Quantum cosmological theories attempt to extend Albert Einstein's theory of gravitation to include quantum theory. There have been many attempts to carry out this extension of Einstein's work and as yet there is no single satisfactory theory. A quantum cosmology is needed in order to draw conclusions about the nature of the initial state of the universe and to interpret the meaning of the idea that it might have quantum-mechanically tunneled out of "nothing," or some version of the quantum vacuum. A quantum cosmological theory is expected to be a particular application of a full theory of quantum gravity (sometimes referred to as a "theory of everything") that would unite and extend all existing theories of the forces of nature. The favored candidate for such a theory at present is M-theory, a version of the theory formally known as superstring theory. Theories of this sort are highly constrained by mathematical requirements of symmetry and finiteness, as well as by the requirement of explaining all known elementary particle physics. Quantum cosmologies lead naturally to the Many-Worlds Interpretation of quantum mechanics.
See also Cosmology, Physical Aspects; Grand Unified Theory; Superstrings
smolin, lee. three roads to quantum gravity. london: weidenfield, 2001.
john d. barrow