On Problems with Cosmography in Cosmic Dark Ages

Understand

Quasars show considerable promise as standard candles in a high-redshift window beyond Type Ia supernovae.

• Recently, Risaliti, Lusso \& collaborators \cite{Risaliti:2018reu, Lusso:2019akb, Lusso:2020pdb} have succeeded in producing a high redshift Hubble diagram ($ z \lesssim 7$) that supports "a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat $\Lambda$CDM model up to $z \sim 1.5-2$, 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 $z \sim 1.5-2$.