Decaying dark matter mimicking time-varying dark energy

A Lambda CDM model with dark matter that decays into inert relativistic energy on a time scale longer than the Hubble time will produce an expansion history that can be misinterpreted as stable dark matter with time-varying dark energy. We calculate the corresponding spurious equation of state parameter, (w) over tilde (phi), as a function of redshift, and show that the evolution of (w) over tilde (phi) depends strongly on the assumed value of the dark-matter density, erroneously taken to scale as a(-3). Depending on the latter, one can obtain models that mimic quintessence ((w) over tilde (phi) > -1), phantom models ((w) over tilde (phi) < -1), or models in which the equation of state parameter crosses the phantom divide, evolving from <(w)over tilde>(phi) > -1 at high redshift to (w) over tilde (phi) < -1 at low redshift. All of these models generically converge toward <(w)over tilde>(phi) approximate to -1 at the present. The degeneracy between the Lambda CDM model with decaying dark matter and the corresponding spurious quintessence model is broken by the growth of density perturbations.