grand unified theory

n. (Abbr. GUT)
A theory of elementary forces that unites the weak, strong, electromagnetic, and gravitational interactions into one field theory and views the known interactions as low-energy manifestations of a single unified interaction.

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Attempts to unify three fundamental interactions—strong, electromagnetic, and weak—with a postulate that the three forces, with the exception of gravity, can be unified into one at some very high energy. The basic idea is motivated by the incompleteness of the electroweak theory of S. Weinberg, A. Salam, and S. Glashow, which has been extremely successful in the energy region presently accessible with the use of accelerators, and by the observation that the coupling constant for strong nuclear forces becomes smaller as energy increases whereas the fine-structure constant (α = 1/137) for electromagnetic interactions is expected to increase with energy. See also Gravitation; Strong nuclear interactions; Weak nuclear interactions.

The simplest grand unification theory (GUT), proposed by H. Georgi and Glashow, is based on the assumption that the new symmetry that emerges when the three forces are unified is given by a special unitary group SU(5) of dimension 24. This symmetry is not observable in the low-energy region since it is badly broken. In this model, as in most GUTs, the coupling constants for the three interactions merge into one at an energy of about 10¹⁴ GeV. Quarks and leptons belong to the same multiplets, implying that distinctions between them disappear at the energy of 10¹⁴ GeV or above. In addition to the known 12 quanta of strong, electromagnetic, and weak interactions, there appear, in this model, 12 new quanta with the mass of 10¹⁴ GeV. These generate new but extremely weak interactions that violate baryon- and lepton-number conservation. The most spectacular prediction of GUTs is the instability of the proton, which is a consequence of baryon-number (and lepton-number) violation. See also Lepton; Proton; Quarks; Symmetry breaking; Symmetry laws (physics).

GUTs, in general, explain why the charge of the electron is precisely that of the proton with the opposite sign. Massive neutrinos are a distinct possibility in GUTs, and the smallness of their mass can also be understood. See also Cosmology; Neutrino.

According to the scenario based on the GUTs, the universe underwent a phase transition when its temperature cooled to 10²⁷ K, which corresponds to 10¹⁴ GeV in energy and to the first 10⁻³⁵ s after the big bang. The phase transition caused an exponential expansion (10³⁰-fold in 10⁻³² s) of the universe, which explains why the observed 3 K microwave background radiation is uniform (the horizon problem), and why the universe behaves as if space is practically flat (the flatness problem). See also Big bang theory; Inflationary universe cosmology; Phase transitions; Universe.

In spite of its theoretical triumph and spectacular predictions, the simple SU(5) model is practically untested by experiment and appears to be incomplete or even incorrect. No experimental evidence of proton decay has been established, and the problems which GUTs leave unsolved are numerous. See also Elementary particle; Fundamental interactions; Supergravity; Supersymmetry.

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A theory that describes the behavior of matter at temperatures that existed only in the first fraction of a second after the Big Bang. In these theories, the strong, weak, and electromagnetic forces are unified. The greatest triumph of GUTs is that they explain the absence of antimatter in the universe. (See also unified field theory.)