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
\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 $z \sim 1.5-2$. 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.
