WMAP five-year constraints on lepton asymmetry and radiation energy density:implications for Planck

Abstract

In this paper we set bounds on the radiation content of the Universe and neutrinoproperties by using the WMAP (Wilkinson microwave anisotropy probe)five-year CMB (cosmic microwave background) measurements complementedwith most of the existing CMB and LSS (large scale structure) data(WMAP5+All), imposing also self-consistent BBN (big bang nucleosynthesis) constraints on theprimordial helium abundance.We consider lepton asymmetric cosmological models parametrized by the neutrino degeneracy parameterξν and the variation of the relativistic degrees of freedom,ΔNeffoth, due to possible other physical processes occurring between BBN and structure formationepochs.We get a mean value of the effective number of relativistic neutrino species ofNeff = 2.98 2.273.60 1.654.37, providing an important improvement over the similar result obtained fromWMAP5+BAO+SN+HST (BAO: baryonic acoustic oscillations; SN: supernovae; HST: Hubble Space Telescope) data(Komatsu et al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0547]).We also find a strong correlation betweenΩmh2 andzeq, showing that weobserve Neff mainlyvia the effect of zeq, rather than via neutrino anisotropic stress as claimed by the WMAP team (Komatsuet al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0547]).WMAP5+All data provide a strong bound on the helium mass fraction ofYp = 0.2486 ± 0.0085 (68% CL), thatrivals the bound on Yp obtained from the conservative analysis of the present data on helium abundance.For the neutrino degeneracy parameter we find a bound of−0.216≤ξν≤0.226 (68% CL), which represents an important improvement over the similar result obtained byusing the WMAP three-year data.The inclusion in the analysis of LSS data reduces the upper limit of the neutrino mass tomν<0.46 eV (95% CL) with respect to the values obtained from the analysis from WMAP5-only data(Dunkley et al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0586]) andWMAP5+BAO+SN+HST data (Komatsu et al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0547]).We forecast that the CMB temperature and polarization measurements observed with highangular resolutions and sensitivities by the future Planck satellite will reduce the errors onξν and Yp down to (68% CL) and σ(Yp) = 0.013 (68% CL) respectively, values fully consistent with the BBN bounds on these parameters.This work has been done on behalf of Planck-LFI (low frequency instrument)activities.