a New Inflationary Cosmology.

The standard "hot big bang" cosmology is unable to naturally explain some important features of our universe. To assure that a universe like ours results, very special conditions in the early stages must be postulated. Many causally disconnected regions must have appeared to be in thermal equilibrium to produce the currently observed homogeneity and isotropy (the "horizon problem") and the energy density must have been finely tuned to account for its present nearly critical value (the "flatness problem"). The standard cosmology offers no explanation for why these conditions should have existed. In addition, when typical grand unified models are incorporated, an abundance of superheavy monopoles is produced which greatly exceeds experimental bounds (the "monopole problem"). The spontaneous symmetry breaking phase transitions in the early universe predicted by grand unified models are studied. Some of these models predict that during the phase transition the universe experienced a period where potential energy dominated the total energy density and caused the size of the universe to increase exponentially with time. It is shown that in special models the exponential expansion can continue even after the phase transition is well underway and, in these cases, once the transition is completed and the universe is describable by the standard Friedmann-Robertson-Walker cosmology, the necessary special conditions of the standard hot big bang model have been realized in a natural way and the monopoles are not overabundant. The general mechanism demonstrated looks promising as a solution to the horizon, flatness, and monopole problems, although specific models considered so far are not completely satisfactory. The standard SU(5) grand unified model and a supersymmetric model which employs the "Written reverse hierarchy mechanism" to dynamically generate the grand unification scale are studied in detail.