**4.2. Redshift Distortions**

This is a statistical measure of large-scale peculiar velocities from
extended redshift surveys alone, under the assumption of global isotropy
in real space and linear biasing. The comparison of radial and angular
fluctuations yields a measure of
_{m}^{0.6}/*b*.
There are several ways to implement this idea, using correlation
functions, power spectra, or expansion in spherical harmonics and Bessel
functions (reviews:
[35],
[42]).

*New developments:* Redshift surveys larger than before have
become available, for example PSCZ from IRAS to a flux limit of 0.6 Jy,
and the Las Campanas Redshift Survey of optical galaxies in the south.
Even larger surveys are planned for the next few years, such as
the Two-Degree Field (2DF) and the Sloan Digital Sky Survey (SDSS)
(see [43]).
These catalogs will drastically reduce the cosmic scatter.

*Pro:* Measurements of redshifts are inexpensive;
there is no need for independent distances.
One can therefore use surveys of large volumes in order to beat the
measurement errors and the cosmic scatter.

*Con:*
In the mildly-nonlinear regime where the interpretation of
distortions is straightforward, the noise in the observations
[*e.g.*, the
(,
) diagram] is bad and difficult to quantify.
With current data, the estimate suffers from large cosmic scatter.
The method is subject to galaxy density biasing. At best it
measures the bias-contaminated parameter
, not
_{m}.
The estimated
by this method may be systematically different from the
estimated
by other methods
[44].

*Current Results:*
The best estimates for IRAS galaxies span a large range:
0.5
_{I}
1.2
[43].
The current samples do not yet probe a sufficiently fair volume of the
universe, and there are indications for systematic effects near the
flux limit.