Do the Spirits Rise?

Understand

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.