How a brane cosmological constant can trick us into thinking thatw<−1

Observations exploring the contemporary cosmic acceleration have sparked interest in dark energy models possessing equations of state with $w<-1$. We review how the cosmic expansion history of a Dvali-Gabadadze-Porrati (DGP) braneworld model with a standard brane cosmological constant can mimic that of ordinary 4D gravity with $w<-1$ “phantom” dark energy for observationally relevant redshifts. We reinterpret the effective phantom nature of the dark energy as arising from dynamical-screening of the brane cosmological constant in DGP. This unusual variety of expansion history is thus possible without violating the null-energy condition, without ghosts and without any big rip, in a model which seems no more contrived than most evolving dark energy models. We indicate ways by which one may observationally test this effective $w<-1$ possibility, and differentiate it from ordinary phantom dark energy.