Conceptions of
real space and
redshift space were introduces in observational cosmology due to the fact that observed redshifts of extragalactic objects are not ``pure'' cosmological redshifts (caused by expansion of the Universe), but are contaminated by gravitationally-induced peculiar velocities of the objects. Speaking about the distances in real space one means that these distances are not contaminated by peculiar velocities (in contrast to
redshift-space distances). In fact, only the parallel (to the line-of-sight) components,
, of the real- and redshift-space distances,
and
, differ, while the transversal (perpendicular to the line-of-sight) components,
, are assumed to be equivalent.