weakened to ∼1.2 standard deviations. Parametrizing the dark radiation component through the effective number of relativistic degrees of freedom Neff, we find Neff=3.98±0.37 at 68% c.l. with the Hubble Space Telescope prior and Neff=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.