Big Bang Nucleosynthesis and
the Creation of the Lightest Elements

     When the Universe was less than one second old, it was so hot that protons and neutrons were unable to stick together to form nuclei. In a nucleus, the strong nuclear force holds these particles together. But in this extraordinarily hot plasma, the protons and neutrons moved so rapidly that they simply bounced off one another. Even if two had managed to stick together briefly, a collision with another proton or neutron would have smashed the two apart.
     By the time the Universe was a few seconds old, however, it had cooled to the point where some protons and neutrons moved slowly enough to allow the strong nuclear force to begin to bind them together. Three minutes later, deuterium, helium-3, helium-4, lithium-7 and beryllium-7 nuclei had been forged. Cosmologists call this event Big Bang nucleosynthesis.
     The processes that generate the helium-four are given in the following images loaded from Craig Tyler's webpage

deuterium production
Figure 1: A proton and a neutron fuse to form deuterium.

helium-3 pathway
Figure 2: The deuterium binds with a proton to yield helium-3.
The helium-3 then fuses with a neutron to produce helium-4.

tritium pathway
Figure 3: Alternatively, the deuterium binds with a neutron to yield tritium.
The tritium then fuses with a proton to produce helium-4.

     Depending of the fractional mass density of protons and neutrons, nuclear physics predicts fairly precisely the abundances of these light nuclei. By examining the quantities of hydrogen, deuterium, helium-3, helium-4 and lithium-7 in the Universe today, astrophysicists can determine the fractional mass density of protons and neutrons and whether the Friedmann-Robertson-Walker model is consistent. Inconsistencies are not found and the deduced proton-neutron fractional mass density is in rough agreement with other measurements made by astronomers. It turns out that proton-neutron mass density is a few percent of the critical mass density so that its contribution to W is less than about 8% and probably about 5%. Big Bang nucleosynthesis provides confirmation of the Standard Model of cosmology when the Universe was just a few minutes old.

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