luminosity–volume test(V/Vm test) A method of testing for evolutionary effects in a sample of extragalactic sources of known redshift. All sources should be known over a particular region of the sky down to a certain flux density level. The volume, V, within the redshift of each object, together with the volume V m within the maximum redshift out to which that object could have been and yet still remain in the sample, are computed from a cosmological model. If no evolution is present and the correct cosmological model is employed, then V ought to be uniformly distributed between 0 and V m. The mean of V /V m is then ½. If the mean differs significantly from ½ for all reasonable cosmological models, then evolution must be occurring; this is the case for quasars. See also luminosity function.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006