Inhomogeneous nucleosynthesis with neutron diffusion

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Abstract
It was recently proposed that a critical, i.e., closure, baryon density could be compatible with helium and deuterium observations, if this baryon density were suitably inhomogeneous during cosmic nucleosynthesis. In this scenario, neutrons would diffuse out of the high-density regions, which would lead to a nonstandard nucleosynthesis, with reduced helium production and improved deuterium survival, and could restore the agreement with observations, which is lost in homogeneous closure models. We have used a numerical model which shows that this proposal is not viable. Our model combines inhomogeneous nucleosynthesis and neutron diffusion self-consistently to study element formation in inhomogeneous situations in which diffusion is important. We find that the proposal fails: it is very difficult to lower the helium production significantly or to raise the final deuterium abundance, because the neutrons diffuse back to the high-density region once nucleosynthesis begins there.