1992

According to standard Big Bang nucleosynthesis theory the light elements - deuterium, helium 3, helium 4 and lithium 7 - owe their origin in whole or part to nuclear reactions that took place in the whole Universe about two minutes after the Big Bang. Indeed the theory has been remarkably successful in explaining and predicting the primordial abundances of these elements relative to hydrogen. Furthermore, these abundances can be used to estimate the mean baryonic (i.e. ordinary matter) density of the Universe today. The primordial helium mass fraction Y is also related to two other fundamental physics parameters: the number of light neutrino "flavours" -- the three known ones correspond to the three known leptons: the electron, the muon, and the tau meson - and the half life of the neutron. Consequently, an accurate determination of Y simultaneously provides an important test of standard Big Bang nucleosynthesis theory, and sets cosmological constraints on the values of these two parameters.

Researchers used the INT and the AAT to measure the primordial helium abundance from the emission lines of HII galaxies. Combined with selected data from the literature relating to extragalactic HII regions in general, they determined Y to be 0.228±0.005 or Y < 0.242 with 95 per cent confidence. This value places in turn an upper limit of 10.4 minutes on the half-life of the neutron and limits the number of neutrino species to < 3.2. These limits based on purely astronomical work, have been confirmed by experiments in the laboratory (10.25 minutes and 3.01±0.01 species). In the course of this work, HII galaxies with broad Wolf-Rayet emission features were excluded from the analysis since they often showed higher helium abundance, supporting a suggestion that there may be additional local sources of helium in the form of winds from Wolf-Rayet stars.

NUCLEOSYNTHESIS