On problems with cosmography in cosmic dark ages

Quasars show considerable promise as standard candles in a high-redshift window beyond Type Ia supernovae. Recently, Risaliti, Lusso & collaborators [1-3] have succeeded in producing a high redshift Hubble diagram (z less than or similar to 7) that supports a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat Lambda CDM model up to z similar to 1.5 - 2, but strong deviations emerge at higher redshifts. This conclusion hinges upon a log polynomial expansion for the luminosity distance. In this note, we demonstrate that this expansion (or improvements thereof) typically can only be trusted up to z similar to 1.5 - 2. As a result, a breakdown in the validity of the expansion may be misinterpreted as a (phantom) deviation from flat Lambda CDM. We further illustrate the problem through mock data examples. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

Quasars show potential as standard candles at high redshifts beyond Type Ia supernovae. Recent research has found that the Hubble diagram of quasars matches the standard flat Lambda CDM model up to z around 1.5-2, but deviates at higher redshifts. The reliability of the polynomial expansion for luminosity distance may only be trusted up to around z 1.5-2.