  We present a decaying dark matter scenario where the daughter products are a
single massless relativistic particle and a single, massive but possibly
relativistic particle. We calculate the velocity distribution of the massive
daughter particle and its associated equation of state and derive its dynamical
evolution in an expanding universe. In addition, we present a model of decaying
dark matter where there are many massless relativistic daughter particles
together with a massive particle at rest. We place constraints on these two
models using supernovae type Ia observations. We find that for a daughter
relativistic fraction of 1% and higher, lifetimes of at least less than 10 Gyrs
are excluded, with larger relativistic fractions constraining longer lifetimes.

Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology