  Proceeding of the "HACK100" Conference, 6-10 June 2022, Trieste, Italy - In
recent years, quasars have been shown to be reliable standardizable candles,
thanks to the non-linear relation between their X-rays and ultraviolet
luminosity. Quasars are also very numerous and they are found at all the
observed redshifts: this allows us to investigate the expansion rate of the
Universe and the cosmological parameters in a previously almost untested
redshift range ($z\sim2-7$). At redshift higher than 1.5, the Hubble Diagram of
quasars shows a significant tension with the predictions of the $\Lambda$CDM
model. I will show how detailed optical/UV and X-rays spectroscopic analysis
can be used (i) to obtain more precise distance estimates, and (ii) to derive
information about the physical process behind the luminosities relation, and
discuss the cosmological implementations.
