Redshift-space distortions arise because observed galaxy redshifts include peculiar velocity as well as cosmological redshift. Distances inferred solely from expansion therefore distort clustering along the line of sight. Coherent large-scale motion and small-scale velocity dispersion produce different effects on the inferred matter power spectrum.
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Redshift-space distortions are the apparent changes in the observed positions and velocities of astronomical objects due to the effects of cosmic expansion and the motion of galaxies within structures such as galaxy clusters. These distortions arise when we observe distant galaxies and characterize their positions using redshift, which refers to the stretching of light from objects that are moving away from us.