Primordial deuterium abundance is one of the sharpest tests of Big Bang nucleosynthesis. Its predicted value depends on two nuclear reactions: proton-neutron radiative capture p(n,γ)d and deuteron photodisintegration d(p,γ)³He. The cross sections at cosmological energies must be known precisely, but experiments at these energies are difficult and extrapolations from higher energies introduce systematic uncertainties.
This paper uses a consistent two-body potential framework — the Malfliet-Tjon interaction — to calculate both reactions with E1 and M1 transitions. A single scaling parameter controlling low-energy scattering dynamics is constrained by the p(n,γ) data and propagated consistently to the d(p,γ). This consistency is the point: the same nuclear physics governs both reactions, so they shouldn't be fit independently.
The resulting abundance, D/H = 2.479 × 10⁻⁵, agrees with observations from metal-poor damped Lyman-α systems. But modest variations of the scaling parameter produce significant changes in the predicted ratio. The deuterium abundance is sensitive to small changes in the nuclear dynamics — which means it remains a powerful discriminator between nuclear models, not just between cosmologies.