Phase transitions in the very early universe

Phase transitions in the early universe are discussed, emphasizing the GUT phase transition. The essential properties of GUTs are reviewed, stressing those relevant for cosmology. The standard scenario of the very early universe is described, and the monopole, horizon, and flatness problems are discussed. The original inflationary universe scenario, which offers a possible solution to these problems, is described. The problems of this scenario are explained, and the new inflationary universe, which avoids them, is discussed. A summary of the calculation of density fluctuations in the new inflationary universe is presented.