This book contains the lectures delivered at the Second Paris Cosmology Colloquium (second ‘Journée Cosmologie’) held at the Observatoire de Paris from June 2 to 4, 1994.[The first ‘Journée Cosmologie’ was held from May 18 to 19, 1993.]The purpose of these meetings is to cover selected topics of high current interest in the interplay between cosmology and fundamental physics and to allow easy and fruitful mutual contacts and communication. 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 Colloguia are informal meetings which bring together physicists, astrophysicists and astronomers.Topics covered in the Second Paris Cosmology Colloquium included: fundamental strings and cosmic strings in cosmology, evolution out of the equilibrium of quantum fields in primordial cosmology, dark matter, as well as other subjects of high current interest related to black holes and quantum gravity.Sessions in the 1994 meeting lasted for three days leaving time for private discussions and for enjoying the beautiful parisian campus of the Observatoire de Paris (built on orders from Colbert and to plans by Claude Perrault from 1667 to 1672). Sessions took place at the ‘Salle du Conseil’ in the historic Perrault building under the portraits of Laplace, Le Verrier, Lalande, Arago, Delambre and Louis XIV.

The purpose of this meeting was to cover selected topics of high current interest in the interplay between cosmology and fundamental physics. It brought together physicists, astrophysicists and astronomers and allowed easy and fruitful mutual contacts and communication among them. Topics covered this year include: phase transitions in cosmology and evolution out of the equilibrium of quantum fields, fundamental strings and cosmic strings in cosmology, dark matter and large scale structure, black holes and quantum gravity.

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.