Neutrinos from the Big Bang

The standard Big Bang cosmology predicts the existence of an, as yet undetected, relic neutrino background, similar to the photons observed in the cosmic microwave background. If neutrinos have mass, then such relic neutrinos are a natural candidate for the dark matter of the universe, and indeed were the first particles to be proposed for this role. This possibility has however been increasingly constrained by cosmological considerations, particularly of large-scale structure formation, thus yielding stringent bounds on neutrino masses, which have yet to be matched by laboratory experiments. Another probe of relic neutrinos is primordial nucleosynthesis which is sensitive to the number of neutrino types (including possible sterile species) as well to any lepton asymmetry. Combining such arguments with the experimental finding that neutrino mixing angles are large, excludes the possibility of a large asymmetry and disfavours new neutrinos beyond those now known. 1 Relic neutrinos and the cosmological mass bound Several years before neutrinos had even been experimentally detected, Alpher, Follin & Hermann [1] noted that they would have been in thermal equilibrium in the early universe “. . . through interactions with mesons” at temperatures above 5MeV; below this temperature the neutrinos “. . . freeze-in and continue to expand and cool adiabatically as would a pure radiation gas”. These authors also observed that the subsequent annihilation of e pairs would heat the photons but not the decoupled neutrinos, so by conservation of entropy Tν/T would decrease from its high temperature value of unity, down to (4/11)1/3 at T ≪ me [1]. (Thus the present density ofmassless relic neutrinos (neglecting a possible chemical potential i.e. lepton–antilepton asymmetry) would be nν nγ = (