Growth index in the γδCDM model

Abstract

To better distinguish the nature of H₀ and S₈ tensions, it is necessary to separate the effects of expansion and the growth of structure. The growth index γ was identified as the most important parameter that characterizes the growth of density fluctuations independently of the effects of cosmic expansion. In the ΛCDM model, analyses performed with various cosmological datasets indicate that the growth index has to be larger than its theoretically predicted value. Cosmological models based on f(R) gravity theories have scale-dependent growth indices, whose values are even more at odds with the growth rate data. In this work, we evaluate the growth index in the γδCDM model both theoretically and numerically. Although based on f(R) gravity theory, we show through several analyses with different combinations of datasets that the growth index in the γδCDM model is very close in value to the ΛCDM and the ωCDM models. The growth of structure is suppressed in the γδCDM model, which is formulated with the extended gravitational growth framework. Upon analyzing cosmological data, we ascertain that the γδCDM model is equally competitive as the ΛCDM and the ωCDM models.