Phantom dark energy from nonlocal infrared modifications of general relativity

We discuss the cosmological consequences of a model based on a nonlocal infrared modification of Einstein equations. We find that the model generates a dynamical dark energy that can account for the presently observed value of Ω_DE, without introducing a cosmological constant. Tuning a free mass parameter m to a value m ≃0.67H_{0, we reproduce the observed value ΩDE≃0.68. This leaves us with no free parameter, and we then get a pure prediction for the equation of state parameter of dark energy. Writing wDE(a)=w0+(1-a)wa}, we find w0≃-1.04 and w_a≃-0.02, consistent with the Planck data and on the phantom side. We also argue that nonlocal equations of the type that we propose must be understood as purely classical effective equations, such as those derived in semiclassical gravity for the in-in matrix elements of the metric. As such, any apparent ghost instability in such equations only affects the classical dynamics, but there is no propagating degree of freedom associated to the ghost and no issue of ghost-induced quantum vacuum decay.