Omega_B from Big Bang Nucleosynthesis vs CMB

Big Bang nucleosynthesis provides an independent measure of the cosmic baryon density Ω_b through the sensitive dependence of light element abundances — particularly deuterium and helium-4 — on the baryon-to-photon ratio η. The primordial deuterium abundance measured in high-redshift, low-metallicity quasar absorption systems yields Ω_b h² = 0.02166 ± 0.00015, while the Planck CMB power spectrum analysis gives Ω_b h² = 0.02242 ± 0.00014. The two values are consistent at roughly the two-sigma level, but the mild tension has motivated scrutiny of whether systematic effects in either measurement are responsible. If the discrepancy is physical rather than systematic, it implies either a modification to BBN physics, a non-standard photon entropy, or a difference between the baryon density at the BBN epoch (z ~ 10⁸) and at decoupling (z ~ 1100).

Successive Collision Theory addresses this mild tension through the thermalization history of the collision debris. In SCT, the photon-baryon plasma that produced the CMB formed from the thermalized kinetic energy of the two colliding pockets, which deposited energy into the overlap volume instantaneously across the collision front. This instantaneous energy deposition set the initial baryon-to-photon ratio η in the thermalized plasma. However, the pre-existing matter from the pockets — stars, compact objects, and processed gas — contributed baryons to the thermalized medium without contributing an equal share to the photon entropy, because their internal energy states were partially thermalized through mechanisms different from the primary photon-gas coupling. The effective η available to BBN nucleosynthesis therefore differs slightly from the η that Planck infers from the CMB power spectrum, which samples the photon-baryon ratio at the epoch of decoupling after all thermalization had completed.

The SCT prediction is that the BBN η is slightly lower than the CMB η because a fraction of the baryons that are counted in the CMB power spectrum were locked in pre-existing compact objects during the BBN epoch and became available to the photon-baryon plasma only through subsequent stellar and gravitational processes. This shifts the BBN-inferred Ω_b downward relative to the CMB value by an amount proportional to the fraction of baryons in pre-existing compact form at z ~ 10⁸. The observed ~1.5-sigma discrepancy between BBN and CMB baryon densities is therefore a direct measure of this pre-existing compact object baryon fraction in the debris field — a physically meaningful signal rather than a statistical fluctuation, and one that is consistent with the pre-existing matter populations required to explain early JWST galaxies and SMBHs.