Cosmic microwave anisotropies from topological defects in an open universe.

We present a general formalism for computing Cosmic Background Radiation (CBR) and density uctuations in open models with stii sources. We decompose both the metric uctuations and the uctuations in the stress-energy tensor into scalar, vector and tensor modes. We nd analytic Green's functions for the linearized Einstein equations in the presence of stii sources and use this formalism to estimate the amplitude and harmonic spectrum of microwave background uctuations produced by topological defects in an open universe. Unlike innationary models that predict a at universe and a spectrum of CBR uctuations that is enhanced at large angular scales, defect models predict that CBR uctuations are suppressed on angular scales larger than that subtended by the curvature scale. In an = 0:2 ? 0:4 universe, these models, when normalized to the amplitude of CBR uctuations observed by COBE, require a moderate bias factor, 2 ? 3, to be compatible with the observed uctuations in galaxy counts. In these models, accurate predictions can be made which are testable through CBR experiments in the near future. A CBR measurement of would then be possible, up to the limit imposed by cosmic variance. We discuss some of the philosophical implications of an open model and propose a solution to the atness problem.