Redshift-space distortions

Redshift-space distortions are apparent changes in galaxy clustering caused by peculiar velocities shifting galaxies’ inferred positions along the line of sight.

Peculiar velocity: A celestial object's motion relative to the average expansion of the universe. Its line-of-sight component shifts a galaxy's inferred radial position.
Growth rate of cosmic structure: The rate at which density fluctuations in the universe grow over cosmic time. Coherent infall makes this rate measurable through clustering anisotropy.
Hubble's law: The relation between a galaxy's recession velocity and its distance in the expanding universe. Observed redshift combines expansion with peculiar motion around this relation.
Real-space clustering: The spatial distribution and correlations of matter or galaxies in physical cosmological coordinates. It is the underlying clustering pattern before redshift-derived radial shifts.
Kaiser effect: A large-scale enhancement of clustering along the line of sight caused by coherent infall into overdense regions. It describes the anisotropic compression produced by coherent peculiar velocities.
Growth index: A parameterization of how the growth rate of cosmic structure depends on the matter density. Redshift-space measurements constrain its value and probe departures from general relativity.
Cosmological redshift: The stretching of light's wavelength as the universe expands during its journey to an observer. It supplies the expansion signal that peculiar velocities perturb.
Redshift measurement error: Uncertainty in an object's redshift caused by instrumental limits, spectral features, or analysis methods. It can broaden radial positions like velocity dispersion, but is not a physical motion.
Finger of God effect: An apparent elongation of galaxy groups along the line of sight caused by their members' random velocities. It is the small-scale elongation caused by virial motions.
Baryon acoustic oscillations: A preferred comoving scale in matter clustering, originating in sound waves in the early universe. Analyses model velocity distortions alongside this standard-ruler signal.

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