Cosmic structure formation in hybrid inflation models

A wide class of inflationary models, known as hybrid inflation models, may produce topological defects during a phase transition at the end of the inflationary epoch. We point out that, if the energy scale of these defects is close to that of grand unification, then their effect on cosmic structure formation and the generation of microwave background anisotropies cannot be ignored. Therefore, it is possible for structure to be seeded by a combination of the adiabatic perturbations produced during inflation and active isocurvature perturbations produced by defects. Since the two mechanisms are uncorrelated the power spectra can be computed by a weighted average of the individual contributions. We investigate the possible observational consequences of this with reference to general hybrid inflation models and also a specific model based on supergravity. These mixed perturbation scenarios have some novel observational consequences and these are discussed qualitatively.