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), 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 where SCT differs: the acoustic physics that converts peak heights into Ω_b assumes the ΛCDM sound speed, while SCT's coherent acoustic regime modifies c_s² through the baryon-loading structure of the CAR relation, with R_b in the numerator rather than the denominator. Fitting ΛCDM acoustics to a CAR sky biases the inferred baryon density, and the direction of the small bias is exactly what the gap records: the CMB chain reading high relative to the nuclear truth.
In epistemic honesty, the quantitative re-derivation of Ω_b under the full CAR acoustic chain is flagged as outstanding computational work (the SCT-modified Boltzmann pipeline), so SCT's claim here is directional and structural rather than numerically closed: BBN clean, CMB inference model-laden, gap small and chain-dependent.
This is the same CAR acoustics behind the sound-horizon reconciliation and the derived baryon-to-photon ratio. There is no need to invoke new physics between BBN and recombination; the physics between them is the inference chain itself.
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 was nuclear systematics and the chain-bias reading is unnecessary, while a BBN value drifting further with shrinking errors sharpens it. Decisively: the completed SCT Boltzmann pipeline must reproduce the observed peak heights with the BBN Ω_b; failure to do so falsifies the CAR acoustic account of the gap.