The primordial abundance of 4He: Evidence for non-standard big bang nucleosynthesis

Abstract

We present a new determination of the primordial helium mass fraction Yp , 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 Yp = 0.2565 ± 0.0010 (stat.) ± 0.0050 (syst.). This value is higher at the 2σ level than the value given by standard big bang nucleosynthesis, implying deviations from it. The effective number of light neutrino species N ν is equal to 3.68+0.80-0.70 (2σ) and 3.80+0.80-0.70 (2σ) for a neutron lifetime τn 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. © 2010. The American Astronomical Society. All rights reserved.