In cosmology, the Planck epoch is the earliest period of time in the history of the universe, from zero to 10^-43 seconds (one Planck time), during which all four fundamental forces were unified and elementary particles did not exist.

Standard quantum mechanics says that it is meaningless to speak of durations shorter than Planck time or distances shorter than one Planck length, how far light travels in one Planck time—about 1.6 * 10^-35 meters. Accordingly, the history of the universe should be measured, not from zero, but from one Planck time. Likewise, the volume of the universe would start at one Planck length in diameter rather than at zero, and, thus, there was never a singularity of infinite density. As yet (2005), there is no generally accepted theory that unifies quantum mechanics and relativistic gravity. A future complete theory may or may not allow more to be said about the Planck epoch.

See also

• Big bang
• Timeline of cosmology
• Unified field theory
• Planck particle