Viable secret neutrino interactions with ultralight dark matter
  Several anomalies in neutrino oscillation experiments point to the existence
of a $\sim 1\,$eV sterile neutrino $\nu_s$ mixing with $\nu_e$ at the level of
$U_{e4}\cong 0.1$, but such a neutrino is strongly disfavored by constraints on
additional light degrees of freedom ($\delta N_{\rm eff}$) and total neutrino
mass ($\sum_\nu m_\nu$) from cosmology. "Secret neutrino interactions" that
have been invoked to suppress the cosmological production of $\nu_s$ typically
falter, but recently it was pointed out that $\nu_s$ could get a large mass in
the early universe by coupling to ultralight dark matter $\phi$, which can
robustly suppress its production. The model has essentially two free
parameters: $m_\phi$, and $m_{s,0}$, the mass of the sterile neutrino at early
times, enhanced by its coupling to $\phi$. I determine the parameter regions
allowed by limits on $\delta N_{\rm eff}$ and $\sum_\nu m_\nu$ from the cosmic
microwave background and big bang nucleosynthesis, using a simplified yet
accurate treatment of neutrino oscillations in the early universe. This
mechanism could have an important impact on laboratory experiments that suggest
oscillations with sterile neutrinos.
