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 helium-four (4He) are:

p + n → 2H + γ
Step 1 (deuterium production): A proton and a neutron fuse to form deuterium (2H)

2H + 2H → 3He + n    or    2H + p → 3He + γ
3He + 2H → 4He + p
Step 2a (helium-3 pathway): The deuterium binds with itself or with a proton to yield helium-3 (3He)
The helium-3 then fuses with deuterium to produce helium-4

2H + 2H → 3H + p
3H + 2H → 4He + n
Step 2b (tritium pathway): Alternatively, the deuterium binds with itself to yield tritium (3H)
The tritium and deuterium fuses to produce helium-4

(In these diagrams, γ represents a gamma ray photon.)

     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 Ω 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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