Growth of perturbations in nonlocal gravity with non-$\Lambda$CDM background

TL;DR

This paper re-analyzes a nonlocal gravity model's perturbation growth, showing its predictions depend heavily on the background cosmology, and finds it can be a viable alternative to standard cosmology.

Contribution

It demonstrates that the growth of perturbations in a nonlocal gravity model varies significantly with different background expansion histories, highlighting the importance of background choice.

Findings

Growth predictions are lower with non-$\Lambda$CDM backgrounds.

The model shows comparable growth data consistency to GR.

Background choice critically affects the model's fit to growth data.

Abstract

We re-analyze the nonlocal gravity model of Deser and Woodard which was proposed to account for the current phase of cosmic acceleration. We show that the growth of perturbations predicted by this nonlocal gravity model when its background evolution is fixed by some particular non-$\Lambda$CDM models can be substantially lower than when its background is fixed by $\Lambda$CDM. This can be seen when we consider the background expansion by a dark energy model with a slightly less negative equation of state with respect to cosmological constant. Our results hints towards a fact that the choice of the background expansion can play a crucial role how this nonlocal gravity model can fit the growth history data. While the growth data might show better consistency to GR models (among the background models we studied so far), it seems the nonlocal gravity model studied in this work is able to show comparable consistency to the growth data as well. Showing this consistency can be considered as a significant result since this model can stand as a distinguishable alternative to the standard model of cosmology.