1803.02383
Constraining Density Fluctuations with Big Bang Nucleosynthesis in the era of precision cosmology
Barrow, Scherrer
Reexamine big bang nucleosynthesis with large-scale baryon density inhomogeneities when the length scale of the density fluctuations exceeds the neutron diffusion length (~1e7-8 cm at BBN), and the amplitude of the fluctuations is sufficiently small to prevent gravitational collapse. In this limit, the final light element abundances can be determined by simply mixing the abundances from regions with different baryon/photon ratios without interactions. Examine gaussian, lognormal, and gamma distributions for the baryon/photon ratio, eta. Find that the deuterium and lithium-7 abundances increase with the RMS fluctuation in eta, while the effect on He-4 is much smaller. Observational upper limits on the primordial D abundance constrain the RMS fluctuation in eta to be less than 10-20% of the mean value of eta. Larger RMS fluctuations can be obtained for less physically realistic distributions for eta. This provides with a new limit on the graininess of the early universe.
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