Is the Hubble diagram of quasars in tension with concordance cosmology?
Hermano Velten, S. Gomes
Abstract
Recently, Risaliti and Lusso [Nat. Astron. 3, 272 (2019)] reported new measurements of the expansion rate of the Universe by constructing the Hubble diagram of 1598 quasars in the redshift range $0.5<z<5.5$. It is claimed a $4\ensuremath{\sigma}$ tension with the standard concordance $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ concerning both the fractionary matter density ${\mathrm{\ensuremath{\Omega}}}_{m0}$ and the dark energy equation of state parameter ${w}_{\mathrm{de}}$ standard values. In this work we promote an independent analysis of the same data set using a model-independent estimator for cosmic acceleration. Our results corroborate that the source of such tension can be related to the ${\mathrm{\ensuremath{\Omega}}}_{m0}$ value with a reasonable indication of a higher ${\mathrm{\ensuremath{\Omega}}}_{m0}$ value (${\mathrm{\ensuremath{\Omega}}}_{m0}\ensuremath{\gtrsim}0.4$). On the other hand, we find that the role played by ${w}_{\mathrm{de}}$ on the claimed tension is weak. We also discuss the use of this estimator as a ``quality tool'' to test the robustness of Hubble diagrams. We conclude claiming that the quasars data cannot yet be seen as a reliable cosmological tool since they cannot even state the universe experienced an accelerated expansion phase.