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 (₁h^2, , , ₄, _, h, n) plus the calibration uncertainties. We consider flat ₌+_=1 cosmological models with two identical degenerate neutrino species, , |_{}|=|_{}| and a small ₄. We assign weak top-hat priors on the electron-neutrino degeneracy parameter ₄ and ₁h^2 based upon allowed values consistent with the nucleosynthesis constraints as a function of,. The change in the background neutrino temperature with degeneracy is also explicitly included, and Gaussian priors for h=0. 720. 08 and the experimental calibration uncertainties are adopted. The marginalized likelihood functions show a slight (0. 5) preference for neutrino degeneracy. Optimum values with two equally degenerate and neutrinos imply, =1. 0-₁. ₀ (₀. ₅) ^+0. 8 (1), from which we deduce ₄=0. 09-₀. ₀₉^+0. 15, and ₁h^2=0. 021-₀. ₀₀₂^+0. 06. The 2 upper limit becomes, <~2. 1, which implies ₄<~0. 30, and ₁h^2<~0. 030. For only a single large-degeneracy species the optimal value is |_{}| or |_{}|=1. 4 with a 2 upper limit of |_{}| or |_{}|<~2. 5.