In ΛCDM the baryon acoustic oscillation (BAO) scale is treated as a single, nearly redshift-independent “standard ruler” set by early-universe sound waves in a hot dense plasma, so BAO measurements at different epochs and along different directions should all match the same comoving sound horizon once geometric effects are accounted for (Eisenstein et al. 2005; Planck Collaboration 2020). However, increasingly precise galaxy, quasar, and Lya BAO data hint at mild but nontrivial inconsistencies in the inferred BAO scale between tracers, redshifts, and analysis methods, challenging the idea of a perfectly universal global ruler within the minimal ΛCDM framework and motivating extensions such as evolving dark energy, spatial curvature, or systematics that are difficult to reconcile self-consistently (Aubourg et al. 2015; Alam et al. 2021).