Primordial nucleosynthesis

Primordial nucleosynthesis is the formation of light atomic nuclei during the universe’s first minutes. It produced most cosmic hydrogen and helium, along with trace deuterium, helium-3, and lithium-7.

Big Bang nucleosynthesis timeline: A sequence of early-universe stages in which nuclear reactions formed light elements as the cosmos cooled. Its changing temperature explains when nuclear reactions began, stalled, and ended.
Big Bang: The expansion of the universe from an early hot, dense state into the cosmos observed today. Primordial nucleosynthesis was an early stage in the universe’s evolution after the Big Bang.
Big Bang nucleosynthesis constraints on cosmology: Limits on cosmological parameters inferred by comparing predicted and observed primordial light-element abundances. Agreement between predictions and measurements tests the standard account of the early universe.
Stellar nucleosynthesis: The production of new atomic nuclei through nuclear reactions inside stars and their remnants. It explains later element production, unlike the early-universe origin of primordial light nuclei.
Neutron–proton ratio: The relative abundance of neutrons and protons in a system. The ratio at the onset of nuclear reactions largely fixed the resulting helium abundance.
Baryon: A composite particle made of three quarks, including protons and neutrons. Baryons supplied the matter from which primordial atomic nuclei formed.
Baryon-to-photon ratio: The ratio of baryons to photons in the universe, a measure of cosmic baryon abundance. Light-element yields depend sensitively on this ratio and thereby constrain ordinary matter density.
Triple-alpha process: A stellar nuclear reaction sequence that fuses three helium-4 nuclei into carbon-12. Stars can build carbon from helium, a pathway unavailable during primordial nucleosynthesis.
Deuterium bottleneck: The delay in light-element formation caused by deuterium’s photodissociation in the hot early universe. Its end opened the way for rapid assembly of heavier light nuclei.
Thermal equilibrium: A state in which interacting components share a common temperature and detailed processes balance. Early equilibrium governed particle abundances before expansion disrupted weak interactions.

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