Grand unified theory

In particle physics, a model in which, at high energies, the three gauge interactions of the Standard Model (those that define electromagnetism, the strong and weak nuclear forces) or forces, are merged into a single force. This unified force has never been directly observed. However, many GUT models theorize its existence. If this unification is possible, it also raises the possibility that there was a Grand unification epoch in the very early universe. At this time, these three forces would be one unified force. 

Experiments have shown, that at higher energies, the electromagnetic and weak nuclear force merge into one force.  GUT models predict that at even higher energies, their is an electronuclear interaction. This would be a unification of the strong nuclear interaction with the electroweak interaction. This GUT interaction would be characterized by one larger gauge symmetry. Thus, there are several force carriers, however, one unified coupling constant. A unification of gravity with the electronuclear interaction would give us a TOE or Theory of Everything. GUTs are often seen as an intermediate towards a TOE. 

The particles predicted by GUTs are expected to have extremely high mass. This mass would be around the GUT scale of 10^16 GeV. This is just a few orders of magnitude lower than the Planck scale of 10^19 GeV. Thus, it is well beyond the reach of particle collider experiments in the foreseeable future. The particles predicted by GUT models, thus, cannot be observed directly. Instead, the effects of grand unification could become apparent through indirect observations. 

GUT models that aim to be realistic are quite complicated. This is because they need to introduce additional fields and interactions, or even additional dimensions of space. 

SU(5) is the simplest GUT. This is the smallest simple Lie group that can contain the Standard Model. These group symmetries allow the reinterpretation of several known particles. This includes the photon, W and Z boson and the gluon. They would be viewed as different states if a single particle field. 

SO(10) is the next simple Lie group which contains the Standard Model. The unification of matter is even more complete here.