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Paper   IPM / Physic / 16451
School of Physics
  Title:   On cosmography in the cosmic dark ages: are we still in the dark?
1.  A. Banerjee
2.  E. O Colgáin
3.  M. Sasaki
4.  M.M. Sheikh-Jabbari
5.  T. Yang
  Status:   Preprint
  Year:  2020
  Supported by:  IPM
Quasars and gamma-ray bursts show considerable promise as standard candles in a high-redshift window beyond Type Ia supernovae. Recently, Risaliti, Lusso & collaborators [,,] have succeeded in producing a high redshift Hubble diagram ( z <~7) that supports "a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat ΛCDM model up to z  ∼ 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 typically can only be trusted up to z  ∼ 1.5−2. As a result, a breakdown in the validity of the expansion may be misinterpreted as a (phantom) deviation from flat ΛCDM. We further illustrate the problem through mock data examples.

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