Grand unification theory
Grand unification, grand unified theory, or GUT is a theory in physics that unifies the strong interaction and electroweak interaction.
Proposed theories
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 mainstream GUT models are:
Note: 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.
Current status
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.
GUT theories are based on the idea of a "desert" with no new physics of several orders of magnitude in the renormalization group. However, it's a bit suspicious to run the renormalization group backwards because we don't know what new physics lie there and the property of universality suggests we can't tell easily either.
See also
