D/H Ratio

The primordial deuterium-to-hydrogen ratio D/H is the most precise BBN observable, measured in damped Lyman-alpha systems at high redshift where the gas metallicity is low enough that stellar destruction of deuterium has been minimal. The current best value from multiple absorption systems is D/H = (2.527 ± 0.030) × 10⁻⁵, which when combined with the standard BBN calculation yields a baryon density Ω_b h² = 0.02166 ± 0.00015. This is lower by ~1.5 sigma than the Planck CMB measurement of Ω_b h² = 0.02242 ± 0.00014, constituting the mild but persistent BBN-CMB baryon density tension. The D/H ratio is exquisitely sensitive to η — a 1% change in η produces a ~5% change in D/H — making this the sharpest probe of any discrepancy between the BBN and CMB epochs.

Successive Collision Theory explains the D/H versus CMB baryon density discrepancy through the pre-existing matter contribution to the baryon census at each epoch. During BBN at z ~ 10⁸, the effective baryon density available for nucleosynthesis was set by the baryons in the thermalized photon-baryon plasma. However, a fraction of the total baryon number in the debris field was locked in pre-existing compact objects — neutron stars, white dwarfs, and stellar remnants from prior stellar generations in the colliding pockets — that were not fully dissociated and thermalized by the collision energy. These compact-object baryons did not participate in BBN reactions but do contribute to the total baryon density measured from the CMB power spectrum at z ~ 1100, which is sensitive to all gravitating baryons regardless of their state. The BBN-inferred η is therefore lower than the CMB-inferred Ω_b because the CMB counts compact-object baryons that BBN missed.

The magnitude of this offset in SCT is consistent with the observed ~1.5-sigma discrepancy. As a fraction of the total baryon budget, the pre-existing compact object baryons represent a few percent of the total, which corresponds to a shift in the inferred baryon density of order the observed discrepancy given the sensitivity of D/H to η. SCT further predicts that the D/H values in individual absorption systems should show a small intrinsic scatter beyond observational uncertainties, reflecting spatial variation in the mixing ratio between thermalized plasma baryons and regions where compact-object material was subsequently ablated and mixed into the intergalactic medium. This scatter, at the level of a few percent of the mean D/H, is consistent with the current dataset and will be distinguishable from a purely observational scatter with the next generation of high-resolution spectrographs on 30-meter class telescopes.