In
physical cosmology,
Big Bang nucleosynthesis (abbreviated
BBN, also known as
primordial nucleosynthesis) refers to the production of nuclei other than those of the lightest isotope of
hydrogen (
hydrogen-1,
1H, having a single
proton as a nucleus) during the early phases of the
universe. Primordial
nucleosynthesis is believed by most cosmologists to have taken place from 10 seconds to 20 minutes after the
Big Bang, and is calculated to be responsible for the formation of most of the universe's
helium as the
isotope helium-4 (
4He), along with small amounts of the hydrogen isotope
deuterium (
2H or D), the
helium isotope
helium-3 (
3He), and a very small amount of the
lithium isotope
lithium-7 (
7Li). In addition to these stable nuclei, two unstable or
radioactive isotopes were also produced: the heavy hydrogen isotope
tritium (
3H or T); and the
beryllium isotope
beryllium-7 (
7Be); but these unstable isotopes later decayed into
3He and
7Li, as above.