Is the Hubble diagram of quasars in tension with concordance cosmology?

Abstract

Recently Risaliti \& Lusso [Nature Astron. 3 (2019) 3 272] 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 $4\sigma$ tension with the standard concordance $\Lambda$CDM concerning both the fractionary matter density $\Omega_{m0}$ and the dark energy equation of state parameter $w_{de}$ 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 $\Omega_{m0}$ value with a reasonable indication of a higher $\Omega_{m0}$ value ($\Omega_{m0} \gtrsim 0.4$). On the other hand, we find that the role played by $w_{de}$ 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 can not yet be seen as a reliable cosmological tool since they can not even state the universe experienced an accelerated expansion phase.