COSMOLOGICAL CONSTRAINTS ON NEUTRINO DEGENERACY

A large lepton asymmetry may be hidden in degenerate neutrinos. The cosmological consequences of neutrino degeneracy are explored with particular attention to the effects on primordial nucleosynthesis. Degenerate neutrinos will increase the density and expansion rate of the Universe so that cosmological data provides constraints on neutrino degeneracy. Although degenerate big-bang nucleosynthesis (DBBN) has two more free parameters (the electron-neutrino degeneracy and the degeneracy in non-electron-neutrinos) than is available in standard big-bang nucleosynthesis (SBBN), the cosmological constraints restrict the permitted range of these parameters. In particular, we find that a baryon-dominated, critical density Universe (OMEGA(B) = 1) is not permitted. A combination of bounds from DBBN and the density and expansion rate of the Universe constraints the e-neutrino degeneracy (-0.06 less than or similar to xi(e) less than or similar to 1.1), the mu, tau-neutrino degeneracy (\xi(mu,tau)\less than or similar to 6.9) and, the nucleon-to-photon ratio (2.8 less than or similar to eta-10 less than or similar to 19).