Chronology of the universe

The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The history begins 13.863 billion years ago when the universe was extremely dense and hot, before any matter formed. The earliest stage in physical models is a vast expansion of space in a tiny fraction of a microsecond, followed by a slower expansion and the creation of matter and antimatter particles. These particles almost completely annihilate each other leaving energy and a small remainder of matter in the form of a quarks-gluon plasma. As the expansion continues, this plasma condenses into a sea of protons and neutrons reacting with neutrinos. By the one second mark the temperature has dropped enough that neutrinos can no longer react; this enormous number of primordial neutrinos continue to stream across the universe to this day.

By 6s the expansion cooling allows electrons and positrons to annihilate, creating another pulse of energy. Then the nuclei of the first elements form, primarily hydrogen and helium with a bit of lithium. After rapid changes in the 3 minutes of expansion, the universe cools for thousands of years to reach a temperature where atoms are stable. An intense set of photons become free; they had been absorbed and emitted by the plasma but they interact comparatively less with the atoms. These photons are visible today as the background temperature of the universe.

The next phase of the universe involved the gradual gravitational collapse of hydrogen gas. Eventually the force of gravity compressed and heated the gas, igniting nuclear fusion to create the first stars. Collections of stars formed galaxies and the larger structures we see today.

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