Rothman and Matzner [48] considered primordial nucleosynthesis in anisotropic
cosmologies, solving the strong reaction equations leading to
He. They find that the concentration of
He increases with increasing shear; this is due to time scale
effects and the competition between dissipation and enhanced
reaction rates from photon heating and neutrino blue shifts.
Their results have been used to place a limit on anisotropy at
the epoch of nucleosynthesis. Kurki-Suonio and Matzner [38] extended this work to include 30 strong 2-particle reactions
involving nuclei with mass numbers
, and to demonstrate the effects of anisotropy on the
cosmologically significant isotopes
H,
He,
He and
Li as a function of the baryon to photon ratio. They conclude
that the effect of anisotropy on
H and
He is not significant, and the abundances of
He and
Li increase with anisotropy in accord with [48].
Furthermore, it is possible that neutron diffusion, the
process whereby neutrons diffuse out from regions of very high
baryon density just before nucleosynthesis, can affect the
neutron to proton ratio in such a way as to enhance deuterium and
reduce
He compared to a homogeneous model. However, plane symmetric,
general relativistic simulations with neutron diffusion [39] show that the neutrons diffuse back into the high density
regions once nucleosynthesis begins there - thereby wiping out
the effect. As a result, although inhomogeneities influence the
element abundances, they do so at a much smaller degree then
previously speculated. The numerical simulations also demonstrate
that, because of the back diffusion, a cosmological model with a
critical baryon density cannot be made consistent with helium and
deuterium observations, even with substantial baryon
inhomogeneities and the anticipated neutron diffusion effect.
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Computational Cosmology: from the Early Universe to the
Large Scale Structure
Peter Anninos http://www.livingreviews.org/lrr-1998-9 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |