- 
seconds after the Big Bang) are in good agreement
with the standard
model of our Universe (see § 2.2).
However, it is interesting to investigate other
more general models to assert the role of shear
and curvature on the nucleosynthesis process.
Rothman and Matzner [108
]
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 [88] 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 [108].
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 [89] 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.
|
Computational Cosmology: from the Early Universe to the Large Scale Structure Peter Anninos http://www.livingreviews.org/lrr-2001-2 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |