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

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 4σ tension with the standard concordance ΛCDM concerning both the fractionary matter density Ωm0 and the dark energy equation of state parameter wde 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 Ωm0 value with a reasonable indication of a higher Ωm0 value (Ωm0 ≳ 0.4). On the other hand, we find that the role played by wde 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.