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We present a simple, yet accurate approximation for calculating the cosmicmicrowave background anisotropy power spectrum in adiabatic models. It consistsof solving for the evolution of a two-fluid model until the epoch ofrecombination and then integrating over the sources to obtain the CMBanisotropy power spectrum. The approximation is useful both for a physicalunderstanding of CMB anisotropies, as well as for a quantitative analysis ofcosmological models. Comparison with exact calculations shows that the accuracyis typically better than 20 percent over a large range of angles andcosmological models, including those with curvature and cosmological constant.Using this approximation we investigate the dependence of the CMB anisotropieson the cosmological parameters. We identify six dimensionless parameters thatuniquely determine the anisotropy power spectrum within our approximation. CMBexperiments on different angular scales could in principle provide informationon all these parameters. In particular, mapping of the Doppler peaks wouldallow an independent determination of baryon mass density, matter mass densityand Hubble constant.Comment: 12 pages, AAS Latex, ApJL, submitted 1994, preprint MIT-CSR-94-1

A two-fluid approximation for calculating the cosmic microwave background anisotropies