Initial singularity

[1] The instant immediately following the initial singularity is part of the Planck epoch, the earliest period of time in the history of our 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 stage of the universe, and general relativity on its own fails to make accurate predictions.

[1][2] 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.

[3] The cosmic microwave background is evidence that the universe expanded from a very hot, dense state, possibly a singularity of infinite density.

[4] Various new models of what preceded and caused the Big Bang have been proposed as a result of the problems created by quantum mechanics.