G2 manifold

From Wikipedia, the free encyclopedia

In differential geometry, a G₂ manifold is a seven-dimensional Riemannian manifold with holonomy group G₂. The group $G_{2}$ is one of the five exceptional simple Lie groups. It can be described as the automorphism group of the octonions, or equivalently, as a proper subgroup of special orthogonal group SO(7) that preserves a spinor in the eight-dimensional spinor representation or lastly as the subgroup of the general linear group GL(7) which preserves the non-degenerate 3-form $\phi$ , the associative form. The Hodge dual, $\psi =*\phi$ is then a parallel 4-form, the coassociative form. These forms are calibrations in the sense of Harvey-Lawson, and thus define special classes of 3 and 4 dimensional submanifolds.

Properties

If M is a $G_{2}$ -manifold, then M is:

Ricci-flat,
orientable,
a spin manifold.

History

Manifold with holonomy $G_{2}$ was firstly introduced by Edmond Bonan in 1966, who constructed the parallel 3-form, the parallel 4-form and showed that this manifold was Ricci-flat. The first complete, but noncompact 7-manifolds with holonomy $G_{2}$ were constructed by Robert Bryant and Salamon in 1989. The first compact 7-manifolds with holonomy $G_{2}$ were constructed by Dominic Joyce in 1994, and compact $G_{2}$ manifolds are sometimes known as "Joyce manifolds", especially in the physics literature.

Connections to physics

These manifolds are important in string theory. They break the original supersymmetry to 1/8 of the original amount. For example, M-theory compactified on a $G_{2}$ manifold leads to a realistic four-dimensional (11-7=4) theory with N=1 supersymmetry. The resulting low energy effective supergravity contains a single supergravity supermultiplet, a number of chiral supermultiplets equal to the third Betti number of the $G_{2}$ manifold and a number of U(1) vector supermultiplets equal to the second Betti number.

See also: Calabi-Yau manifold, Spin(7) manifold

References

E. Bonan, (1966), "Sur les variétés riemanniennes à groupe d'holonomie G2 ou Spin(7)", C. R. Acad. Sci. Paris 262: 127–129 .
Bryant, R.L. (1987), "Metrics with exceptional holonomy", Annals of Mathematics (Annals of Mathematics) 126 (2): 525–576, doi:10.2307/1971360, JSTOR 1971360 .
Bryant, R.L.; Salamon, S.M. (1989), "On the construction of some complete metrics with exceptional holonomy", Duke Mathematical Journal 58: 829–850 .
M. Fernandez; A. Gray (1982), "Riemannian manifolds with structure group G2", Ann. Mat.Pura Appl. 32: 19–845 .
Harvey, R.; Lawson, H.B. (1982), "Calibrated geometries", Acta Mathematica 148: 47–157, doi:10.1007/BF02392726 .
Joyce, D.D. (2000), Compact Manifolds with Special Holonomy, Oxford Mathematical Monographs, Oxford University Press, ISBN 0-19-850601-5 .
Karigiannis, Spiro (2011), "What Is . . . a G₂-Manifold?", AMS Notices 58 (04): 580–581 .

This article is issued from Wikipedia. The text is available under the Creative Commons Attribution/Share Alike; additional terms may apply for the media files.