Constraints on the lepton asymmetry and radiation energy density: Implications for PLANCK

Abstract

By using most of the present CMB and LSS measurements and the BBN constraints on the primordial helium abundance, Y_p, we set bounds on the radiation content of the Universe and neutrino properties. 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_{oth}^{eff}, due to possible other physical processes that occurred between BBN and structure formation epoch. We found that present CMB and LSS data constraints the neutrino degeneracy parameter at \xi_{\nu} \leq 0.722, implying a lepton asymmetry of the neutrino background {\cal L}_{\nu} \leq 0.614 (2-\sigma). We also found \Delta N^{eff}_{oth}=0.572^{+1.972}_{-1.780}, the contribution to the effective number of relativistic neutrino species N^{eff}=3.058^{+1.971}_{-1.178} and a primordial helium abundance Y_p=0.249^{+0.014}_{-0.016} (2-\sigma errors). These results bring an important improvement over the similar ones obtained by using WMAP~1-year and older LSS data or the WMAP~3-year data alone and the standard primordial helium abundance value Y_p=0.24, relaxing the stringent BBN constraint on the neutrino degeneracy parameter (\xi_{\nu} \leq 0.07). We forecast that the CMB temperature and polarization maps observed with high angular resolutions and sensitivity by the future Planck Mission will constraint the primordial primordial helium abundance at Y_p=0.247 \pm 0.002 (2-\sigma errors) in agreement with the most stringent limits on Y_p given by the BBN and the neutrino degeneracy parameter at \xi_{\nu} \leq 0.280 (2-\sigma), not excluding the possibility of larger lepton asymmetry. This work has been done on behalf of Planck-LFI activities.