How Big Bang nucleosynthesis works
Big Bang nucleosynthesis (BBN) is a key component of the Big Bang model that explains how the light nuclei deuterium, helium-3, helium-4 and lithium-7 were created during the first few minutes of the universe. Big Bang theory states that the universe started out some 13.7 billion years ago in a very hot and dense state that has been expanding and cooling ever since. As described by Einstein's general theory of relativity, the rate of expansion depends on the amount of mass and energy the universe contains. Before BBN took place – when the universe was less than 1 s old – matter and energy existed in the form of a hot, dense gas of fundamental particles. As the universe cooled, particles with progressively less energy populated the universe so that by 1 s only protons, neutrons and lighter stable particles were present. Weak interactions between both protons and neutrons and the much lighter electrons, positrons and neutrinos maintained a thermal equilibrium that fixed the relative numbers of neutrons and protons at a certain value. After this, the temperature of the gas dropped to about 8 × 109 K, thereby preventing further weak interactions. From this time onwards, there remained one neutron (n) for every six protons (i.e. hydrogen nuclei, 1H).
During the next few minutes, nuclei formed. Deuterium nuclei (2H) were produced by collisions between protons and neutrons, and further nuclear collisions led to every neutron grabbing a proton to form the most tightly bound type of light nucleus: helium-4. This process was complete after about five minutes, when the universe became too cold for nuclear reactions to continue. Tiny amounts of deuterium, helium-3 and beryllium- 7 were produced as by-products, with the latter undergoing beta decay to form lithium-7. Almost all of the protons that were not incorporated into helium-4 nuclei remained as free particles, and this is why the universe is close to 25% helium and 75% hydrogen by mass everywhere we look. The other nuclei are less abundant by several orders of magnitude.
By measuring the intensity of atomic spectral lines in astrophysical objects, astronomers can infer the number of nuclei of a given type per hydrogen nucleus. These nuclear abundances produced during BBN depend on the density of matter (or baryon density) during those first few minutes, which can be related directly to the baryon density we see today. Any effect that changes the early thermal evolution of the universe or the interactions between the nuclei would also leave traces in the abundances, which means BBN provides an important probe of the early universe.
If we assume that only the particles and forces contained in the Standard Model of particle physics were present during BBN, then the baryon density measured by NASA's WMAP mission (and corroborated by the deuterium abundance) determines the initial chemical composition of the universe: mostly hydrogen, with roughly 0.08 helium-4 atoms, 10–5 deuterium atoms, 10–5 helium-3 atoms and 10–10 lithium atoms per hydrogen atom, but no detectable amount of anything else. All the other elements in the cosmos were synthesized much later inside stars or in cosmic-ray collisions.