Phase transitions in gauge theories and cosmology

Reviews phase transitions in super-dense matter, which consists of particles interacting in accordance with the unified gauge theories of weak, strong and electromagnetic interactions. It is shown that at a sufficiently large temperature a phase transition takes place after which almost all elementary particles in the hot super-dense matter become massless and weak interactions become long-range like electromagnetic interactions. Analogous phenomena may take place with an increase of fermion density in cold dense matter, and also in the presence of external fields and currents. Phase transitions in gauge theories lead to a time dependence of the masses of particles, of coupling constants and of the cosmological term in the expanding Universe, to the appearance of a domain structure of vacuum, to substance energy non-conservation, to a possibility of obtaining the 'hot' Universe starting with a 'cold' one, and to some other unusual effects important for cosmology and for elementary particle physics.