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| Paper IPM / P / 16451 |
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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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