Infall and Omega

Virialized structures in the Universe produce velocity perturbations of galaxies which are located near them. The amplitude of these perturbations (delta v)/v depends on the background density of the Universe (OMEGA) and the density contrast ( delta rho/rho ) interior to the shell upon which the galaxy is located. The unperturbed velocity in this case is given by v = Hr where r is the distance from the source which is causing the perturbation.

The linear approximation to this perturbation, which works in the limit of small delta rho/rho is the following:

We can apply this equation to the case of Virgo Infall as schematically shown below:

The Virgo cluster is a nearby cluster or region of mass overdensity. Our galaxy feels the gravitational pull of Virgo and therefore our expansion velocity, with respect to Virgo is retarded. The diagram above shows three components:

For now ,let's just consider our infall into Virgo.