Ever since the discovery of the microwave background dipole by Smoot, Gorenstein & Muller (1977) and the pioneering measurements of galaxy motions by Rubin et al. (1976), the study of cosmic flows, or deviations from a smooth Hubble flow due to large-scale gravitational perturbations, has been recognized as one of the most powerful constraints to cosmological scenarios (Peebles 1980, Dekel 1994, Strauss & Willick 1995). Indeed, under the assumption that cosmic structure originated from small-amplitude density fluctuations that were amplified by gravitational instability, the peculiar velocity v and mass density contrast are together linked in the linear regime by a deceptively simple expression (from mass conservation in linear perturbation theory):
The mean square bulk velocity on a scale R is easily calculated in Fourier space as:
where P(k) is the mass fluctuation power spectrum and 2(kR) is the Fourier transform of a top-hat window of radius R. Measurements of galaxy PVs can thus directly constrain m, the shape and amplitude of the power spectrum, and test assumptions about the statistical properties of the initial fluctuations and gravitational instability as the engine of perturbation growth.
The last major workshop on Cosmic Flows in July 1999 in Victoria, BC (Courteau, Strauss, & Willick 2000; hereafter CFW2000) came at a time when important new data sets and critical modeling of the "biasing" relation between the galaxy and mass distribution were just being released. Fundamental questions debated at the conference, and central to all cosmological investigations based on cosmic flows, included (1) : (1) What is the amplitude of bulk flows on the largest scales probed? (2) Can velocity analysis provide accurate estimates of m?, and (3) What is the value of m? The last two years have seen significant progress providing nearly definitive answers to each of the 3 questions above, as we discuss in the remainder of this review.
Detailed information about cosmic flows can be found in the Cosmic Flows 1999 workshop proceedings (CFW2000), including the conference review by Dekel (2000). Also in Willick (1999) and Dekel (1999), as well as Willick (2000).
1 Discussions about the measurements of the small-scale velocity dispersion and the coldness of the velocity field also figured prominently in the workshop agenda but we do not offer any update below, for lack of space. The interested reader should read CFW2000. Back.