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. ) suggested a value of m close to unity. A more recent analysis , 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.
6 See http://www.mso.anu.edu.au/6dFGS/ Back.