Revisiting cosmological bounds on sterile neutrinos
  We employ state-of-the art cosmological observables including supernova
surveys and BAO information to provide constraints on the mass and mixing angle
of a non-resonantly produced sterile neutrino species, showing that cosmology
can effectively rule out sterile neutrinos which decay between BBN and the
present day. The decoupling of an additional heavy neutrino species can modify
the time dependence of the Universe's expansion between BBN and recombination
and, in extreme cases, lead to an additional matter-dominated period; while
this could naively lead to a younger Universe with a larger Hubble parameter,
it could later be compensated by the extra radiation expected in the form of
neutrinos from sterile decay. However, recombination-era observables including
the Cosmic Microwave Background (CMB), the shift parameter $R_{CMB}$ and the
sound horizon $r_s$ from Baryon Acoustic Oscillations (BAO) severely constrain
this scenario. We self-consistently include the full time-evolution of the
coupled sterile neutrino and standard model sectors in an MCMC, showing that if
decay occurs after BBN, the sterile neutrino is essentially bounded by the
constraint $\sin^2\theta \lesssim 0.026 (m_s/\mathrm{eV})^{-2}$.
