Screening of cosmological constant in non-local cosmology

摘要

We consider a model of non-local gravity with a large bare cosmological constant, Λ, and study its cosmological solutions. The model is characterized by a function f(ψ) = f _0e ~(αψ), where ψ = □ ~(-1)R and α is a real dimensionless parameter. In the absence of matter, we find an expanding universe solution a ∝ t ~n with n < 1, that is, a universe with decelerated 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 α > α _(cr) ≈ 0.17. This is quite a weak condition. We argue that the solution is stable against the inclusion of matter fields. Thus our solution opens up new possibilities for solution to the cosmological constant problem.