abstract: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 during the early phases of the universe. Primordial nucleosynthesis is believed by most cosmologists to have taken place between approximately 10 seconds until 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as isotope He-4, along with small amounts of deuterium (H-2 or D), the helium isotope He-3, and a very small amount of the lithium isotope Li-7.