More Is Different: Reconciling eV Sterile Neutrinos with Cosmological Mass Bounds

Understand

It is generally expected that adding light sterile species would increase the effective number of neutrinos, $N_{eff}$.

• In this paper we discuss a scenario that $N_{eff}$ can actually decrease due to the neutrino oscillation effect if sterile neutrinos have self-interactions.
• We specifically focus on the eV mass range, as suggested by the neutrino anomalies.
• With large self-interactions, sterile neutrinos are not fully thermalized in the early Universe because of the suppressed effective mixing angle or matter effect.