Strong Lensing, Dark Matter and H 0 Estimate

Gravitational lensing represents a powerful tool to estimate the cosmological parameters and the distribution of dark matter. I will describe the main observable quantities, concentrating on strong lensing, that manifests its effect through the formation of spectacular events, like multiple quasars, Einstein rings and arcs in clusters of galaxies. In events where a quasar is lensed by an intervening galaxy, it is possible to give an estimate of the Hubble constant H 0 , by choosing a mass density model for the lens, thus allowing to constrain the dark matter content.