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We construct an updated extended compilation of distinct (but possiblycorrelated) $f\sigma_8(z)$ Redshift Space Distortion (RSD) data publishedbetween 2006 and 2018. It consists of 63 datapoints and is significantly largerthan previously used similar datasets. After fiducial model correction weobtain the best fit $\Omega_{0m}-\sigma_8$ $\Lambda$CDM parameters and showthat they are at a $5\sigma$ tension with the correspondingPlanck15/$\Lambda$CDM values. Introducing a nontrivial covariance matrixcorrelating randomly $20\%$ of the RSD datapoints has no significant effect onthe above tension level. We show that the tension disappears (becomes less than$1\sigma$) when a subsample of the 20 most recently published data is used. Apartial cause for this reduced tension is the fact that more recent data tendto probe higher redshifts (with higher errorbars) where there is degeneracyamong different models due to matter domination. Allowing for a nontrivialevolution of the effective Newton's constant as$G_{\textrm{eff}}(z)/G_{\textrm{N}} = 1 + g_a \left(\frac{z}{1+z}\right)^2 -g_a \left(\frac{z}{1+z}\right)^4$ ($g_a$ is a parameter) and fixing a \plcdmbackground we find $g_a=-0.91\pm 0.17$ from the full $f\sigma_8$ dataset whilethe 20 earliest and 20 latest datapoints imply $g_a=-1.28^{+0.28}_{-0.26}$ and$g_a=-0.43^{+0.46}_{-0.41}$ respectively. Thus, the more recent $f\sigma_8$data appear to favor GR in contrast to earlier data. Finally, we show that theparametrization $f\sigma_8(z)=\lambda \sigma_8 \Omega(z)^\gamma /(1+z)^\beta$provides an excellent fit to the solution of the growth equation for both GR($g_a=0$) and modified gravity ($g_a\neq 0$).

Evolution of the $f\sigma_8$ tension with the Planck15/$\Lambda$CDM determination and implications for modified gravity theories