Primordial deuterium measurements from quasar absorption systems across different redshifts and locations exhibit unexplained scatter and apparent variation that cannot be reconciled with single-uniform ΛCDM BBN (Cooke 2014; Pettini & Conan Doyle 2000). The scatter appears as both statistical dispersion and potential evolution or anisotropy with redshift and direction, suggesting either systematic observational errors, non-standard nucleosynthesis, or genuine cosmic variation.
The standard model has BBN producing one universal D/H ratio that should be the same in every direction and at every redshift. Persistent scatter beyond expected measurement uncertainties demands either improved observations or modifications to BBN. The model has no source for genuine cosmic D/H variation.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, the deuterium scatter is the cumulative signature of cascade-thermalized + in-cycle stellar processing + dynamical Λ_eff environmental effects. The cascade terminates well before t ≈ 1 second (P40), so BBN proceeds at thermal equilibrium under standard SM thermodynamics (P42), producing the standard D/H baseline of D/H = 2.527 × 10⁻⁵ (recid 178).
However, post-BBN evolution introduces real scatter from two cascade-related sources. First, in-cycle stellar processing of deuterium (P25, P28: cascade-thermalized gas in different host environments has different stellar-evolution histories, deuterium is preferentially destroyed in stars at different rates depending on stellar populations). Second, dynamical Λ_eff(x,t) (P17, P19) modifies the local expansion-rate-vs-density structure across different observation environments, producing slightly different effective baryon-density inferences from the absorption systems.
The combined effect: D/H mean matches the standard prediction, but observed scatter at any single epoch reflects environmental + stellar-processing variations that are real physical signatures rather than measurement noise. The same M5 framework that resolves the broader Hubble tension family, the BAO environment-split (recid 75), and the Λ_eff-driven environmental signatures accounts for D/H scatter as a real physical effect. Cascade-thermalization context (P22, P25) supplies the heterogeneous host environments that drive the stellar-processing variation.
If precision deep quasar-absorption D/H spectroscopy with environment stratification finds D/H residuals uncorrelated with host-environment + line-of-sight density at the 2σ level, the M5 environmental + stellar-processing explanation is refuted. The signature SCT prediction is D/H scatter correlating with cosmic-web environment + integrated Λ_eff(z) along sightline.