Dark Energy and Decaying Dark Matter

We discuss a cosmology in which cold dark-matter particles decay into relativistic particles. Such decays lead to a bulk viscosity in the cosmic fluid. This bulk viscosity enters the cosmic energy equation as an effective negative pressure. We investigate whether this negative pressure is of sufficient magnitude to account fo the observed cosmic acceleration. We have shown that a single decaying species in a Λ = 0, flat, dark-matter dominated cosmology can not reproduce the observed magnitude-redshift relation from Type Ia supernovae. However, late decaying particles may be able to account for a significant fraction of the apparent cosmic acceleration.