Big Bang nucleosynthesis

Big Bang nucleosynthesis is the formation of light atomic nuclei during the first minutes of cosmic expansion. It produced nearly all primordial deuterium and helium, along with trace lithium.

Primordial nucleosynthesis: The formation of atomic nuclei in the early universe, including Big Bang nucleosynthesis. This broader process also encompasses any proposed nuclear production beyond the first minutes.
Primordial abundance: The proportion of chemical or isotopic species present before later astrophysical processing. Big Bang nucleosynthesis predicts these initial proportions for the light elements.
Big Bang: The hot, dense early state and subsequent expansion of the universe. Nucleosynthesis was a brief stage in the universe's evolution after the Big Bang.
Stellar nucleosynthesis: The production of new atomic nuclei through nuclear reactions inside stars. Stars produce many heavier elements, whereas Big Bang nucleosynthesis mainly made the lightest nuclei.
Neutron–proton ratio: The relative abundance of neutrons and protons in a system. Its value when nuclear reactions began largely determined the helium-4 abundance.
Deuterium: A hydrogen isotope whose nucleus contains one proton and one neutron. Its primordial abundance is especially sensitive to the cosmic density of ordinary matter.
Standard Model of particle physics: The theory describing known elementary particles and three of their fundamental interactions. Its particles and interactions govern the weak and nuclear reactions in standard predictions.
Triple-alpha process: A stellar reaction sequence that fuses three helium-4 nuclei into carbon-12. This later route builds carbon, a nucleus not produced in significant amounts by the Big Bang.
Deuterium bottleneck: The delay in early-universe nuclear reactions caused by deuterium's destruction by energetic photons. Nuclei could assemble efficiently only after expansion cooled photons enough for deuterium to survive.
Helium-4: A helium isotope with two protons and two neutrons in its nucleus. Its predicted primordial mass fraction provides a robust test of early-universe physics.

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