Abstract
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 ${H}_{0}=73.8\ifmmode\pm\else\textpm\fi{}2.4\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ at 68% c.l.. Here we repeat this kind of analysis assuming a prior of ${H}_{0}=68\ifmmode\pm\else\textpm\fi{}2.8\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ 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 ${H}_{0}=69.7\ifmmode\pm\else\textpm\fi{}2.5\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ at 68% c.l. obtained from CMB measurements under assumption of the standard $\ensuremath{\Lambda}\mathrm{CDM}$ model. We show that with the MS ${H}_{0}$ prior the evidence for dark radiation is weakened to $\ensuremath{\sim}1.2$ standard deviations. Parametrizing the dark radiation component through the effective number of relativistic degrees of freedom ${N}_{\mathrm{eff}}$, we find ${N}_{\mathrm{eff}}=3.98\ifmmode\pm\else\textpm\fi{}0.37$ at 68% c.l. with the Hubble Space Telescope prior and ${N}_{\mathrm{eff}}=3.52\ifmmode\pm\else\textpm\fi{}0.39$ at 68% c.l. with the MS prior. We also discuss the implications for current limits on neutrino masses and on primordial Helium abundances.
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