Cosmic time
The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The metric expansion of space is estimated to have begun 13.8 billion years ago. The time since the Big Bang is also known as cosmic time. For the purposes of this summary, it is convenient to divide the chronology of the universe into four parts:
The Planck epoch is an era in traditional (non-inflationary) big bang cosmology wherein the temperature was so high that the four fundamental forces—electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction—were one fundamental force. Little is understood about physics at this temperature; different hypotheses propose different scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on the theory of general relativity, which is thought to break down for this epoch due to quantum effects.
In inflationary cosmology, times before the end of inflation (roughly 10−32 second after the Big Bang) does not follow the traditional big bang timeline. Models attempting to formulate processes of the Planck epoch are speculative proposals for "New Physics". Examples include the Hartle–Hawking initial state, string landscape, string gas cosmology, and the ekpyrotic universe.
As the universe expanded and cooled, it crossed transition temperatures at which forces separate from each other. These can be regarded as phase transitions much like condensation and freezing phase transitions of ordinary matter. The grand unification epoch began when gravitation separated from the gauge forces. The non-gravitational physics of this epoch would be described by a so-called grand unified theory (GUT). The grand unification epoch ended when the GUT forces further separate into the strong and electroweak forces.
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