In physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. It is believed to be responsible for the formation of hydrogen (H-1 or simply H), its isotope deuterium (H-2 or D), the helium isotopes He-3 and He-4, and the lithium isotope Li-7. Big Bang nucleosynthesis begins about one minute after the Big Bang, when the universe has cooled enough to form stable protons and neutrons, after baryogenesis. From simple thermodynamical arguments, one can calculate the fraction of protons and neutrons based on the temperature at this point. This fraction is in favour of protons, because the higher mass of the neutron results in a spontaneous decay of neutrons to protons with a half-life of about 15 minutes.