On the Reliability of Quasars as Cosmological Distance Indicators

The goal of the present paper is to assess the usefulness of quasars as cosmological distance indicators. We calibrate, in a model-independent way, the non-linear relation between X-ray and UV emission to derive quasar luminosity distances. Using this calibration, we construct the Hubble diagram up to redshift $z \sim 7.5$, and test the ability of quasars to constrain the $\Lambda$CDM and $\omega$CDM models at low and high redshifts, in combination with the Type Ia supernova Pantheon Plus sample, as well as the latest results from DESI and Planck Compressed data. We find consistency with previous studies in the values of $\gamma$ and $\beta$ when using a quasar subsample with $z<1.43$. However, when the data is used to constrain the cosmological models, we find that quasars fail as reliable distance indicators, even when combined with other independent cosmological datasets.