Phantom dark energy from non-local infrared modifications of General
  Relativity
  We discuss the cosmological consequences of a model based on a non-local
infrared modification of Einstein equations. We find that the model generates a
dynamical dark energy that can account for the presently observed value of
$\Omega_{\rm DE}$, without introducing a cosmological constant. Tuning a free
mass parameter $m$ to a value $m\simeq 0.67 H_0$ we reproduce the observed
value $\Omega_{\rm DE}\simeq 0.68$. This leaves us with no free parameter and
we then get a pure prediction for the EOS parameter of dark energy. Writing
$w_{\rm DE}(a)=w_0+(1-a) w_a$, we find $w_0\simeq-1.04$ and $w_a\simeq -0.02$,
consistent with the Planck data, and on the phantom side. We also argue that
non-local 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.
