Cosmography and flat ΛcDM tensions at high redshift

Risaliti, Lusso, and collaborators have constructed a high-redshift Hubble diagram of supernovae (SNe), quasars (QSO), and gamma-ray bursts (GRBs) that shows a "∼4σ tension with the ΛCDM model"based on a log polynomial cosmographic expansion [1,2]. In this work, we demonstrate that the log polynomial expansion generically fails to recover flat ΛCDM beyond z∼2, thus undermining the ∼4σ tension claim. Moreover, through direct fits of both the flat ΛCDM and the log polynomial model to the SNe+QSO+GRB data set, we confirm that the flat ΛCDM model is preferred. Ultimately, we trace the tension to the QSO data and show that a best fit of the flat ΛCDM model to the QSO data leads to a flat ΛCDM universe with no dark energy within 1σ. This marks an irreconcilable tension between the Risaliti-Lusso QSOs and flat ΛCDM.