The initial singularity was a gravitational singularity of seemingly infinite density thought to have contained all the mass and space-time of the Universe before quantum fluctuations caused it to rapidly expand in the Big Bang and subsequent inflation, creating the present-day Universe. The instant immediately following the initial singularity is part of the Planck epoch, the earliest period of time in the history of the universe.
Traditional models of the Universe
The use of only general relativity to predict what happened in the beginnings of the Universe has been heavily criticized, as quantum mechanics becomes a significant factor in the high-energy environment of the earliest Universe, and general relativity on its own fails to make accurate predictions. In response to the inaccuracy of considering only general relativity, as in the traditional model of the Big Bang, alternative theoretical formulations for the beginning of the Universe have been proposed, including a string theory-based model in which two branes, enormous membranes much larger than the Universe, collided, creating mass and energy.
It is impossible to see the singularity or the actual Big Bang itself, as time and space did not exist inside the singularity and, therefore, there would be no way to transmit any radiation from before the Big Bang to the present day. Although there is no direct evidence for a singularity of infinite density, the cosmic microwave background is evidence that the universe expanded from a very hot, dense state.
Alternatives to the singularity
Various new models of what preceded and caused the Big Bang have been proposed as a result of the problems created by quantum mechanics. One model, using loop quantum gravity, aims to explain the beginnings of the Universe through a series of Big Bounces, in which quantum fluctuations cause the Universe to expand. This procreation also predicts a cyclic model of universes, with a new universe being created after an old one is destroyed, each with different physical constants. Another procreation based on M-theory and observations of the cosmic microwave background (CMB), states that the Universe is but one of many in a multiverse, and has budded off from another universe as a result of quantum fluctuations, as opposed to our Universe being all that exists.
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