Screening of cosmological constant in non-local gravity

Understand

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'.