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We investigate cosmological structure formation seeded by topological defects which may form during a phase transition in the early universe. First we derive a partially new, local and gauge invariant system of perturbation equations to treat microwave background and dark matter uctuations induced by topological defects or any other type of seeds. We then show that this system is well suited for numerical analysis of structure formation by applying it to seeds induced by uctuations of a global scalar eld. Our numerical results cover a larger dynamical range than previous investigations and are complementary to them since we use substantially di erent methods. The resulting microwave background uctuations are compatible with older simulations. We also obtain a scale invariant spectrum of uctuations although with somewhat higher amplitude. On the other hand, our dark matter results yield a smaller bias parameter compatible with b 2 on scales of 20h ?1 Mpc in contrast to previous work which yielded to large bias factors. Our conclusions are thus more positive. According to the aspects analyzed in this work, global topological defect induced uctuations yield viable scenarios of structure formation and do better than standard CDM on large scales.

The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark matter and texture are discussed and compared with recent observational results of the COBE satellite. A couple of important statistical parameters are determined. The fluctuations are slightly non gaussian. The quadrupole anisotropy is 1.5±1.2×10−5 and the fluctuations on a angular scale of 10 degrees are (3.8±2.6)×10−5 . The COBE are within about one standard deviation of the typical texture + CDM model discussed in this paper. Furthermore, we calculate fluctuations on intermediate scales (about 2 degrees) with the result ∆T /T (θ ∼ 2 o ) = 3.9 ± 0.8) × 10 −5 . Collapsing textures are modeled by spherically symmetric field configurations. This leads to uncertainties of about a factor of 2.