A precision calculation of the effective number of cosmological neutrinos

The neutrino energy density of the Universe can be conveniently parametrized in terms of the so-called effective number of neutrinos. N-v(eff). This parameter enters in several cosmological observables. In particular it is an important input in those numerical codes, like CMBFAST. which are used to study the Cosmic Microwave Background anisotropy spectrum. By studying the neutrino decoupling with Boltzmann equations. one can show that this quantity differs from the number of massless neutrino species for an additional contribution due to a partial heating of neutrinos during the e(+/-) annihilations. leading to non-thermal features in their final distributions, In this Letter we review the different results obtained in the literature and perform a new analysis which takes into account. in a fully consistent way, the QED corrections at finite temperature to the photon and e(+/-) plasma equation of state. The value found for three massless active neutrinos is N-v(eff)=3.0395, in perfect agreement with the recommended value used in CMBFAST. N-v(eff)=3.04. We also discuss the case of additional relativistic relics and massive active neutrinos. (C) 2002 Elsevier Science B.V. All rights reserved.