Glossary term: Big Bang Nucleosynthesis
Description: Nucleosynthesis refers to processes where heavier atomic nuclei are formed from lighter ones via nuclear fusion. Big Bang Nucleosynthesis (BBN), sometimes called Primordial Nucleosynthesis, is a brief instance of nucleosynthesis during our Universe's hot and dense Big Bang phase, nearly 14 billion years ago. According to our current cosmological models, BBN began in the first few seconds of the early Universe and lasted for a few minutes. The timing is directly linked to the rate at which the early Universe was expanding and cooling down. Earlier than those first seconds, the Universe had been too hot for nuclei more complex than hydrogen to survive. At the end of those few minutes, the Universe was not hot and dense enough for nuclear fusion to continue. After the end of BBN, about 25% by mass of atomic nuclei was in the form of helium-4 (a particularly stable isotope of helium), and 75% was hydrogen. Closer inspection shows that there must also have been small traces of deuterium (a hydrogen isotope), helium-3 (another helium isotope), and isotopes of lithium: lithium-6 and lithium-7. The amounts of each element produced during BBN depend only on basic cosmological parameters, and thus constitute a prediction of the cosmological models. Given that over the following nearly 14 billion years, significant additional nucleosynthesis has taken place, notably in the interiors of stars, it is a challenge to try and estimate initial element abundances from present-day observational data. For helium-4, helium-3, deuterium, and lithium-6, the cosmological BBN predictions agree very well with the reconstructions from observations. For lithium-7, there is a marked difference, but it is not clear at this time whether that indicates a problem with our understanding of BBN or a problem with the attempts to estimate the initial lithium-7 abundance.
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