  Dynamical observational probes of the growth of density perturbations
indicate that gravity may be getting weaker at low redshifts $z$. This evidence
is at about $2-3\sigma$ level and comes mainly from weak lensing data that
measure the parameter $S_8=\sigma_8 \sqrt{\Omega_{0m}/0.3}$ and redshift space
distortion data that measure the growth rate times the amplitude of the linear
power spectrum parameter $f\sigma_8 (z)$. The measured $f\sigma_8$ appears to
be lower than the prediction of General Relativity (GR) in the context of the
standard $\Lambda$CDM model as defined by the Planck best fit parameter values.
This is the well known $f\sigma_8$ tension of $\Lambda$CDM, which constitutes
one of the two main large scale challenges of the model along with the $H_0$
tension. We review the observational evidence that leads to the $f\sigma_8$
tension and discuss some theoretical implications. If this tension is not a
systematic effect it may be an early hint of modified gravity with an evolving
effective Newton's constant $G_{eff}$ and gravitational slip parameter $\eta$.
We discuss such best fit parametrizations of $G_{eff}(z)$ and point out that
they can not be reproduced by simple scalar-tensor and $f(R)$ modified gravity
theories because these theories generically predict stronger gravity than
General Relativity (GR) at low $z$ in the context of a $\Lambda$CDM background
$H(z)$. Finally, we show weak evidence for an evolving reduced absolute
magnitude of the SnIa of the Pantheon dataset at low redshifts ($z<0.1$) which
may also be explained by a reduced strength of gravity and may help resolve the
$H_0$ tension.
