In this paper, we investigate the consistency of a decreasing Geff(z) with some viable scalar-tensor and f(R) theories. We stress that f(R) theories generically can not lead to a decreasing Geff(z) for any cosmological background. For scalar-tensor models we deduce that in the context of a ΛCDM background a decreasing Geff(z) is not consistent with a large Brans-Dicke parameter ωBD,0 today. This inconsistency remains and amplifies in the presence of a phantom dark energy equation of state parameter. However, it can be avoided for w>-1. We also find that any modified gravity model with the required decreasing Geff(z) and Geff,0=G, would have a characteristic signature in its growth index γ.
If you use any of the above codes or the figures in a published work please cite the following paper:
Consistency of Modified Gravity with a Decreasing Geff in a ΛCDM Background
Radouane Gannouji, Lavrentios Kazantzidis, Leandros Perivolaropoulos and David Polarski
Phys.Rev. D98 (2018) no.10, 104044, arxiv:1809.07034.
Any further questions/comments are welcome.