Light sterile neutrinos after BICEP-2

The recent discovery of B-modes in the polarization pattern of the Cosmic Microwave Background by the BICEP2 experiment has important implications for neutrino physics. We revisit cosmological bounds on light sterile neutrinos and show that they are compatible with all current cosmological data provided that the mass is relatively low. Using CMB data, including BICEP-2, we find an upper bound of ms < 0.85 eV (2σ Confidence Level). This bound is strengthened to 0.48 eV when HST measurements of H0 are included. However, the inclusion of SZ cluster data from the Planck mission and weak gravitational measurements from the CFHTLenS project favours a non-zero sterile neutrino mass of 0.44+0.11−0.16 eV. Short baseline neutrino oscillations, on the other hand, indicate a new mass state around 1.2 eV. This mass is highly incompatible with cosmological data if the sterile neutrino is fully thermalised (Δχ2 > 10). However, if the sterile neutrino only partly thermalises it can be compatible with all current data, both cosmological and terrestrial.