Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

3.2. Microwave Anisotropies

A second argument for nonbaryonic dark matter is associated with the upper limits on and detections of anisotropies in the cosmic microwave background (CMB). To form the observed large-scale structure through purely gravitational processes, the amplitude of the fluctuations in the matter density at decoupling must have exceeded a minimum value; this implies a minimum amplitude for the CMB anisotropies which may contravene observations for a purely baryonic model. The anisotropies are reduced in a model dominated by nonbaryonic dark matter ( >> _b), partly because the density fluctuations start growing earlier (from when the dark matter dominates the density) and partly because they continue growing for a longer period (fluctuations freezing out at a redshift z ^-1). Despite this argument, it is not clear that the anisotropy constraints require to be as large as 1 - especially if one relinquishes scale-invariant fluctuations - because both the amplitude and angular scale of the anisotropies are reduced in a low density Universe owing to the effects of radiation pressure at decoupling (Coles & Ellis 1994). In the past few years, therefore, much attention has focused on baryon-dominated models with "primeval isocurvature" fluctuations (Peebles 1987a, b). The fluctuations are assumed to have a power-law form and the problem is to determine whether one can choose a spectral index n which simultaneously matches the COBE anisotropies at 10°-90° (Smoot et al 1992) and the large-scale structure data (Cen et al 1993). One can already place strong constraints on the combination of _b and n (Efstathiou et al 1992, Gouda & Sugiyama 1992), and some researchers claim that baryon-dominated models are already excluded (Chiba et al 1993).