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Impact of H prior on the evidence for dark radiation

Physical Review D - Particles, Fields, Gravitation and Cosmology, ISSN: 1550-7998, Vol: 86, Issue: 4
2012
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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 =73.8±2.4km/s/Mpc at 68% c.l.. Here we repeat this kind of analysis assuming a prior of H =68±2.8km/s/Mpc at 68% c.l., derived from a median statistics (MS) analysis of 537 non-CMB H measurements from Huchra's compilation. This prior is fully consistent with the value of H =69.7±2.5km/s/Mpc at 68% c.l. obtained from CMB measurements under assumption of the standard ΛCDM model. We show that with the MS H 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 N , we find N =3.98±0.37 at 68% c.l. with the Hubble Space Telescope prior and N =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. © 2012 American Physical Society.

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