keV Sterile Neutrino Dark Matter from Singlet Scalar Decays: The Most
  General Case
  We investigate the early Universe production of sterile neutrino Dark Matter
by the decays of singlet scalars. All previous studies applied simplifying
assumptions and/or studied the process only on the level of number densities,
which makes it impossible to give statements about cosmic structure formation.
We overcome these issues by dropping all simplifying assumptions (except for
one we showed earlier to work perfectly) and by computing the full course of
Dark Matter production on the level of non-thermal momentum distribution
functions. We are thus in the position to study all aspects of the resulting
settings and apply all relevant bounds in a reliable manner. We have a
particular focus on how to incorporate bounds from structure formation on the
level of the linear power spectrum, since the simplistic estimate using the
free-streaming horizon clearly fails for highly non-thermal distributions. Our
work comprises the most detailed and comprehensive study of sterile neutrino
Dark Matter production by scalar decays presented so far.
