Decaying dark matter mimicking time-varying dark energy
  A $\Lambda$CDM model with dark matter that decays into inert relativistic
energy on a timescale 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,
$\widetilde w_\phi$, as a function of redshift, and show that the evolution of
$\widetilde w_\phi$ 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 ($\widetilde w_\phi > -1$), phantom
models ($\widetilde w_\phi < -1$) or models in which the equation of state
parameter crosses the phantom divide, evolving from $\widetilde w_\phi > -1$ at
high redshift to $\widetilde w_\phi < -1$ at low redshift. All of these models
generically converge toward $\widetilde w_\phi \approx -1$ 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.
