content_image/345673.jpg content_image/345677.jpg content_image/345679.jpg
<img src='content_image/345673.jpg'>

the ΛCDM model in the Einstein gravity. 

Figures 6 show evolution of γ ( z ) together with that of G$_{eff}$/G for different values of k . In the early high-redshift regime, γ ( z ) takes a constant value identical to the ΛCDM model be- cause f ( R ) gravity is indistinguishable from the Einstein gravity plus a positive cosmological constant then. It gradually decreases in time, reaches a minimum, and then increase again towards the present epoch. We can understand this tendency from the evolution equation for γ ( z )[36], 

<img src='content_image/345677.jpg'>

where Ω$_{DE}$ = 1 − Ω$_{m}$ is the density parameter of dark energy based on (8). In the high- redshift era when Ω$_{DE}$ is small, the above equation may be approximated as 

<img src='content_image/345679.jpg'>

