Cosmography and flat ACDM 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 similar to 4 sigma tension with the ACDM 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 ACDM beyond z similar to 2, thus undermining the similar to 4 sigma tension claim. Moreover, through direct fits of both the flat ACDM and the log polynomial model to the SNe + QSO + GRB data set, we confirm that the flat ACDM model is preferred. Ultimately, we trace the tension to the QSO data and show that a best fit of the flat ACDM model to the QSO data leads to a flat ACDM universe with no dark energy within 1 sigma. This marks an irreconcilable tension between the Risaliti-Lusso QSOs and flat ACDM.