Super-acceleration as Signature of 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<-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 non-interacting will infer an effective dark energy fluid with w<-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 LambdaCDM 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. 
Subinoy Das, Pier Stefano Corasaniti, Justin Khoury