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Planck epoch
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). It is believed that, due to the extraordinarily small scale of the universe at that time, quantum effects of gravity dominated physical interactions. 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.
Modern cosmology now suggests that the Planck epoch may have inaugurated a period of unification, known as the grand unification epoch, and that symmetry breaking then quickly led to the era of cosmic inflation, the Inflationary epoch, during which the universe greatly expanded in scale over a very short period of time.
==Theoretical ideas==
As there presently exists no widely accepted framework for how to combine quantum mechanics with relativistic gravity, science is not currently able to make predictions about events occurring over intervals shorter than the Planck time or distances shorter than one Planck length, the distance light travels in one Planck time—about 1.616 × 10−35 meters. Without an understanding of quantum gravity, a theory unifying quantum mechanics and relativistic gravity, the physics of the Planck epoch are unclear, and the exact manner in which the fundamental forces were unified, and how they came to be separate entities, is still poorly understood. Three of the four forces have been successfully integrated in a common framework, but gravity remains problematic. If quantum effects are ignored, the universe starts from a singularity with an infinite density. This conclusion could change when quantum gravity is taken into account. String theory and loop quantum gravity are leading candidates for a theory of unification, which have yielded meaningful insights already, but work in noncommutative geometry and other fields also holds promise for our understanding of the very beginning.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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