Quasars and dark energy

Dark energy can be studied with a number of probes, broadly classified as standard candles and standard rulers. The use of several independent probes is needed to get accurate and reliable results since every probe has its particular systematic problems/biases. Quasars do not seem to be obvious standard candles but there are two ways to pursue. The first method is based on the selection of special sub-class of quasars, which radiate precisely at Eddington ratio.  The second method can use all quasars, but requires to perform the monitoring of the quasar variability of the continuum and of the emission lines. Reverberation studies (i.e. the measurements of the time delay of the line emission with respect to the continuum) allow to establish the absolute monochromatic luminosity of a given source independently of its redshift. We developed a theory which explains the delay-luminosity relation known for nearby sources and allows to extrapolate this type of relation towards higher black hole mass sources, like quasars at redshifts of a few. Preliminary studies are being done using our data from the dedicated campaign with the 11-m SALT telescope. These high-quality data allow to study the spectra in much detail (including the spectra decomposition into lines and Fe II pseudo-continuum) in order to determine the possible systematic errors. Later the method will be applied to expected spectroscopic surveys; in addition, current estimates indicate that the project can also be undertaken with a photometric survey of the quality of LSST. The advantage of using quasars lies in their broad range of redshifts, large number of available sources and weak evolution of their metallicity with redshift.