Revival of the Deser-Woodard nonlocal gravity model: Comparison of the original nonlocal form and a localized formulation

Abstract

We examine the origin of two opposite results for the growth of perturbations in the Deser-Woodard (DW) nonlocal gravity model. One group previously analyzed the model in its original nonlocal form and showed that the growth of structure in the DW model is enhanced compared to general relativity (GR), and thus concluded that the model was ruled out. Recently however, another group has re-analyzed it by localizing the model and found that the growth in their localized version is suppressed even compared to the one in GR. The question was whether the discrepancy is originated from an intrinsic difference between the nonlocal and localized formulations, or is due to their different implementations of the sub-horizon limit. We show that the nonlocal and local formulations give the same solutions for the linear perturbations as long as the initial conditions are set the same. The different implementations of the sub-horizon limit lead to different transient behaviors of some perturbation variables, however they do not affect much the growth of matter perturbations at the sub-horizon scale. In the meantime, we also report an error in the numerical calculation code of the former group, and verify that after fixing the error the nonlocal version also gives the suppressed growth. Finally, we discuss two alternative definitions of the effective gravitational constant taken by the two groups and some open problems.