Big Bang nucleosynthesis

The first nuclei

Big Bang nucleosynthesis, or BBN, refers to the formation of light nuclei during the first few minutes of cosmic history. The early universe was hot and dense enough for nuclear reactions, but expansion and cooling limited how far fusion could proceed.

BBN mainly produced hydrogen, helium-4, deuterium, helium-3, and lithium-7. Heavier elements were made later in stars and stellar explosions.

Neutrons and protons

At very high temperatures, neutrons and protons interconverted through weak interactions such as

u_eleftrightarrow p+e^-$$ and related reactions. As the universe cooled, these reactions froze out, leaving a neutron-to-proton ratio of roughly $$rac{n}{p}simrac{1}{6}$$ before neutron decay further reduced it. ## Deuterium bottleneck To build helium, the universe first needed deuterium: $$p+n ightarrow D+gamma$$ But at early times, high-energy photons destroyed deuterium as soon as it formed. Only after the universe cooled enough could deuterium survive. This delay is called the deuterium bottleneck. Once deuterium survived, nuclear reactions rapidly built helium-4. ## Helium abundance Almost all surviving neutrons ended up in helium-4 because it is very tightly bound. A simple estimate of the helium mass fraction uses the neutron-proton ratio. If $n/papprox1/7$ by the time nucleosynthesis proceeds, then the helium mass fraction is roughly $$Y_papproxrac{2(n/p)}{1+n/p}approx0.25$$ This means about 25 percent of ordinary matter by mass became helium-4. ## Why not heavier elements? BBN did not produce many heavy elements because the universe expanded and cooled quickly, and there are no stable nuclei with mass numbers 5 or 8. These gaps block easy buildup to heavier nuclei. Stars later overcome these limitations through long timescales, high densities, and special resonances such as the triple-alpha process. ## Baryon density The predicted abundances depend on the density of ordinary matter, or baryons. Deuterium is especially sensitive to baryon density. More baryons allow more efficient burning of deuterium into helium, leaving less deuterium behind. Comparing predicted and observed abundances provides a measurement of the cosmic baryon density. ## Agreement with observations The observed abundances of helium-4 and deuterium agree well with BBN predictions and with baryon density inferred from the cosmic microwave background. Lithium-7, however, shows a discrepancy between prediction and some observations, known as the lithium problem. ## The big idea Big Bang nucleosynthesis explains the primordial abundance of light elements. In the first few minutes, the universe made mostly hydrogen and helium, with traces of deuterium, helium-3, and lithium. BBN is a major success of the Big Bang model and a sensitive probe of ordinary matter density.