The best D/H measurements anchor BBN beautifully at 2.53 × 10⁻⁵ (Cooke 2014), but the full ensemble scatters beyond formal uncertainties with hints of redshift variation, and one global nucleosynthesis event permits exactly one primordial ratio. The mean is a triumph; the dispersion is undiagnosed.
The model's BBN runs once, everywhere, identically, so every absorber must return the same primordial D/H up to measurement error and astration. Scatter beyond that budget forces a choice between unmodeled systematics and the unthinkable, genuinely varying primordial conditions, and the model cannot survive the second option.
SCT locks the primordial half as firmly as the standard model does, and for the same reason made stronger: the cascade terminated before one second (P40), BBN ran at thermal equilibrium under standard physics (P42), and the SCT expansion rate at T = 1 MeV matches the ΛCDM rate to one part in 10³⁹, so the framework registers the identical D/H of 2.527 × 10⁻⁵ as a fixed point. There is no collision-flavored mechanism for primordial variation, and SCT stakes its BBN sector on there being none, the same disciplined position held on lithium (recid 169).
What SCT adds is the second half: the deposited universe processes its deuterium unevenly. Absorption systems sit on a web whose reservoirs were heterogeneous from deposition (P25, P34), with star formation igniting early and unevenly on that geography (recid 111, recid 125), so astration histories vary sightline by sightline more richly than a smoothly assembled universe would arrange. The registered expectation: the ensemble scatter should resolve into measurement systematics plus environment-correlated astration, tracking each absorber's metallicity and web position, with the floor of the D/H distribution, the most pristine systems, converging exactly on the one primordial number. The cascade-termination chemistry is in Paper 1, From Chaos to Convergent Foundations.
Keystone economy: P40 and P42 fix the ratio; P25 diversifies the histories that obscure it.
The shared-jeopardy kill: confirmed D/H variation in systems of effectively zero metallicity, where astration cannot operate, would demonstrate genuinely varying primordial conditions and refute the Plasma Equivalence chain (P29, P40) directly, for SCT as fatally as for ΛCDM. The added half is separately exposed: if the scatter shows no correlation with absorber metallicity or environment as the census grows, the heterogeneous-astration reading loses its signature.