Rothman and Matzner [54] 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 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 [44] 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 [54].
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 [45] 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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Physical and Relativistic Numerical Cosmology
Peter Anninos http://www.livingreviews.org/lrr-1998-2 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |