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

In this paper we set bounds on the radiation content of the Universe and neutrino properties by using the WMAP ( Wilkinson microwave anisotropy probe) five-year CMB ( cosmic microwave background) measurements complemented with most of the existing CMB and LSS ( large scale structure) data ( WMAP5 + All), imposing also self-consistent BBN ( big bang nucleosynthesis) constraints on the primordial helium abundance. We consider lepton asymmetric cosmological models parametrized by the neutrino degeneracy parameter xi(nu) and the variation of the relativistic degrees of freedom, Delta N-eff(oth), due to possible other physical processes occurring between BBN and structure formation epochs. We get a mean value of the effective number of relativistic neutrino species of N-eff = 2.98 (2.27) (3.60) (1.65) (4.37), providing an important improvement over the similar result obtained from WMAP5 + 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 between Omega(m)h(2) and z(eq), showing that we observe Ne. mainly via the effect of zeq, rather than via neutrino anisotropic stress as claimed by the WMAP team ( Komatsu et al ( WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [ 0803.0547]). WMAP5 + All data provide a strong bound on the helium mass fraction of Y-p = 0.2486 +/- 0.0085 ( 68% CL), that rivals the bound on Y-p obtained from the conservative analysis of the present data on helium abundance. For the neutrino degeneracy parameter we. nd a bound of -0.216 <= xi(nu) <= 0.226 ( 68% CL), which represents an important improvement over the similar result obtained by using the WMAP three-year data. The inclusion in the analysis of LSS data reduces the upper limit of the neutrino mass to m(nu) < 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]) and WMAP5 + 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 high angular resolutions and sensitivities by the future Planck satellite will reduce the errors on xi(nu) and Y-p down to sigma(xi(nu)) similar or equal to 0.089 ( 68% CL) and sigma( Y-p) = 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.