Cosmic flow

Cosmic flow is the large-scale motion of galaxies relative to one another, combining the expansion of the universe with gravitationally driven motions toward and away from dense regions.

Hubble–Lemaître law: The relation between a galaxy’s distance and its recession speed caused by cosmic expansion. It describes the expansion component that cosmic-flow measurements must separate from local motion.
Cosmological redshift: The stretching of light to longer wavelengths as the universe expands during its journey to an observer. It provides the redshift signal from which expansion and line-of-sight motion are inferred.
Local Group: The small galaxy group containing the Milky Way, Andromeda, and dozens of smaller galaxies. Its members share motions shaped by their mutual gravity rather than uniform expansion.
Peculiar velocity survey: An observational program measuring galaxy distances and redshifts to estimate departures from cosmic expansion. Such surveys turn individual motions into maps of coherent flow.
Peculiar velocity: An object’s velocity relative to the smooth expansion of the universe. It isolates the gravitationally driven component of a galaxy’s observed motion.
Cosmic distance ladder: A sequence of calibrated methods for measuring distances beyond the Solar System. Independent distances are needed to compare observed recession speeds with expansion predictions.
Virgo Cluster: A nearby rich cluster of galaxies in the constellation Virgo, central to the Virgo Supercluster region. Its gravitational pull contributes to the measured motion of the Local Group.
Tully–Fisher relation: An empirical relation between a spiral galaxy’s rotation speed and intrinsic luminosity. It estimates galaxy distances for large peculiar-velocity samples.
Gravitational instability: The growth of density differences as gravity draws matter toward regions that are already denser. It turns small early-universe density variations into the structures that redirect galaxy motions.
Standard candle: An astronomical object whose intrinsic brightness can be estimated to infer its distance. Standard candles supply distances that expose departures from the Hubble–Lemaître relation.

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