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 re-examination 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 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 non-zero 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.