Cosmic expansion and deviations from it

Distance determinations of field galaxies and clusters are discussed. For field galaxies, if corrected for selection effects, one finds H 0 ≈ 50. Cluster distances are less vulnerable to selection effects. Based on globular clusters, novae, supernovae, the D n –σ relation, 21 cm-line widths and on the requirement that the largest spirals in the cluster be larger than the Galaxy and M31, the distance of the Virgo cluster is found to be 21.3 Mpc. Unreliable distance indicators (e.g. brightest stars in the I- and K-band, planetary nebulae, and surface brightness fluctuations) are discussed in some detail and excluded. The "cosmic" velocity of the Virgo cluster, which the cluster would have if all peculiar motions were absent, is inferred from distant clusters, whose relative distances to the Virgo cluster are known. The result, v Virgo CMB = 1179 ± 17 km s −1 , combined with the cluster distance, yields the asymptotic value of H 0 (cosmic) = 55 ± 2. This value holds, beyond ~3 Mpc, at all scale lengths without significant variations. The expansion is highly linear. Deviations from the pure Hubble flow are considered at different scale lengths. Peculiar motions grow with distance, with σ v = 50-70 km s −1 at scales <10 Mpc, and reaching 220 km s −1 at the Virgo distance. The largest streaming velocity of 620 km s −1 is required by the CMB dipole; this motion is mirrored by clusters at an effective distance of ~6500 km s −1 . The comoving volume is probably even smaller. The three-dimensional σ v for cluster centers is ~590 km s −1 or less. The local acceleration requires a density parameter of Ω 0 = 0.1–0.2 which may be entirely due to baryonic matter.