Abstract: Sterile neutrinos in the eV mass range are often invoked to explain terestrial neutrino experiments. Given the preferred mass and mixing angle, such a sterile neutrino will thermalise completely in the early universe and are at odds with observations of the large scale structure. A partially thermalised sterile neutrino is still allowed however, and such a neutrino can also alleviate some of the tension found in current datasets such as BICEP2 and Planck. I will explain the current cosmological constraints on sterile neutrinos and then discuss two different ways of inhibiting sterile neutrino production: A large lepton asymmetry in the active neutrino sector, or a new hidden gauge boson in the sterile neutrino sector.
