The BAO standard ruler is drifting between its users. Galaxy, quasar, and Lyman-α tracers return mildly different scales, and the Lyman-α forest has pulled away from the galaxy scale at the 1.5-to-2σ level across surveys, while the single Planck-calibrated expectation, r_d = 147.09 ± 0.26, is itself an inference through the ΛCDM chain rather than a measurement (Aubourg 2015; Alam 2021; DESI 2024). A ruler that should be one number is behaving like a family of numbers.
The model computes a single sound horizon from early-universe physics and propagates it through a single global expansion history. Both halves are assumptions: one r_d for all tracers, one H(z) for all environments. Any genuine drift between tracers has nowhere to go except systematics or new dark-sector physics, and the proposed extensions each repair one dataset by straining another.
SCT keeps the ruler and changes the carrying. By the Plasma Equivalence Theorem the collision-thermalized plasma reaches recombination in the standard thermodynamic state, so there is exactly one drag horizon, computed with the textbook sound speed from ω_b and ω_m alone: r_d = 146.8 ± 5 Mpc, within 1σ of DESI's 147 (P29, P30). An earlier SCT reading set the ruler with the coherent (CAR) sound speed, c_s² = (1 + R_b)/3, yielding 149.1 Mpc; the registered ledger now retires that step as a category error, and the coherence physics with its cascade-derived R_b = 0.2545 ± 0.032 lives in the late-time sector (S₈ = 0.783, intrinsic-alignment bias) rather than inside the ruler.
The drift between tracers is the M5 half: dynamical Λ_eff(x,t) makes the expansion history mildly environment- and epoch-dependent (P17, P18, P19), so the angular-diameter-distance mapping that converts the one ruler into an observed scale differs slightly between tracers sampling different redshifts and different density environments. The registered prediction is concrete: 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 drift between tracers is not noise to be modeled away; it is the one ruler reporting the structure of the Λ_eff field it travels through.
PROVISIONAL (ruler component). r_d = 146.8 ± 5 Mpc awaits independent third-party verification of the Boltzmann pipeline. The earlier CAR-at-recombination outputs (149.1 Mpc and the 161 to 178 Mpc analytic integrals) are superseded, and the 28 Mpc gap the old ledger carried is resolved as the category error itself. The M5 environmental component is grounded separately.
Two registered kills. DESI Year 5 measuring r_d above 150.5 or below 145.0 Mpc at 3σ refutes the standard-horizon value directly. Environment-split BAO finding void and cluster tracers consistent to better than 0.05 percent in D_A at the same redshift rules out the predicted 0.1 to 0.3 percent Λ_eff shift and removes the M5 half. Both legs must survive; either failing breaks the resolution.