Screening of cosmological constant in non-local cosmology
  We consider a model of non-local gravity with a large bare cosmological
constant, $\Lambda$, and study its cosmological solutions. The model is
characterized by a function $f(\psi)=f_0 e^{\alpha\psi}$ where
$\psi=\Box^{-1}R$ and $\alpha$ is a real dimensionless parameter. In the
absence of matter, we find an expanding universe solution $a\propto t^n$ with
$n<1$, that is, a universe with decelarated expansion without any fine-tuning
of the parameter. Thus the effect of the cosmological constant is effectively
shielded in this solution. It has been known that solutions in non-local
gravity often suffer from the existence of ghost modes. In the present case we
find the solution is ghost-free if $\alpha>\alpha_{cr}\approx0.17$. This is
quite a weak condition. We argue that the solution is stable against the
includion of matter fields. Thus our solution opens up new possibilities for
solution to the cosmological constant problem.
