Non-local gravity and dark energy
  We discuss a nonlocal modification of gravity obtained adding a term $m^2
R\,\Box^{-2}R$ to the Einstein-Hilbert action. We find that the mass parameter
$m$ only affects the non-radiative sector of the theory, while the graviton
remains massless, there is no propagating ghost-like degree of freedom, no vDVZ
discontinuity, and no Vainshtein radius below which the theory becomes strongly
coupled. For $m={\cal O}(H_0)$ the theory therefore recovers all successes of
GR at solar system and lab scales, and only deviates from it at cosmological
scales. We examine the cosmological consequences of the model and we find that
it automatically generates a dynamical dark energy and a self-accelerating
evolution. After fixing our only free parameter $m$ so to reproduce the
observed value of the dark energy density today, we get a pure prediction for
the dark energy equation of state, $w_ {\rm DE}\simeq-1.14$. This value is
consistent with the existing data, and could also resolve the possible tension
between the Planck data and local measurements of the Hubble parameter.
