Superacceleration as the signature of a dark sector interaction

We show that an interaction between dark matter and dark energy generically results in an effective dark-energy equation of state of $w<\ensuremath{-}1$. This arises because the interaction alters the redshift dependence of the matter density. An observer who fits the data treating the dark matter as noninteracting will infer an effective dark-energy fluid with $w<\ensuremath{-}1$. We argue that the model is consistent with all current observations, the tightest constraint coming from estimates of the matter density at different redshifts. Comparing the luminosity and angular-diameter distance relations with $\ensuremath{\Lambda}\mathrm{CDM}$ and phantom models, we find that the three models are degenerate within current uncertainties but likely distinguishable by the next generation of dark-energy experiments.