How a Brane Cosmological Constant Can Trick Us into Thinking That W < −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 4–dimensional 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. E-mail: [email protected] E-mail: [email protected]