Screening of cosmological constant in non-local cosmology

TL;DR

This paper explores a non-local gravity model that naturally shields a large cosmological constant, yielding a decelerated universe solution without fine-tuning and avoiding ghost instabilities under certain conditions.

Contribution

It introduces a specific exponential form of non-local function that provides ghost-free, stable solutions shielding the cosmological constant without fine-tuning.

Findings

Achieves decelerated expansion without fine-tuning

Identifies conditions for ghost-free solutions

Suggests a new approach to the cosmological constant problem

Abstract

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.