Cosmology with AGN: can we use quasars as standard candles?

The nonlinear relation between X-ray and UV luminosity in quasars can be used to estimate their distance. Recently, we have shown that, despite the large dispersion of the relation, a Hubble diagram made of large samples of quasars can provide unique constraints on cosmology at high redshift. Furthermore, the dispersion of the relation is heavily affected by measurement errors: until now we have used serendipitous X-ray observations, but dedicated observations would significantly increase the precision of the distance estimates. We discuss the future role of XMM in this new field, showing (a) the fundamental contribution of the Serendipitous Source Catalog and of large surveys and (b) the breakthrough advancements we may achieve with the observation of a large number of Sloan Digital Sky Survey (SDSS) quasars at high redshift: every similar to 10 quasars observed at z similar to 3 would be equivalent to discovering a supernova at that redshift.