In physical cosmology, the Big Bang nucleosynthesis was an event that happened during the Big Bang. It made elements heavier than hydrogen when the Universe was young. Most cosmologists believe that it happened roughly 10 seconds to 20 minutes after the Big Bang. It is also calculated to have made most of the helium in the Universe and a very small amount of lithium. It also made two radioactive isotopes. They are tritium and beryllium-7.