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 $H_0=73.8\pm 2.4\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ at 68% c.l.. Here we repeat this kind of analysis assuming a prior of $H_0=68\pm 2.8\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ 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 $H_0=69.7\pm 2.5\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ at 68% c.l. obtained from CMB measurements under assumption of the standard $\Lambda \mathrm{CDM}$ model. We show that with the MS $H_0$ prior the evidence for dark radiation is weakened to $\sim 1.2$ standard deviations. Parametrizing the dark radiation component through the effective number of relativistic degrees of freedom $N_{\mathrm{eff}}$, we find $N_{\mathrm{eff}}=3.98\pm 0.37$ at 68% c.l. with the Hubble Space Telescope prior and $N_{\mathrm{eff}}=3.52\pm 0.39$ at 68% c.l. with the MS prior. We also discuss the implications for current limits on neutrino masses and on primordial Helium abundances.

• Received 7 June 2012

DOI:https://doi.org/10.1103/PhysRevD.86.043520