A New Consistency Test for the $Λ$CDM Model using Radial and Transverse BAO Measurements

Abstract: We present a calibration-free consistency test of spatially flat $Λ$CDM based on baryon acoustic oscillation (BAO) distance measurements. The method forms ratios of BAO distances -- the Hubble distance $Ð(z)$, the comoving angular diameter distance $\DM(z)$, and the volume-averaged distance $\DV(z)$ -- so that the sound horizon scale cancels, and then maps each observed ratio to an effective flat-$Λ$CDM matter density parameter, $\OmL$, defined as the value of $Ω_{\rm M}$ that reproduces the measured ratio within $Λ$CDM. Flat $Λ$CDM predicts that $\OmL$ should be independent of redshift and of the particular ratio used. For ratios involving the integrated distances $\DM$ and $\DV$, we associate them with well-defined effective line-of-sight redshift intervals using a redshift-matching strategy based on the integral mean value theorem. We apply the test to BAO measurements from the Dark Energy Spectroscopic Instrument (DESI) Data Release~1 and Data Release~2, propagating the full published BAO covariance matrices into all derived ratios and $\OmL$ constraints. Within current uncertainties, the inferred $\OmL$ values are broadly consistent with a redshift-independent constant, providing an internal consistency check of flat $Λ$CDM that can be strengthened straightforwardly as BAO measurements improve.