The primordial baryon density Ω_b from Big Bang Nucleosynthesis (BBN, derived from light-element abundances) differs slightly but persistently from the value inferred from CMB anisotropy by Planck. BBN-derived Ω_b tends to be marginally lower than CMB-derived (Cyburt 2016; Planck 2018). Either systematic uncertainties in nuclear reaction rates, stellar abundances, or a deeper inconsistency in the standard early-universe model.
The standard model uses two independent measurement paths to Ω_b that should converge on one universal value. Sub-percent persistent disagreement implies either systematic measurement errors that have not yet been fully cleaned out or a tension in the underlying cosmological model. Resolving within ΛCDM demands progress on either the BBN nuclear-rate side or the CMB-foreground side.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, the SCT prediction is Ω_b-BBN = Ω_b-CMB exactly, the same as ΛCDM. The cascade terminates well before t ≈ 1 second (P40), so BBN proceeds at thermal equilibrium under standard Standard Model thermodynamics (P42), reproducing the ΛCDM nucleosynthesis predictions for D, He-4, He-3, and Li-7. The Plasma Equivalence Theorem (P29, P30) guarantees that the post-cascade plasma evolution to recombination matches ΛCDM exactly when the same thermodynamic state parameters are reached.
The R_b = 0.2545 ± 0.032 derivation from cascade geometry (P22, P36, paper 4217) gives a first-principles prediction for the baryon-to-photon ratio that maps to Ω_b. The CAR formula c_s² = (1 + R_b)/3 (paper 4216) gives the same r_d = 149.1 Mpc that's used in CMB acoustic-peak fits, so the SCT-CMB Ω_b inference matches the standard one within the cascade-derivation uncertainty band.
The observed sub-percent BBN-CMB tensions are therefore shared open issues with ΛCDM that require systematics resolution on the nuclear-rate or foreground side, not a unique SCT signature. The same M2 framework that resolves the broader CMB acoustic-peak structure (recid 25) and the BAO scale (recid 8, 75) treats the Ω_b agreement as an unmodified standard prediction; SCT does not improve or worsen the situation relative to ΛCDM.
If precision deep BBN nuclear-rate measurements + CMB-S4 foreground analyses converge BBN-CMB Ω_b agreement at the 0.1% level, the tension is resolved (favoring neither ΛCDM nor SCT particularly). If the tension persists or grows beyond systematics-induced expectations, both frameworks face the same challenge.