The First Three Minutes
Within minutes of the Big Bang, the universe was a hot, dense plasma of protons, neutrons, electrons, and photons. As temperature dropped below ~10⁹ K, nuclear reactions began fusing these particles into the lightest elements. This epoch of Big Bang nucleosynthesis (BBN) lasted roughly 20 minutes and determined the primordial chemical composition of the cosmos — about 75% hydrogen, 25% helium-4, with trace amounts of deuterium, helium-3, and lithium-7.
The Deuterium Bottleneck
Nucleosynthesis cannot proceed until deuterium (the first stepping stone to heavier nuclei) survives photodisintegration. Despite deuterium's low binding energy (2.22 MeV), the enormous photon-to-baryon ratio (~10¹⁰) means that even at temperatures well below this threshold, the high-energy tail of the photon distribution destroys deuterium efficiently. Only when T drops to ~0.8×10⁹ K does deuterium accumulate — this 'bottleneck' delays nucleosynthesis and allows more neutrons to decay, reducing the final helium yield.
Abundance Predictions
BBN predictions depend on a single free parameter: the baryon-to-photon ratio η, now precisely measured from the CMB. With η = 6.1×10⁻¹⁰, the theory predicts Yp ≈ 0.247, D/H ≈ 2.5×10⁻⁵, and ³He/H ≈ 10⁻⁵. These predictions match observations beautifully for hydrogen, helium, and deuterium — but lithium-7 is overpredicted by a persistent factor of ~3, a discrepancy known as the cosmological lithium problem.
A Window into Particle Physics
BBN's sensitivity to the expansion rate makes it a powerful particle-physics probe. Extra light species (like a fourth neutrino) would speed expansion, increasing the freeze-out neutron fraction and overproducing helium. The agreement between predicted and observed abundances constrains the effective number of neutrino species to Neff = 2.99 ± 0.17, confirming three light neutrino flavors and placing stringent limits on exotic particles and forces in the early universe.