Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology
Abstract
We calculate and display the primordial light-element abundances as a function of a neutrino degeneracy parameter common to all flavors. It is the only unknown parameter characterizing the thermal medium at the primordial nucleosynthesis epoch. The observed primordial helium abundance Yp is the most sensitive cosmic ``leptometer.'' Adopting the conservative Yp error analysis of Olive and Skillman implies -0.04 0.07 whereas the errors stated by Izotov and Thuan imply =0.0245+-0.0092 (1 sigma). Improved determinations of the baryon abundance have no significant impact on this situation. A determination of Yp that reliably distinguishes between a vanishing or nonvanishing is a crucial test of the cosmological standard assumption that sphaleron effects equilibrate the cosmic lepton and baryon asymmetries.
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