COBE background radiation anisotropies and large-scale structure in the Universe

The microwave background anisotropics detected by the COBE DMR experiment provide our first detailed information about primordial fluctuations. Their properties suggest that the Universe is flat, and had Gaussian initial fluctuations with a scaleinvariant spectrum. We discuss the constraints imposed on such theoretical models by the COBE measurements, by observations of galaxy clustering, and by the observed streaming motions of galaxies. When normalized to match the COBE results, models with Ω = 1 and with more large-scale power than the standard cold dark matter (CDM) model predict lower streaming motions than are observed, but agree well with the dynamics of clustering on smaller scales. Unbiased Ω = 1 CDM models fit the COBE data and the streaming motions, but are less easily reconciled with galaxy clustering data on either small or large scales. Spatially flat CDM models with Ω ∼ 0.2 and a cosmological constant require the mass to be substantially more clustered than the galaxies in order to be consistent with COBE and with observed streaming motions. They are then in conflict, however, with dynamical measurements on smaller scales.