Seconds after the Big Bang, the hot, dense early universe underwent nuclear fusion, creating primordial hydrogen, helium, and trace amounts of deuterium and lithium. Nucleosynthesis predictions depend sensitively on baryon density and the expansion rate, allowing observational tests of cosmological models and constraints on dark matter.
You know from Hubble's law that the universe is expanding, and running that expansion backward implies an early universe that was much hotter and denser than today. Big Bang nucleosynthesis (BBN) describes the brief window — roughly from 10 seconds to 20 minutes after the Big Bang — when temperatures were high enough for nuclear fusion but low enough for newly formed nuclei to survive. This is the period that set the chemical starting conditions for the entire universe.
Before nucleosynthesis began, the universe was a soup of protons, neutrons, electrons, and photons in thermal equilibrium. As the temperature dropped below about 10 billion kelvin, neutrons and protons began fusing. The process started with the simplest reaction: a proton and neutron combining to form deuterium (heavy hydrogen). Deuterium then served as a stepping stone to helium-4, the most stable light nucleus, through a chain of reactions. The process also produced small amounts of helium-3, lithium-7, and trace beryllium. Crucially, the lack of stable nuclei at mass 5 and mass 8 created a bottleneck — there was no efficient pathway to build heavier elements. By about 20 minutes after the Big Bang, the universe had cooled too much for further fusion, and the process froze out.
The resulting abundances are remarkably specific: roughly 75% hydrogen and 25% helium by mass, with deuterium at about one part in 30,000 and lithium-7 at roughly one part in 10 billion. These predictions depend almost entirely on a single free parameter — the baryon-to-photon ratio, which measures how much ordinary matter exists relative to radiation. A higher baryon density means more collisions, faster deuterium processing, and slightly more helium. The sensitivity of deuterium abundance to baryon density makes it an especially precise cosmological probe: measuring primordial deuterium in distant gas clouds pins down the total amount of ordinary matter in the universe.
The agreement between BBN predictions and observations is one of the strongest pieces of evidence for the hot Big Bang model. The predicted helium abundance matches what astronomers measure in the most chemically pristine regions of the universe. The deuterium measurements from quasar absorption spectra independently confirm the baryon density derived from the cosmic microwave background. This concordance also constrains what the universe *cannot* be made of: since BBN accounts for all the ordinary (baryonic) matter, any additional mass needed to explain galaxy rotation curves and large-scale structure must be non-baryonic — providing independent evidence for dark matter from nuclear physics alone.