How did primordial nucleosynthesis produce helium in the early universe?

Primordial nucleosynthesis occurred when the early universe was approximately 1.5 minutes old and the temperature had dropped to ${10}^9$ K. At this stage, the environment became cool enough for neutrons to be captured by protons, forming deuterium nuclei. Previously, any deuterium formed would have been immediately broken apart by the intense heat.1

Once deuterium was stable, a rapid chain of nuclear reactions ensued, combining these deuterium nuclei and remaining neutrons with protons to assemble helium nuclei. While some neutrons decayed into protons and electrons over time, the majority were incorporated into helium. Sophisticated calculations accounting for these concurrent processes predict a primordial helium-mass fraction of approximately 25 percent, a value that aligns closely with astronomical observations and serves as a key success of the hot big bang theory.1