3.8. Peculiar velocities
Deviations from the Hubble flow directly probe the mass fluctuations
in the Universe, and hence provide a powerful probe of the dark
matter. Peculiar velocities are deduced from the difference between
the redshift and the distance of a galaxy. The observational difficulty
is in accurately measuring distances to galaxies. Even the best distance
indicators (e.g., the Tully-Fisher relation) give an error of 15% per
galaxy, hence limiting the application of the method at large
distances. Peculiar velocities are mainly sensitive to
m, not to
or
quintessence. Extensive analyses in the early 1990s (e.g., Ref.
[38])
suggested a value of
m
close to unity. A more recent analysis
[39], which
takes into account non-linear corrections, gives
8
m0.6 = 0.49 ± 0.06 and
8
m0.6 = 0.63 ± 0.08 (90% errors)
for two independent data sets. While at present
cosmological parameters derived from peculiar velocities are strongly
affected by random and systematic errors, a new generation of surveys
may improve their accuracy. Two promising approaches are the 6dF
near-infrared survey of 15,000 peculiar
velocities 6 and the
kinematic Sunyaev-Zel'dovich effect.