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Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics.It deals with environments in which neither gravitational nor quantum effects can be ignored, [1] such as in the vicinity of black holes or similar compact astrophysical objects, as well as in the early stages of the universe moments after the Big Bang.
Therefore, a theory is needed that integrates relativity theory and quantum theory. [3] Such an approach is attempted for instance with loop quantum cosmology, loop quantum gravity, string theory and causal set theory. [4] In quantum cosmology, the universe is treated as a wave function instead of classical spacetime. [5]
The anti-de Sitter/conformal field theory correspondence, sometimes called Maldacena duality (after ref. [11]) or gauge/gravity duality, is a conjectured relationship between two kinds of physical theories. On one side are anti-de Sitter spaces (AdS) which are used in theories of quantum gravity, formulated in terms of string theory or M-theory.
M-theory is a theory of quantum gravity; and as all others it has not gained experimental evidence that would confirm its validity. [1] It also does not single out our observable universe as being special, and so does not aim to predict from first principles everything we can measure about it.
Since the energy scale of inflation and the Planck scale are relatively close, some of the quantum fluctuations that have made up the structure in our universe were smaller than the Planck length before inflation. Therefore, there ought to be corrections from Planck-scale physics, in particular the unknown quantum theory of gravity.
A theory of quantum gravity is needed in order to reconcile these differences. [16] Whether this theory should be background-independent is an open question. The answer to this question will determine the understanding of what specific role gravitation plays in the fate of the universe. [17]
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 directly on Albert Einstein 's geometric formulation rather than the treatment of gravity as a mysterious ...
Current research on loop quantum gravity may eventually play a fundamental role in a theory of everything, but that is not its primary aim. [41] Loop quantum gravity also introduces a lower bound on the possible length scales. There have been recent claims that loop quantum gravity may be able to reproduce features resembling the Standard Model.