Screening of cosmological constant in non-local gravity
  We discuss a possible mechanism to screen a cosmological constant in
non-local gravity. We find that in a simple model of of non-local gravity with
the Lagrangian of the form, $R+f(\Box^{-1}R)-2\Lambda$ where $f(X)$ is a
quadratic function of $X$, there is a flat spacetime solution despite the
presence of the cosmological constant $\Lambda$. Unfortunately, however, we
also find that this solution contains a ghost in general. Then we discuss the
condition to avoid a ghost and find that one can avoid it only for a finite
range of `time'. Nevertheless our result suggests the possibility of solving
the cosmological constant problem in the context of non-local gravity.
