Helium and Big Bang nucleosynthesis

The existence of a universal primordial helium abundance is a significant piece of evidence in support of there having been a phase of high temperature and density in the early universe, when light elements were created at a temperature of the order of 109 K. While primordial deuterium and 7Li are sensitive to η≡nB/nγ which fixes the cosmic baryon density at the present time, the primordial helium abundance is a test of consistency, assuming that the equivalent number Nν of relativistic neutrino families is 3 and that the weak-interaction coupling constants deduced from the experimentally determined neutron half-life are known. Recent estimates of primordial abundances of all three species are converging to give good agreement with standard (homogeneous) Big Bang nucleosynthesis theory with a cosmic baryon density 0.03≤ΩBh702≤0.04.