COSMOLOGICAL PERTURBATIONS SEEDED BY TOPOLOGICAL DEFECTS - SETTING THE INITIAL CONDITIONS

We consider a perfectly homogeneous and isotropic universe which under goes a sudden phase transition. If the transition produces topological defects, which we assume, perturbations in the geometry and cosmic fl uid also suddenly appear. We apply the standard general relativistic j unction conditions to match the pre-and post-transition eras and thus set the initial conditions for the perturbations. We solve their evolu tion equations analytically in the case when the defects act as a cohe rent source and their density scales like the background density. We s how that isocurvature as well as adiabatic perturbations are created i n a ratio which is independent of the detailed properties of the defec ts. We compare our result to the initial conditions currently used in the literature and show how the cosmic fluid naturally ''compensates'' for the presence of the defects. [S0556-2821(97)01024-2].