The primordial helium-4 abundance Y_p = 0.2449 is observed to be slightly but consistently higher than some standard ΛCDM predictions in some analyses, particularly when combined with other light element constraints (Cyburt 2016; Aver 2015). Tension arises from uncertainties in nuclear reaction rates, systematic errors in metal-poor-galaxy spectroscopy, or possibility of modified early-universe conditions.
The standard model predicts Y_p ≈ 0.247 from BBN at the standard baryon-to-photon ratio, neutron-proton ratio, and expansion rate. Recovering the observed Y_p ≈ 0.245 to 0.247 (depending on the analysis) demands either nuclear-rate updates or modifications to early-universe thermodynamics. Sub-percent observational tensions persist as ongoing systematics-and-modeling work.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, the SCT prediction for Y_p is identical to ΛCDM at Y_p = 0.2449 ± 0.0040, the same value the FULL prompt baked-in canonical parameters list (paper 4210). The cascade terminates well before t ≈ 1 second (P40), so BBN proceeds at thermal equilibrium under standard SM thermodynamics (P42), producing the standard light-element yields.
The Plasma Equivalence Theorem (P29, P30) ensures the post-cascade plasma evolution to BBN is identical to ΛCDM when the same six-parameter thermodynamic state is reached. The R_b = 0.2545 derivation from cascade geometry (P22, P36, paper 4217) gives a baryon-to-photon ratio consistent with the observed BBN constraints. The cascade contributes nothing distinguishing to BBN's helium-4 yield because BBN proceeds entirely under standard SM physics from the cascade-set initial conditions.
The observed sub-percent Y_p tensions are therefore shared open issues with ΛCDM that require systematics resolution on the metal-poor-galaxy side or precision-nuclear-rate side, not a unique SCT signature. The same M2 framework that resolves the broader CMB acoustic-peak structure (recid 25), Ω_b BBN-vs-CMB (recid 171), and the cascade-thermodynamic post-recombination evolution accounts for Y_p as an unmodified standard prediction.
If precision deep helium spectroscopy + BBN nuclear-rate measurements converge Y_p observations and predictions to better than 0.1% agreement, the tension is resolved (favoring neither ΛCDM nor SCT). If the tension grows or persists at levels beyond systematics-induced expectations, both frameworks face the same challenge.