Effective Angular Bias in Cosmological Distance Inference: Empirical Evidence from DESI BAO and the Hubble Tension

This paper investigates a phenomenological geometric approach to the  H0 tension. We explore whether a systematic bias in angular distance inference, arising from effective coarse-grained anisotropic expansion, can partially account for the discrepancy between local distance ladder measurements and early-universe CMB inferences.

We introduce an effective metric parameterization in which directional variations in expansion rates produce an angular weighting bias in the mapping between physical and observed quantities. We demonstrate that observing such a geometry under the strict assumption of statistical isotropy induces a systematic rescaling of the inferred angular diameter distance (DA), yielding the leading-order scaling relation H0global≈H0local(1−ϵ/2)

Inverting this relation using SH0ES and Planck data yields an effective angular bias of ϵ≈0.15. We test this geometric hypothesis using raw Baryon Acoustic Oscillation (BAO) data from the Dark Energy Spectroscopic Instrument (DESI) 2024 Data Release 1. Direct analysis at zeff=0.51 reveals a 1.96σinternal tension between radial (H0=70.08±1.37km s−1 Mpc−1) and transverse (H0=66.94±0.84km s−1 Mpc−1) expansion determinations.

Combined across multiple redshift bins, this 2.1σradial-transverse dichotomy provides suggestive empirical support for an inference bias in angular distance measures, offering a testable phenomenological alternative to modifications of early-universe thermodynamics.