Pantheon+ is the supernova ledger of the Hubble tension, and its precision has converted a discrepancy into a structure. The compilation, 1,701 light curves of Type Ia supernovae spanning redshift 0.001 to 2.26, calibrated through the SH0ES Cepheid distance ladder, delivers H0 = 73.04 +/- 1.04 km/s/Mpc, fully 5 sigma from Planck's CMB-derived 67.4 +/- 0.5 (Brout et al. 2022; Riess et al. 2022). The sample's size and redshift reach close the classic escape routes: calibration systematics are constrained by multiple independent anchor galaxies, dust and color corrections are modeled across populations, and the tension survives every documented cut. The supernovae also anchor the dark energy measurements now showing the DESI evolving-w preference, placing them at the center of both flagship anomalies.
Within ΛCDM the supernova Hubble diagram must be a single smooth function of redshift: one expansion history, one H0, every sightline and host environment statistically identical after standard corrections. The data's residual structure has therefore drawn increasing attention: analyses report host-property and environment correlations in Hubble residuals beyond the standard mass-step correction, low-redshift coherent-flow features that survive peculiar-velocity modeling, and the persistent fact that the inferred w drifts when the sample is partitioned, none fatal individually, collectively suggesting the single-function assumption is leaking. The model has no framework in which supernova distances could legitimately depend on environment, so each correlation is processed as a systematic to be corrected rather than a signal to be read.
The standing is central and sharpening: Pantheon+ and its successors (Union3, DES-SN5YR) disagree with each other at the margins in exactly the low-redshift, environment-sensitive regime, Rubin will multiply the statistics a hundredfold, and the question of whether supernova residuals carry environmental physics is about to become decisively answerable.