On the current status of Modified Gravity
  We revisit the cosmological viability of the Hu $\&$ Sawicki modified gravity
scenario. The impact of such a modification on the different cosmological
observables, including gravitational waves, is carefully described. The most
recent cosmological data, as well as constraints on the relationship between
the clustering parameter $\sigma_8$ and the current matter mass-energy density
$\Omega_m$ from cluster number counts and weak lensing tomography, are
considered in our numerical calculations. The strongest bound we find is
$|f_{R0}| < 3.7 \times 10^{-6}$ at $95\%$~CL. Forthcoming cluster surveys
covering 10,000 deg$^2$ in the sky, with galaxy surface densities of
$\mathcal{O}(10)$~arcmin$^{-2}$ could improve the precision in the
$\sigma_8$-$\Omega_m$ relationship, tightening the above constraint.
