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In physical cosmology, the Planck epoch (or Planck era), named after Max Planck, is the earliest period of time in the history of the universe, from zero to approximately 10-43 seconds (one Planck time), during which quantum effects of gravity were significant. One could also say that it is the earliest moment in time, as the Planck time is perhaps the shortest possible interval of time, and the Planck epoch lasted only this brief instant. At this point approximately 1.37×1010 years ago the force of gravity is believed to have been as strong as the other fundamental forces, which hints at the possibility that all the forces were unified. Inconceivably hot and dense, the state of the universe during the Planck epoch was unstable or transitory, tending to evolve and giving rise to the familiar manifestations of the fundamental forces through a process known as symmetry breaking. Modern cosmology now suggests that the Planck epoch may have inaugurated a period of unification or 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.
[edit] 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.
[edit] Experiments exploring this time
Experimental data casting light on this cosmological epoch has been scant or non-existent until now, but recent results from the WMAP probe have allowed scientists to test hypotheses about the universe's first trillionth of a second (although the cosmic microwave background radiation observed by WMAP originated when the universe was already several hundred thousand years old). Although this interval is still orders of magnitude longer than the Planck time, other experiments currently coming online including the IceCube neutrino detector and the Planck Surveyor probe, promise to push back our 'cosmic clock' further to reveal quite a bit more about the very first moments of our universe's history, hopefully giving us some insight into the Planck epoch itself. Of course, data from particle accelerators provides meaningful insight into the early universe as well. Experiments with the Relativistic Heavy Ion Collider have allowed physicists to determine that the Quark-gluon plasma (an early phase of matter) behaved more like a liquid than a gas, and the Large Hadron Collider soon to come online at CERN will allow us to probe still earlier phases of matter, but no accelerator (current or planned) will allow us to probe the Planck scale directly. However, the more we understand about how matter forms, the more precisely we will be able to interpret what we learn from astrophysical data, and from other sources.
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