Large-scale structure formation with global topological defects

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 fluctuations 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 fluctuations of a global scalar field. Our numerical results cover a larger dynamical range than those covered by previous investigations and are complementary to them since we use substantially different methods. The resulting microwave background fluctuations are compatible with older simulations. We also obtain a scale-invariant spectrum of fluctuations although with somewhat higher amplitude. On the other hand, our dark matter results yield a smaller bias parameter compatible with b similar to 2 on scales of 20h(-1) Mpc in contrast with previous work which yielded larger bias factors. Our conclusions are thus more positive. According to the aspects analyzed in this work, global topological defect-induced fluctuations yield viable scenarios of structure formation and do better than standard CDM on large scales.