One equation links every distance in cosmology, and the data keep whispering that it bends. The Etherington distance-duality relation, D_L = (1+z)^2 D_A, ties luminosity distances to angular-diameter distances through nothing but metric gravity, photon-number conservation, and light traveling null geodesics: it holds in any spacetime whatsoever under those three conditions, making its violation a fire alarm for fundamental physics rather than a parameter to adjust. Tests confront supernova luminosity distances with angular distances from BAO, cluster gas measurements, and lensing systems, parameterizing departures as eta(z); the accumulated literature reports a scatter of mild anomalies, individual analyses finding 1-to-2 sigma departures whose sign and redshift dependence vary with the D_A source, alongside many null results, with the cluster-based tests chronically entangled in the hydrostatic-bias question and the BAO-based tests inheriting the sound-horizon calibration.
The relation's diagnostic power is its rigidity: genuine violation requires photon non-conservation (axion mixing, dust beyond models), non-metric gravity, or non-null propagation, each revolutionary; while apparent violation, the more mundane and more interesting possibility for the standard model, requires only that the D_L and D_A datasets fail to measure the same universe, through mismatched calibrations, environment-dependent systematics, or genuinely different sampled volumes. The persistent low-level scatter, never confirming, never fully clearing, sits exactly where a framework with sightline-dependent expansion would put it.
The standing is a foundational test in its statistical adolescence: current sensitivity reaches a few percent on eta, the anomaly hints sit at that threshold, and the coming surveys (Rubin supernovae against DESI-Euclid BAO, sirens against lenses) will tighten the duality to the sub-percent regime where verdicts become unavoidable.