A reexamination of the cosmological bound to the number of neutrino flavors

Abstract

The predictions of the light element abundances produced primordially in the standard hot big bang cosmological model with three species of light neutrinos ( N ν =3) is consistent with current observational data. If additional species of light neutrinos exist, additional 4He is produced primordially; consistency with observations of 4He leads to an upper bound to N ν which depends on the neutron half-life ( τ 1 2 ), the nucleon-to-photon ratio (or, equivalently, the primordial abundances of deuterium and helium-3) and the primordial mass fraction of 4He ( Y p ). The bounds to these quantities are reexamined and N ν ⩽4.0 is reconfirmed.