Testing inhomogeneous cosmography in our cosmic neighborhood using CosmicFlows-4

TL;DR

This study investigates the convergence issues of third order cosmographic expansion in our local universe using CosmicFlows-4 data, revealing early divergence and emphasizing cautious interpretation of cosmographic parameters.

Contribution

It demonstrates the divergence of cosmographic expansion at low redshifts using realistic local models, highlighting limitations of standard cosmography in our cosmic neighborhood.

Findings

Cosmographic expansion diverges before z=0.1

Standard FLRW cosmography loses information at low redshifts

Caution needed in interpreting cosmographic coefficients

Abstract

The convergence of the third order general cosmographic expansion of the luminosity distance is examined using several versions of a semi-realistic model of our local cosmic neighborhood, based on publicly available density and velocity fields from CosmicFlows-4. The study supports earlier findings that the general cosmographic expansion diverges at surprisingly low redshifts, often well before z = 0.1. By being based on a realistically placed observer within a data-informed cosmic environment, the results underscore that convergence must be a central concern when applying the general cosmographic expansion. By showing all-sky maps of kinematic parameters, the study also highlights the substantial information we lose when relying solely on standard FLRW-based cosmography. Poor convergence does not necessarily render the information extracted by fitting data to the general cosmographic expansion meaningless. Rather, it calls for caution in interpreting this information, particularly regarding the physical meaning of the fitting coefficients, the physical scales they probe and the implicit smoothing introduced by the fit.