More Is Different: Reconciling eV Sterile Neutrinos with Cosmological
  Mass Bounds
  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. As the Universe cools
down, flavor equilibrium between active and sterile species can be reached
after big bang nucleosynthesis (BBN) epoch, but leading to a decrease of
$N_{eff}$. In such a scenario, we also show that the conflict with cosmological
mass bounds on the additional sterile neutrinos can be relaxed further when
more light species are introduced.
