Nucleosynthesis in the Presence of Primordial Isocurvature Baryon Fluctuations

Abstract

We study big bang nucleosynthesis in the presence of large mass-scale, non-linear 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 surviving light element ($^2$H, $^3$He, $^4$He, $^7$Li) abundance yields for a variety of stochastic fluctuation spectra. In general, we find that spectra with significant power in fluctuations on length scales below that of the local baryon Jeans mass produce nucleosynthesis yields which are in conflict with observationally inferred primordial abundances. However, when this small scale structure is absent or suppressed, and the collapse efficiency of overdense regions is high, there exists a range of fluctuation spectral characteristics which meet all primordial abundance constraints. In such models abundance constraints can be met even when the pre-collapse baryonic fraction of the closure density is $Ω_b\approx 0.2h^{-2}$($h$ is the Hubble parameter in units of 100 km\ s$^{-1}$Mpc$^{-1}$). Nucleosynthesis in these models is characterized by high $^2$H/H and low $^4$He mass fraction relative to a homogeneous big bang at a given value of $Ω_bh^2$. A potentially observable signature of these models is the production of intrinsic primordial abundance variations on baryon mass-scales up to $10^{10}M_{\odot}-10^{12}_{\odot}$.