The BAO peak flexes with environment: compressed in overdense regions, stretched in underdense ones, at the percent level or more (Roukema 2015; Neyrinck 2018), beyond what nonlinear evolution and reconstruction comfortably supply. A ruler that reads differently in voids and clusters is not behaving like the rigid early-universe standard it is assumed to be.
The model fixes one comoving sound horizon at the drag epoch and one global expansion history to carry it forward, so environment independence is structural. Any measured environmental flex must be manufactured by late-time nonlinear physics, the very contribution reconstruction was designed to remove, leaving the model explaining its own correction's failure.
SCT leaves the early-universe ruler alone and puts the flex where the data show it: in the carrying. The Plasma Equivalence Theorem (P29, P30) guarantees the post-cascade acoustic physics matches the standard treatment, so the comoving scale stamped at the drag epoch is one number. What varies is the integrated expansion along each line of sight: Λ_eff(x,t) runs enhanced through voids and suppressed through overdensities (P17, P19), so the angular-diameter-distance mapping that converts the one true ruler into a measured peak position differs between void-environment and cluster-environment galaxy pairs. The peak is not moving; the metric carrying it to us is.
The prediction is registered with numbers: void-environment BAO tracers should return a D_A shifted from cluster-environment tracers by 0.1 to 0.3 percent at the same redshift, the direct environment-split signature of the Λ_eff field at its canonical one-percent variability on 100 to 300 Mpc scales. The mechanism is the same M5 engine carrying the Hubble tension's KBC component, the ISW landscape (recid 67), and the global ruler drift (recid 64); the expansion-side structure is laid out in Paper 7, From Chaos To Cosmic Expansion, with the acoustic-sector separation in Paper 3, From Chaos To Concordance Spectra.
Two premises carry it, P17 for the ratio and P19 for its environmental variability, with P29 and P30 standing guard to keep the early universe untouched.
The kill threshold is registered: DESI environment-split BAO finding void-environment and cluster-environment tracers consistent in D_A to better than 0.05 percent at the same redshift rules out the predicted 0.1 to 0.3 percent shift and refutes the M5 reading. Conversely, a split that survives but shows no correlation with sightline density structure would break the mechanism's geography even while leaving an anomaly.