3.5 Nucleosynthesis

Observations of the light elements produced during Big Bang nucleosynthesis following the quark/hadron phase transition (roughly 10–2 – 102 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 [140Jump To The Next Citation Point] considered primordial nucleosynthesis in anisotropic cosmologies, solving the strong reaction equations leading to 4He. They find that the concentration of 4He 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 A ≤ 7, and to demonstrate the effects of anisotropy on the cosmologically significant isotopes 2H, 3He, 4He and 7Li as a function of the baryon to photon ratio. They conclude that the effect of anisotropy on 2H and 3He is not significant, and the abundances of 4He and 7Li 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 4He 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.

  Go to previous page Go up Go to next page