There is a rather strong belief that in the very early Universe when temperatures were very high compared with today, all the forces were unified into a single force. Only when the temperature dropped did these forces separate from each other to leave us with the 4 distinct forces that we see in our present Universe. The process of the forces separating from each other is called spontaneous symmetry breaking and is summarized in the table below.
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*Temperature Conversion: 1 GeV =
Theories that involve the unification of the forces remain theoretical speculations that are as yet unproven, but they have a history. The first forces to be unified were the electric and magnetic forces, unified in Maxwell's equations. There is now strong experimental evidence for the unification of the electromagnetic and weak interactions. Although GUTs are not proven experimentally, and there are many of them, there is strong circumstantial evidence to suggest that a theory at least like a Grand Unified Theory with one single force of nature is required to make sense of the Universe.