## Real space

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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, $\pi$, of the real- and redshift-space distances, $r$ and $s$, differ, while the transversal (perpendicular to the line-of-sight) components, $\sigma$, are assumed to be equivalent.