UC San Diego

We show that a class of inhomogeneous big bang nucleosynthesis models exist which yield light-element abundances in agreement with observational constraints for baryon-to-photon ratios significantly smaller than those inferred from standard homogeneous big bang nucleosynthesis (HBBN). These inhomogeneous nucleosynthesis models are characterized by a bimodal distribution of baryons in which some regions have a local baryon-to-photon ratio η ≈ 3 × 10-10, while the remaining regions are baryon depleted. HBBN scenarios with primordial (2H + 3He)/H ≲ 9 × 10-5 necessarily require that most baryons be in a dark or nonluminous form, although new observations of a possible high deuterium abundance in Lyα clouds may relax this requirement somewhat. The models described here present another way to relax this requirement and can even eliminate any lower bound on the baryon-to-photon ratio.