Grand unification theory

Grand unification, grand unified theory, or GUT is a theory in physics that unifies the strong interaction and electroweak interaction. Several such theories have been proposed, but none is currently universally accepted. An even more ambitious theory that includes all fundamental forces, including gravitation, is termed a theory of everything.

Some common GUT models are:

• Georgi-Glashow model -- SU(5)
• SO(10)
• Flipped SU(5) -- SU(5)×U(1)
• Pati-Salam model -- SU(4)×SU(2)×SU(2)
• Trinification -- SU(3)×SU(3)×SU(3)
• E₆
• Technicolor models

Note that these models refer to Lie algebras not to Lie groups. The Lie group could be [SU(4)×SU(2)×SU(2)]/Z₂, just to take a random example.

GUT models generically predict the existence of topological defects such as monopoles, cosmic strings, domain walls, and others. None have been observed and their absence is known as the monopole problem in cosmology.

As of 2004, there is still no hard evidence nature is described by a GUT theory. In fact, since the Higgs particle hasn't been discovered yet, it's not even certain if the Standard Model is fully accurate.

See also: Grand Unification Energy