Element Diffusion During Cosmological Structure Formation

We consider element diffusion during the linear growth of structure in the intergalactic medium (IGM) before the redshift of reionization. The elements produced during big bang nucleosynthesis (such as D, He-4 or Li-7) condensed in collapsing regions more readily than hydrogen. Diffusion was most effective when the IGM was neutral since the collisional cross-section between neutral species is smaller than the corresponding Coulomb cross-section in an ionized gas. We find that diffusion led to small deviations in element abundances of up to 0.1%. Abundance measurements to this level of precision in low-metallicity environments could provide information about the thermal and ionization history of the primordial gas during the early formation of structure in the IGM.