Hubble flow

Hubble flow is the large-scale recession of galaxies as the universe expands, with recession speed increasing approximately in proportion to distance.

Metric expansion of space: The increase of distances between widely separated, gravitationally unbound points as the universe evolves. It produces the large-scale recession described by Hubble flow without requiring galaxies to move through space.
Comoving distance: A distance measure that factors out cosmic expansion and stays constant for objects moving with the expansion. It distinguishes fixed cosmic coordinates from the increasing physical separation in Hubble flow.
Alexander Friedmann: A Russian physicist who derived expanding and contracting cosmological solutions to Einstein's field equations. His solutions established that general relativity permits a changing cosmic scale.
Observable universe: The region of the universe from which light has had time to reach an observer since the early universe. Its scale and boundary are interpreted within a universe whose distances evolve through expansion.
Gravitationally bound system: A collection of matter whose internal gravitational attraction prevents it from following the general cosmic expansion. Bound systems such as galaxies do not generally expand with the large-scale Hubble flow.
Hubble–Lemaître law: The relation stating that a galaxy’s recession speed is approximately proportional to its distance. It quantifies the distance-dependent recession pattern of Hubble flow.
Proper distance: The physical separation between objects at a specified cosmic time. Unlike comoving distance, it grows with the expansion for galaxies in the flow.
Georges Lemaître: A Belgian priest and physicist who proposed an expanding universe and connected it to galaxy observations. He derived a distance–velocity relation before it became widely associated with Hubble.
Age of the universe: The elapsed time since the universe entered its hot, dense early expansion, estimated at about 13.8 billion years. The measured expansion rate helps constrain how long cosmic expansion has proceeded.
Doppler effect: The change in an observed wave's frequency caused by relative motion between source and observer. It describes motion through space, unlike cosmological redshift caused by expanding space.

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