Observational Constraints on Growth Index with Cosmography

Understand

In the literature, it was proposed that the growth index $\gamma$ is useful to distinguish the scenarios of dark energy and modified gravity.

• In the present work, we consider the constraints on the growth index $\gamma$ by using the latest observational data.
• To be model-independent, we use cosmography to describe the cosmic expansion history, and also expand the general $\gamma(z)$ as a Taylor series with respect to redshift $z$ or $y$-shift, $y=z/(1+z)$.
• We find that the present value $\gamma_0=\gamma(z=0)\simeq 0.42$ (for most of viable $f(R)$ theories) is inconsistent with the latest observational data at high confidence level (C.L.).