Additional light sterile neutrinos and cosmology

Tantalizing cosmological and terrestrial evidence suggests the number of light neutrinos may be greater than 3, motivating a careful reexamination of cosmological bounds on extra light species. Big bang nucleosynthesis constrains the number of relativistic neutrino species present during nucleosynthesis, N-eff(BBN), while measurements of the cosmic microwave background (CMB) angular power spectrum constrain the effective energy density in relativistic neutrinos at the time of matter-radiation equality, N-eff(CMB). There are a number of scenarios where new sterile neutrino species may have different contributions to Delta N-eff(BBN) and Delta N-eff(CMB), for masses that may be relevant to reconciling cosmological constraints with various terrestrial claims of neutrino oscillations. We consider a scenario with two sterile neutrinos and explore whether partial thermalization of the sterile states can ease the tension between cosmological constraints on N-eff(BBN) and terrestrial data. We then investigate the effect of a nonzero neutrino mass on their contribution to the radiation abundance, finding reductions in Delta N-eff(CMB) of more than 5% for neutrinos with masses above 0.5 eV. While the effects we investigate here could play a role, we nevertheless find that two additional light sterile neutrinos species cannot fit all the data at the 95% confidence level. DOI: 10.1103/PhysRevD.87.083515