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):

(1) |

The mean square bulk velocity on a scale *R* is easily calculated in
Fourier space as:

(2) |

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.
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