Sterile neutrino in cosmology

Sterile neutrino, in addition to providing neutrino mass generation, depending on its mass scale, may have many important cosmological implications. eV sterile neutrinos may explain the dark radiation problem and the experimental data of short baseline experiments; sterile neutrinos may also provide baryogenesis through leptogenesis. The main way of obtaining a significant abundance of sterile neutrinos is through their mixing and oscillations with the active neutrinos. Two different mechanisms to suppress the sterile neutrinos thermalization and their eventual large production have been studied: the existence of neutrino asymmetries or the introduction of secret interactions among sterile neutrinos. However, the scenarios of secret self-interactions studied so far are not sufficient to reconcile sterile neutrinos with cosmology. The new picture that we introduce assumes the same secret interactions with a light pseudoscalar as the source of sterile neutrino production by oscillations (when they involve active species) and, in addition, along with the self-interactions we will also consider interactions between sterile neutrino and pseudoscalar particle in the sterile sector.