Multiple measurements of quasars acting as standard probes: Model independent calibration and exploring the dark energy equation of states

Recently, two classes of quasar samples were identified, which are promising as new cosmological probes extending to higher redshifts. The first sample uses the nonlinear relation between the ultraviolet and X-ray luminosities of quasars to derive luminosity distances, whereas the linear sizes of compact radio quasars in the second sample can serve as standardized rulers, providing angular-diameter distances. In this study, under the assumption of a flat universe, we refreshed the calibration of multiple measurements of high-redshift quasars (in the framework of a cosmological-model-independent method with the newest Hubble parameters data). Furthermore, we placed constraints on four models that characterize the cosmic equation of state (w). The obtained results show that: (1) the two quasar samples could provide promising complementary probes at much higher redshifts, whereas compact radio quasars perform better than ultraviolet and X-ray quasars at the current observational level; (2) strong degeneracy between the cosmic equation of state (w) and Hubble constant (H-0) is revealed, which highlights the importance of independent determination of H-0 from time-delay measurements of strongly lensed quasars; (3) together with other standard ruler probes, such as baryon acoustic oscillation distance measurements, the combined QSO+BAO measurements are consistent with the standard Lambda CDM model at a constant equation of state w = -1; (4) ranking the cosmological models, the polynomial parametrization gives a rather good fit among the four cosmic-equation-of-state models, whereas the Jassal-Bagla-Padmanabhan (JBP) parametrization is substantially penalized by the Akaike Information Criterion and Bayesian Information Criterion.

Automated summary

The study identified two classes of quasar samples as new cosmological probes that can extend to higher redshifts. It refreshed the calibration of high-redshift quasars using a cosmological-model-independent method with the newest Hubble parameters data and placed constraints on four models characterizing the cosmic equation of state (w). The results suggest that compact radio quasars may be more suitable for observational purposes compared to ultraviolet and X-ray quasars, and highlight the importance of independently determining the Hubble constant (H-0).