This book contains the lectures delivered at the Fourth Paris Cosmology Colloquium held at the Observatoire de Paris from June 4 - 9, 1997. This Colloquium ‘Phase Transitions in Cosmology’ is the first event of a EUROCONFERENCE series (‘Accompanying Measures’), with the support of the training and Mobility Programme of the Commision of the European Communities.The purpose of the Paris Cosmology Colloquia is to cover selected topics of high current interest in the interplay between cosmology and fundamental physics and to allow easy and fruitful interaction and communication between researchers in these areas. The main aim is to put in contact fundamental theoretical physics (including string theory) with real physical problems that arise in the study of the universe. The Paris Cosmology Colloquia are informal meetings which bring together physicists, astrophysicists and astronomers. Topics covered in this Fourth Paris Cosmology Colloquium included: cosmic microwave background, phase transitions in cosmology and evolution out of the equilibrium quantum fields, the value of the Hubble constant, fundamental strings in primordial cosmology, as well as other subjects of high current interest such as scaling laws in the interstellar medium and in the large scale structure, gravitational lensing and dark matter.

String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point particles that form the basis for the standard model of particle physics. The phrase is often used as shorthand for Superstring theory, as well as related theories such as M-theory. By replacing the point-like particles with strings, an apparently consistent quantum theory of gravity emerges. Moreover, it may be possible to 'unify' the known natural forces (gravitational, electromagnetic, weak nuclear and strong nuclear) by describing them with the same set of equations. Studies of string theory have revealed that it predicts higher-dimensional objects called branes. String theory strongly suggests the existence of ten or eleven (in M-theory) space-time dimensions, as opposed to the usual four (three spatial and one temporal) used in relativity theory.