Nonequilibrium neutrino statistical mechanics in the expanding Universe

Details Nonequilibrium neutrino statistical mechanics in the expanding Universe Nonequilibrium neutrino statistical mechanics in the expanding Universe

Oct 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phrvd..46.3372d&link_type=abstract

Physical Review D (Particles, Fields, Gravitation, and Cosmology), Volume 46, Issue 8, 15 October 1992, pp.3372-3387

Physics

74

Origin, Formation, And Abundances Of The Elements

Scientific paper

We study neutrino decoupling in the early Universe (t~sec,T~MeV) by integrating the Boltzmann equations that govern the neutrino phase-space distribution functions. In particular, we compute the distortions in the νe and νμ/ντ phase-space distributions that arise in the standard cosmology due to e+/- annihilations. These distortions are nonthermal, with the effective neutrino temperature increasing with neutrino momentum, approaching a 0.7% increase for electron neutrinos and a 0.3% increase for μ and τ neutrinos at the highest neutrino momenta, and correspond to an increase in the energy density of νe's of about 1.2% and in the energy density of νμ/ντ's of about 0.5% (roughly one additional relic neutrino per cm-3 per species). The distortion for electron neutrinos is larger than that for μ and τ neutrinos because electron neutrinos couple to e+/-'s through both charged- and neutral-current interactions. Our results graphically illustrate that neutrino decoupling is a continuous process which is momentum dependent. Because of subtle cancellations, these distortions lead to only a tiny change in the predicted primordial 4He abundance, ΔY~=1-2×10-4.

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