THE PRIMORDIAL ABUNDANCE OF 4He: EVIDENCE FOR NON-STANDARD BIG BANG NUCLEOSYNTHESIS

We present a new determination of the primordial helium mass fraction Y-p, based on 93 spectra of 86 low-metallicity extragalactic H (II) regions, and taking into account the latest developments concerning systematic effects. These include collisional and fluorescent enhancements of He I recombination lines, underlying He I stellar absorption lines, collisional and fluorescent excitation of hydrogen lines, and temperature and ionization structure of the H II region. Using Monte Carlo methods to solve simultaneously for the above systematic effects, we find the best value to be Y-p = 0.2565 +/- 0.0010 (stat.) +/- 0.0050 (syst.). This value is higher at the 2 sigma level than the value given by standard big bang nucleosynthesis implying deviations from it. The effective number of light neutrino species N-nu is equal to 3.68(-0.70)(+0.80) (2 sigma) and 3.80(-0.70)(+0.80) (2 sigma) for a neutron lifetime tau(n) equal to 885.4 +/- 0.9 s and 878.5 +/- 0.8 s, respectively, i.e., it is larger than the experimental value of 2.993 +/- 0.011.