Planck epoch

The earliest proposed period of cosmic history, ending around one Planck time after the Big Bang. Quantum-gravity effects are expected to dominate, but no established theory describes it fully.

Quantum gravity: The study of physical theories that consistently describe gravity within quantum mechanics. A complete theory is needed to describe the Planck epoch’s expected quantum-gravitational regime.
Planck time: The unit of time defined by combining the gravitational constant, the speed of light, and Planck’s constant. It marks the approximate duration before which the Planck epoch is placed.
Big Bang: The expansion of the universe from an early hot, dense state into its present form. The Planck epoch is the earliest interval in the Big Bang account of cosmic history.
Planck era: An informal name for the earliest interval of cosmic history when Planck-scale physics is relevant. The phrase is often used interchangeably, though some accounts distinguish an era from a sharply bounded epoch.
Problem of time: The difficulty of defining time consistently in quantum theories of gravity. The usual notion of cosmic time may fail when spacetime itself is quantum.
Planck length: The characteristic length formed from the gravitational constant, the speed of light, and Planck’s constant. It indicates the scale at which quantum-gravity effects are expected to matter.
Big Bang singularity: The singular boundary obtained when classical expanding-universe equations are extrapolated backward. It is the classical limit associated with the epoch’s beginning, not a confirmed physical description.
Radiation-dominated era: A cosmic era in which radiation contributes more energy density than matter. Unlike the Planck epoch, its evolution can be treated with established cosmological physics.
Cosmic singularity: A boundary in classical cosmological models where physical quantities become undefined or diverge. Classical equations extrapolate the universe toward a singularity rather than describing its earliest state.
Planck energy: The characteristic energy scale formed from fundamental constants, associated with quantum gravity. It corresponds to the extreme energies expected in the earliest cosmic conditions.

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