Dark radiation from particle decay: cosmological constraints and
  opportunities
  We study particle decay as the origin of dark radiation. After elaborating
general properties and useful parametrisations we provide model-independent and
easy-to-use constraints from nucleosynthesis, the cosmic microwave background
and structure formation. Bounds on branching ratios and mass hierarchies depend
in a unique way on the time of decay. We demonstrate their power to exclude
well-motivated scenarios taking the example of the lightest ordinary sparticle
decaying into the gravitino. We point out signatures and opportunities in
cosmological observations and structure formation. For example, if there are
two dark decay modes, dark radiation and the observed dark matter with
adjustable free-streaming can originate from the same decaying particle,
solving small-scale problems of structure formation. Hot dark matter mimicking
a neutrino mass scale as deduced from cosmological observations can arise and
possibly be distinguished after a discovery. Our results can be used as a
guideline for model building.
