Speaker
Description
Active Galactic Nuclei (AGNs) are among the few astrophysical populations capable of probing the expansion history of the Universe beyond the redshift range currently accessible to Type Ia supernovae. We investigate their cosmological potential through an analysis of more than $5\times10^{5}$ quasars from the SDSS DR14 catalog spanning redshifts up to $z\sim3.5$. The ultraviolet--X-ray luminosity relation is examined in multiple redshift intervals using independent linear regressions. We obtain slopes in the range $\gamma \simeq 0.59$--$0.61$ and a nearly constant dispersion of $\sigma_{\rm rms}\simeq0.30$ dex, with no statistically significant evidence for redshift evolution. These results support the use of AGNs as high-redshift cosmological tracers. Building on this framework, we develop a multidimensional selection methodology combining photometric, statistical, and cosmological-consistency criteria to identify a photometric AGN golden sample of 255 candidates from SDSS DR17. These sources are incorporated into a joint Type Ia supernova--AGN Hubble diagram, extending the cosmological reconstruction beyond the regime densely populated by supernova observations. The resulting analysis provides a complementary determination of the Hubble constant and enables direct comparisons with contemporary cosmological constraints.