### 3.5 Nucleosynthesis

Observations of the light elements produced during Big Bang nucleosynthesis following the
quark/hadron phase transition (roughly 10^{–2} – 10^{2} s after the Big Bang) are in good agreement with the
standard model of our Universe (see Section 2.2). However, it is interesting to investigate other more
general models to assert the role of shear and curvature on the nucleosynthesis process, and place limits on
deviations from the standard model.
Rothman and Matzner [140] considered primordial nucleosynthesis in anisotropic cosmologies, solving
the strong reaction equations leading to ^{4}He. They find that the concentration of ^{4}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 [109] 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 ^{2}H, ^{3}He, ^{4}He and ^{7}Li as a
function of the baryon to photon ratio. They conclude that the effect of anisotropy on ^{2}H and
^{3}He is not significant, and the abundances of ^{4}He and ^{7}Li increase with anisotropy in accord
with [140].

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 ^{4}He compared to a homogeneous model. However, plane symmetric,
general relativistic simulations with neutron diffusion [110] 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.