The baryon density is measured at three minutes (BBN deuterium plus LUNA's lab-measured burning rate: Ω_b h² near 0.02195 +/- 0.00022) and at 400,000 years (Planck acoustic peaks: 0.02237 +/- 0.00015), and the flagship concordance has opened a 1.5 to 2σ gap, with BBN preferring fewer baryons. Both measurements are precision metrology with shrinking errors and no ΛCDM freedom between them.
One Ω_b serves both epochs through two model-dependent chains, and the CMB chain carries the model's acoustic assumptions: standard sound speed, standard damping, a CDM-loaded photon-baryon fluid. If those assumptions misweigh the peaks even slightly, the inferred Ω_b shifts while BBN's nuclear chain stands clean, and the model has no way to see which chain bent.
SCT trusts the three-minute measurement absolutely: cascade termination before one second (P40) makes SCT nucleosynthesis identical to standard BBN (P42), and thermodynamic state sufficiency carries the abundances forward untouched (P29, P30), so the LUNA-updated BBN Ω_b is the framework's clean number. The recombination chain is standard too: by the same Plasma Equivalence Theorem the collision-thermalized plasma reaches the acoustic epoch in the state the standard equations describe, so SCT adds no freedom between the two epochs either. An earlier SCT reading located the gap in a modified CAR sound speed at recombination; the registered ledger retires that step as a category error, and coherence physics stays in the late-time sector.
What remains is honest shared jeopardy, the same stance SCT takes on lithium-7: a 1.5 to 2σ pull between two precision chains that both frameworks compute identically, most plausibly nuclear-rate or measurement systematics, carrying no cosmological verdict at its current size. The one registered SCT caveat sits in the damping tail: the Planck posterior is computed under ΛCDM assumptions, and the pending SCT-modified Boltzmann run (the same open task that arbitrates N_eff = 2.514) is what will show whether any residual Ω_b shift survives a model-consistent fit.
There is no need to invoke new physics between BBN and recombination; under SCT the physics between them is standard on both ends, which is exactly why the framework can let the gap be small, shared, and boring.
The clean kill: SCT's BBN identity requires the deuterium-based Ω_b to hold; if improved nuclear rates and 30-meter-class D/H measurements move the BBN value to the CMB number, the gap dissolves as nuclear systematics, while a BBN value drifting further at 3σ or more with shrinking errors would break the shared-jeopardy reading and demand a mechanism neither framework registers. Decisively: the completed SCT Boltzmann pipeline must reproduce the observed peak heights with the BBN Ω_b; failure to do so reopens the acoustic sector against SCT.