Cosmology Intertwined III: $f \sigma_8$ and $S_8$
  The standard $\Lambda$ Cold Dark Matter cosmological model provides a
wonderful fit to current cosmological data, but a few tensions and anomalies
became statistically significant with the latest data analyses. While these
anomalies could be due to the presence of systematic errors in the experiments,
they could also indicate the need for new physics beyond the standard model. In
this Letter of Interest we focus on the tension of the Planck data with weak
lensing measurements and redshift surveys, about the value of the matter energy
density $\Omega_m$, and the amplitude or rate of the growth of structure
($\sigma_8,f\sigma_8$). We list a few interesting models for solving this
tension, and we discuss the importance of trying to fit with a single model a
full array of data and not just one parameter at a time.
