ABSTRACT
One of the most important concepts in modern particle theory is that of spontaneous symmetry breaking (SSB). The idea that there are underlying symmetries of Nature that are not manifest in the structure of the vacuum appears to play a crucial role in the unification of the forces. This chapter reviews some general aspects of high-temperature symmetry restoration and phase transitions. It considers a real scalar field described by the Lagrangian density to illustrate some of the features of finite temperature effects. The phenomenon of high-temperature symmetry restoration can be understood in several ways. A physical approach is to consider the effect of finite temperature upon the propagation of a particle. Transition to the true vacuum state by quantum tunnelling occurs through the nucleation of bubbles of the energetically favored asymmetric phase, which then expand outward at the speed of light.
