On Problems with Cosmography in Cosmic Dark Ages

Abstract

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.

4 pages; v2 plots added, matches published version

References in corpus (3)

• Cosmology with gamma-ray bursts: I. The Hubble diagram through the calibrated $E_{\mathrm{p},\mathrm{i}}$ - $E_{\mathrm{iso}}$ correlation
• The Hubble series: Convergence properties and redshift variables
• Cosmology with gamma-ray bursts: II Cosmography challenges and cosmological scenarios for the accelerated Universe

Cited by in corpus (18)

• Challenges for $\Lambda$CDM: An update
• On Larger $H_0$ Values in the CMB Dipole Direction
• Do quasar X-ray and UV flux measurements provide a useful test of cosmological models?
• High-redshift cosmography: Application and comparison with different methods
• Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies
• Cosmic expansion parametrization: Implication for curvature and ${\text{H}}_0$ tension
• Can high-redshift Hubble diagrams rule out the standard model of cosmology in the context of cosmographic method?
• Quasar UV/X-ray relation luminosity distances are shorter than reverberation-measured radius-luminosity relation luminosity distances
• Can the Distance-Redshift Relation Be Determined from Correlations Between Luminosities?
• Cosmographic Footprints of Dynamical Dark Energy
• Effect of extinction on quasar luminosity distances determined from UV and X-ray flux measurements
• Model-independent cosmological inference after the DESI DR2 data with improved inverse distance ladder
• f(T) cosmology against the cosmographic method: A new study using mock and observational data
• Testing the $\Lambda$CDM paradigm with growth rate data and machine learning
• Cosmological distances with general-relativistic ray tracing: framework and comparison to cosmographic predictions
• Redshift-evolutionary X-ray and UV luminosity relation of quasars from Gaussian copula
• Variability in Quasar Light Curves: using quasars as standard candles
• Cosmological parameter constraints using phenomenological symbolic expressions: On the significance of symbolic expression complexity and accuracy