3.6.1. Inflows and Outflows
Relative distance determinations for substantial samples of galaxies in regions where there are complete redshift surveys can vastly increase the the discriminatory power of the data in mapping out the true mass distribution. Mapping of the peculiar velocity field constrains a combination of the large-scale matter density contrast and the cosmological density parameter . Due to the presence of biasing, the large-scale matter density contrast is really measured by the large-scale luminosity density contrast multiplied by b. It is also essential In the highly clustered Universe that we have discussed, regions of mass overdensity will cause a net inflow of material towards the center of the mass overdensity. For large regions of mass underdensity (e.g., large voids) there will be a net outflow of material from the void center. For smaller scale voids, however, the outflow is compressed due to the effects of nearby larger scale voids which have bigger outflows (see van de Weygaert 1995).
The measurements of peculiar velocities has a rich history of producing ambiguous results with a touch of irony. The ambiguous nature of the results is a direct reflection of the methodology which generally produces low S/N indicators of peculiar velocities. The first announced detection of systematic peculiar velocities in the local velocity field was made in 1976 by Vera Rubin and Kent Ford. In general, this result was not believed and many papers came into being explaining the result away on the basis of conspiratorial selection effects or twisted statistics. The main problem at the time with the Rubin-Ford effect was the lack of a framework in which to understand it. The possibility of large scale motions produced by the collective gravitational acceleration of distant aggregate mass concentrations was not considered to be viable in a local Universe where smooth Hubble flow was assumed to dominate.
A year after the Rubin-Ford paper was published, observations of the CMB revealed the possible existence of a dipole anisotropy (DA). By 1979, measurements by George Smoot and colleagues had established the amplitude of the DA to be 600 km s-1. A more modern determination of the DA vector by Smoot et al. (1992) based on COBE data has amplitude 620 km s-1 and director = 10.75 hr and = -28.5 degrees. The DA is best understood as motion of our Galaxy, with amplitude of 600 km s-1, with respect to the frame of reference established by the CMB (which we assume to be an inertial frame). The origin of this motion is most likely due to gravitational acceleration of the Milky Way by a distant mass concentration. This discovered DA establishes that the local Hubble flow does have some noise. The Smoot et al. measurements also established a direction for the DA. Unfortunately, this vector is not directly pointed at the nearest known mass concentration, the Virgo cluster. Additional regions of over density are therefore required to fully explain the DA. The detection of the DA is quite important because it provides confirming evidence of peculiar velocities determined by other, lower S/N means. In fact, without the existence of the DA, claims of peculiar velocities in the local velocity field of amplitude a few hundred km s-1 would probably still be meet with wholesale skepticism. The firm existence of the DA now validates the general concept of large scale peculiar velocities and confirms that the Rubin-Ford conceptual effect is a real one even though their more detailed original claim has not been verified.