Georgi-Glashow model

In 1974, Howard Georgi and Sheldon Glashow propose a Grand unified theory model. 

In the model, the gauge groups of the Standard Model, SU(3) for the strong nuclear force, SU(2) for the weak nuclear force and U(1) for electromagnetism, are combined into a single gauge group SU(5). Then at the grand unification energy or GUT scale, the unified group SU(5) is spontaneously broken into its Standard Model subgroups.