In physical cosmology, the Planck epoch or Planck era is the earliest period of time in the history of the universe, from zero to approximately 10^-43 seconds (Planck time). While there is no proven theory that correctly describes the universe at this period, it is postulated that quantum effects of gravity dominated physical interactions due to the small scale of the universe. During this period, approximately 13.79 billion years ago, gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Inconceivably hot and dense, the state of the universe during the Planck epoch was unstable. As it expanded and cooled, the familiar manifestations of the fundamental forces arose through a process known as symmetry breaking.