Big Bang Nucleosynthesis - Sequence

Sequence

Big Bang nucleosynthesis began a few minutes after the big bang, when the universe had cooled down sufficiently to allow deuterium nuclei (one proton + one neutron) to survive disruption by high-energy photons.(This time is essentially independent of dark matter content, since the universe was highly radiation dominated until much later, and this controls the temperature/time relation). The relative abundances of protons and neutrons follow from simple thermodynamical arguments, combined with the way that the mean temperature of the universe changes over time (if the reactions needed to reach the thermodynamically favoured equilibrium values are too slow compared to the temperature change brought about by the expansion, abundances will remain at some specific non-equilibrium value). Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at this point. The answer is that there are about seven protons for every neutron at the beginning of nucleosynthesis. This fraction is in favour of protons initially, primarily because their lower mass with respect to the neutron favors their production. Free neutrons decay to protons with a half-life of about 15 minutes, but this time-scale is too long to affect the number of neutrons over the period in which BBN took place, primarily because most of the free neutrons had already been absorbed in the first 3 minutes of nucleogenesis—a time too short for a significant fraction of them to decay to protons.

One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies are very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe. Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, making what happens before irrelevant.

As the universe expands, it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. Before nucleosynthesis began, the temperature was high enough for many photons to have energy greater than the binding energy of deuterium; therefore any deuterium that is formed is immediately destroyed (a situation known as the deuterium bottleneck). Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium (at about T = 0.1 MeV), when there is a sudden burst of element formation. However, very shortly thereafter, at twenty minutes after the Big Bang, the universe becomes too cool for any further nuclear fusion and nucleosynthesis to occur. At this point, the elemental abundances are nearly fixed, and only change as some of the radioactive products of BBN (such as tritium) decay.