COBE's constraints on the global monopole and texture theories of cosmic structure formation

Abstract

We report on a calculation of large scale anisotropy in the cosmic microwave background radiation in the global monopole and texture models for cosmic structure formation. We have evolved the six component linear gravitational field along with the monopole or texture scalar fields numerically in an expanding universe and performed the Sachs-Wolfe integrals directly on the calculated gravitational fields. On scales $> 7^\circ$, we find a Gaussian distribution with an approximately scale invariant fluctuation spectrum. The $\Delta T/T$ amplitude is a factor of 4-5 larger than the prediction of the standard CDM model with the same Hubble constant and density fluctuation normalization. The recently reported COBE-DMR results imply that global monopole and texture models require high bias factors or a large Hubble constant in contrast to standard CDM which requires very low $H_0$ and bias values. For $H_0 = 70 {\rm {km\over sec} Mpc^{-1}}$, we find that normalizing to the COBE results implies $b_8 \simeq 3.2\pm 1.4$ (95\% c.l.). If we restrict ourselves to the range of bias factors thought to be reasonable before the announcement of the COBE results, $1.5 \lsim b_8 \lsim 2.5$, then it is fair to conclude that global monopoles and textures are consistent with the COBE results and are a {\it better} fit than Standard CDM.