The initial singularity is a singularity predicted by some models of the

Big Bang theory The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including the ...

to have existed before the Big Bang. The instant immediately following the initial singularity is part of the Planck epoch, the earliest period of time in the history of our universe.

Traditional models of our universe

The use of only

general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

to predict what happened in the beginnings of the universe has been heavily criticized, as

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

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. 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 In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...

-based model in which two branes, enormous membranes much larger than the universe, collided, creating mass and energy. The

cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

is evidence that the universe expanded from a very hot, dense state, possibly a singularity of infinite density.

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 Loop quantum gravity (LQG) is a theory of quantum gravity that incorporates matter of the Standard Model into the framework established for the intrinsic quantum gravity case. It is an attempt to develop a quantum theory of gravity based direc ...

, aims to explain the beginnings of the universe through a series of Big Bounces, in which quantum fluctuations cause the universe to expand. This use of loop quantum gravity also predicts a

cyclic model A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 ...

of universes, with a new universe being created after an old one is destroyed, each with different

physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that cannot be explained by a theory and therefore must be measured experimentally. It is distinct from a mathematical constant, which has a ...

s. These proposals have been criticized as inconsistent with the Borde–Guth–Vilenkin theorem, however their modifications with only one bounce (as opposed to cyclic series of bounces) circumvent this problem (particularly if the contracting phase is empty, i.e. compactified Milne, and (2+1)-dimensional, due to the inherent stabilizing rigidity of vacuum in this case). Another possibility based on

M-theory In physics, M-theory is a theory that unifies all Consistency, consistent versions of superstring theory. Edward Witten first conjectured the existence of such a theory at a string theory conference at the University of Southern California in 1 ...

and observations of the

(CMB) states that the universe is but one of many in a

multiverse The multiverse is the hypothetical set of all universes. Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describ ...

, and has budded off from another universe (e.g., one that macroscopically looks like static empty space) as a result of

quantum fluctuation In quantum physics, a quantum fluctuation (also known as a vacuum state fluctuation or vacuum fluctuation) is the temporary random change in the amount of energy in a point in space, as prescribed by Werner Heisenberg's uncertainty principle. ...

s such as quantum foam, as opposed to our universe being all that exists. Non-quantum classical models without singularities have also been proposed.

References

{{Portal bar, Physics, Astronomy, Stars, Outer space, Science Big Bang Physical cosmology Gravitational singularities