Effects of an Intermediate Mass Sterile Neutrino Population on the Early
  Universe
  The hot and dense early Universe combined with the promise of high-precision
cosmological observations provide an intriguing laboratory for Beyond Standard
Model (BSM) physics. We simulate the early Universe to examine the effects of
the decay of thermally populated sterile neutrino states into Standard Model
products around the time of weak decoupling. These decays deposit a significant
amount of entropy into the plasma as well as produce a population of
high-energy out-of-equilibrium active neutrinos. As a result, we can constrain
these models by their inferred value of $N_{\rm eff}$, the effective number of
relativistic degrees of freedom. In this work, we explore a variety of models
with $N_{\rm eff}{}$ values consistent with CMB observations, but with vastly
different active neutrino spectra which will challenge the standard
cosmological model, affect lepton capture rates on free nucleons, and may
significantly affect Big Bang Nucleosynthesis (BBN).
