Overproduction ofHe4in strongly inhomogeneousΩb=1models of primordial nucleosynthesis

We have investigated inhomogeneous cosmic nucleosynthesis with extremely high contrasts (up to $R={10}^4$) in baryon-number density. Because those models develop sharp structure at the boundaries between the high- and low-density regions, accurate results require a high resolution. We have carried out such high-resolution numerical simulations and find that inhomogeneity in a closure density (${\Omega }_b=1\mathrm{}$) model cannot bring the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ yields down to fit observational constraints. Investigation of the detailed process dynamics shows that neutrons diffuse so rapidly during nucleosynthesis that they cannot effectively be separated from protons for long enough times to significantly suppress ${}_{}{}^4{}_{}{}^{}\mathrm{He}$.