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

TL;DR

This study refines the primordial helium abundance measurement using extensive spectra and advanced systematic effect corrections, finding a value higher than standard predictions, which suggests possible new physics beyond standard cosmology.

Contribution

It provides a new, more precise determination of the primordial helium abundance incorporating comprehensive systematic corrections and Monte Carlo analysis.

Findings

Primordial helium mass fraction Yp = 0.2565±0.0010 (stat.)±0.0050 (syst.)

Yp value exceeds Standard Big Bang Nucleosynthesis predictions at 2sigma

Effective number of neutrino species Nnu > 3, indicating deviations from standard model

Abstract

We present a new determination of the primordial helium mass fraction Yp, based on 93 spectra of 86 low-metallicity extragalactic HII regions, and taking into account the latest developments concerning systematic effects. These include collisional and fluorescent enhancements of HeI recombination lines, underlying HeI stellar absorption lines, collisional and fluorescent excitation of hydrogen lines and temperature and ionization structure of the HII 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 2sigma level than the value given by Standard Big Bang Nucleosynthesis (SBBN), implying deviations from it. The effective number of light neutrino species Nnu is equal to 3.68^+0.80_-0.70 (2sigma) and 3.80^+0.80_-0.70 (2sigma) 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.