Big Bang Nucleosynthesis

Understand

As the early universe expands and cools the rates of the weak interactions that keep neutrinos in thermal equilibrium with the matter and the related rates of the reactions that inter-convert neutrons and protons decrease.

• Eventually, these rates fall below the expansion rate -- they freeze out.
• Likewise, the rates of the strong and electromagnetic nuclear reactions that build up and tear down nuclei, though fast enough to maintain equilibrium early on, slow down and ultimately lead to freeze out.
• Together these freeze out processes comprise the epoch of Big Bang Nucleosynthesis (BBN).