Nucleosynthesis in the presence of primordial isocurvature baryon fluctuations

We study big bang nucleosynthesis in the presence of large mass scale, nonlinear entropy fluctuations. Overdense regions, with masses above the local baryon Jeans mass, are expected to collapse and form condensed objects. Surviving nucleosynthesis products therefore tend to originate from underdense regions. We compute expected survival light-element (2H, 3He, 4He, and 7Li) abundance yields for a variety of stochastic fluctuation spectra. In general, we find that spectra with significant power in fluctuations on mass scales below that of the local baryon Jeans mass (∼105 Mcircle-dot) produce nucleosynthesis yields which are in conflict with observationally inferred primordial abundances. Only when this small-scale structure is absent or suppressed, and the collapse efficiency of overdense regions is high, can there exist a range of fluctuation spectral characteristics which conceivably could meet all primordial abundance constraints. Even then, the primordial abundance of 7Li would have to be larger than 7Li/H≅3x10-10. In such models, abundance constraints could be met even when the precollapse baryonic fraction of the closure density is Ωb≅0.2 h-2 (h is the Hubble parameter in units of 100 kms-1 Mpc-1). Nucleosynthesis in these models is characterized by high 2H/H and low 4He mass fraction relative to a homogeneous big bang at a given value of Ωb h2. A potentially observable signature of these models is the production of intrinsic primordial abundance variations on baryon mass scales up to 1010-1012 Mcircle-dot. copyright 1995 The American Astronomical Society