Cosmic strings: Large-scale mass fluctuations and velocity fields

Abstract

In this paper we calculate the large scale mass fluctuations and peculiar velocity fields in the cosmic string scenario in different dark matter-dominated universes. By using Turok et al.'s string density fluctuation spectra before gravitational growth and the usual linear perturbation theory, we get the mean square root mass fluctuation 〈( δM M ) 2(M)〉 1 2 and obtain the first calculation of large-scale peculiar velocity fields in the cosmic string picture. If we assume galaxies trace the mass distribution, normalization of δM/ M at scale 8 Mpch −1 requires Gμ > 10 −5, which is too large to be allowed by Big Bang nucleosynthesis theory and the observed background radiation anisotropy, where μ is the mass per unit string length. Pancaking at z = 3 in neutrino-dominated universes requires even larger Gμ. The peculiar velocity decreases more slowly with increasing distance than in usual cosmological models, but this is still not enough to explain Collins et al.'s observed data. Our results indicate that cosmic string density fluctuations alone cannot form such large scale structures as observed.