Observational evidence for dark matter from peculiar velocities in the local Universe

Abstract

Deviations from the Hubble flow in the local Universe provide a direct measure of the underlying total mass distribution, assuming the gravitational instability picture. A comparison of the mass distributions deduced from the peculiar velocities with the galaxy distribution could tell us the amount of dark matter on large scales. We discuss the origin of motion of the Local Group with respect to the Cosmic Background Radiation and review the peculiar velocity field deduced from distances to hundreds of elliptical and spiral galaxies, including a new results for the Shapley Supercluster. The density parameter Ω0 deduced from the relation between the peculiar velocity and acceleration (as inferred from galaxies) suggests an open universe if light traces mass. However, it is still possible that there is enough matter to close the Universe if the galaxy distribution is more strongly clustered (‘biased’) than the mass distribution. We also considered the possibility of a non‐zero cosmological constant λ and ‘anti‐bias’.