The early universe
The early universe , lasting around 377,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter , so most of it quickly annihilates, leaving a small excess of matter in the universe. At about one second, neutrinos decouple ; these neutrinos form the cosmic neutrino background . If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge – including protons and neutrons – and from about 3 minutes, conditions are suitable for nucleosynthesis : around 25% of the protons and all the neutrons fuse into heavier elements , mainly helium-4 . By 20 minutes, the universe is no longer hot enough for fusion, but far too hot for neutral atoms to exist or photons to travel far. It is ...