Big Bang nucleosynthesis

In physical cosmology, Big Bang nucleosynthesis (also known as primordial nucleosynthesis, and abbreviated as BBN)^[1] is 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. This type of nucleosynthesis is thought by most cosmologists to have occurred from 10 seconds to 20 minutes after the Big Bang.^[2] It is thought to be responsible for the formation of most of the universe's helium (as isotope helium-4 (⁴He)), along with small fractions of the hydrogen isotope deuterium (²H or D), the helium isotope helium-3 (³He), and a very small fraction of the lithium isotope lithium-7 (⁷Li). In addition to these stable nuclei, two unstable or radioactive isotopes were produced: the heavy hydrogen isotope tritium (³H or T) and the beryllium isotope beryllium-7 (⁷Be). These unstable isotopes later decayed into ³He and ⁷Li, respectively, as above.

Elements heavier than lithium are thought to have been created later in the life of the Universe by stellar nucleosynthesis, through the formation, evolution and death of stars.