Constraints on neutrino degeneracy from the cosmic microwave background and primordial nucleosynthesis

We reanalyze the cosmological constraints on the existence of a net universal lepton asymmetry and neutrino degeneracy based upon the latest high resolution CMB sky maps from BOOMERANG, DASI, and MAXIMA-1. We generate likelihood functions by marginalizing over (Ω_bh ²,ξ_vμ,τ,Ω_Λ,h,n) plus the calibration uncertainties. We consider flat Ω_M+Ω _Λ = 1 cosmological models with two identical degenerate neutrino species, ξ_vμ,τ ≡ |ξ_vμ| = |ξ_vτ| and a small ξ_ve We assign weak top-hat priors on the electron-neutrino degeneracy parameter ξ _ve and Ω_bh² based upon allowed values consistent with the nucleosynthesis constraints as a function of ξ_{vμ τ}. The change in the background neutrino temperature with degeneracy is also explicitly included, and Gaussian priors for h = 0.72±0.08 and the experimental calibration uncertainties are adopted. The marginalized likelihood functions show a slight (0.5σ) preference for neutrino degeneracy. Optimum values with two equally degenerate μ. and τ neutrinos imply ξ _vμτ = 1.0_-1.0(0.5σ)+0.8(1σ), from which we deduce ξ_ve = 0.09_-0.09+^0.15and Ω_bh²=0.021_-0.002^+0.06. The 2σ uPPer limit becomes ξ_veμτ≤.2.1, which implies ξ_ve≤0.30, and Ωbh²≤0.030. For only a single large-degeneracy species the optimal value is |ξ_vμ| or |ξ_vτ|= 1.4 with a 2σ upper limit of |ξ _vμ| or |ξ_vτ|≤2.5.