Which light elements formed during the first minutes after the big bang?

During the first few minutes of the big bang, the universe underwent a process known as primordial nucleosynthesis, which resulted in the formation of the lightest chemical elements.1

As the universe expanded and cooled to a temperature of approximately ${10}^9$ K—roughly 1.5 minutes after the big bang—conditions became suitable for nuclear reactions to occur without the immediate destruction of the resulting nuclei. The primary elements formed during this period include:1

• Hydrogen: The most abundant element, existing primarily as individual protons.21
• Helium: Specifically the isotope helium-4, which was synthesized as neutrons were captured by protons to form deuterium and subsequently assembled into helium nuclei.1
• Trace elements: Small amounts of other light nuclei, such as lithium, beryllium, and boron, were also produced.2

Standard calculations indicate that this process resulted in a primordial composition of approximately 75 percent hydrogen and 25 percent helium by mass, a prediction that aligns closely with astronomical observations of the early universe.31