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Many physicists believed that all four forces were once unified at high energy levels but as the universe reduced into a lower-energy states the inherent symmetry between the forces began to break down.this broken symmetry caused the creation of four distinct forces of nature. The goal of the unified theory is an attempt to look back in time to when these forces where unified as one. In this thesis I tried to give an overview of the challenges which is againsted theorists in the way of unification. In the first chapter explores the theory that try to unify the electromagnetic and weak forces are investigated.it is done by considering guage principle and spontaneous symmetry breaking.in the...