Quasars and their emission lines as cosmological probes

Quasars are the most luminous stable sources in the Universe. They are currently
observed out to redshift z ~ 7 when the Universe was less than one tenth of its
present age. Since their discovery 50 years ago astronomers have dreamed of using
them as standard candles. Unfortunately quasars cover a very large range (8 dex) of
luminosity making them far from standard. I briefly review several methods that can
potentially exploit quasars properties and allow us to obtain useful constraints on
principal cosmological parameters. Using the 4D Eigenvector 1 formalism there is a
way to effectively isolate quasars radiating near the Eddington limit. If the
Eddington ratio is known, under several assumptions it is possible to derive
distance independent luminosities. I briefly discuss and the main statistical and
systematic errors involved, and whether these "standard Eddington candles" can be
actually used to constrain cosmological models.