Cosmic Strings and Domain Walls in Models with Goldstone and Pseudo-Goldstone Bosons

Topological vacuum structures are investigated in models with spontaneous breaking of exact and approximate global symmetries. The cosmological evolution of the structure is discussed. A spontaneous breaking of an exact global U(1) symmetry gives rise to vacuum strings which can produce cosmological density fluctuations leading to galaxy formation. In a simplified axion model, the vacuum structures are strings connected by domain walls. They decay before they can dominate the universe.