Do the Spirits Rise?

TL;DR

This paper investigates a nonlocal gravity model that explains cosmic acceleration without dark energy and examines whether ghost instabilities can be excited over time, finding strong evidence against such excitation.

Contribution

It provides an analysis showing that the scalar ghost mode in the nonlocal gravity model remains unexcited during evolution, supporting its viability.

Findings

The nonlocal gravity model explains cosmic acceleration without dark energy.

The scalar ghost mode does not get excited during the universe's evolution.

The model avoids ghost-related instabilities despite having a ghost in the localized version.

Abstract

A nonlocal gravity model based on $\frac1{\square} R$ achieves the phenomenological goals of generating cosmic acceleration without dark energy and of suppressing the growth of perturbations compared to the $\Lambda$CDM model. Although the localized version of this model possesses a scalar ghost, the nonlocal version does not suffer from any obvious problem with ghosts. Here we study the possibility that the scalar ghost mode might be excited through time evolution, even though it is initially absent. We present strong evidence that this does not happen.