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.