Recessional velocity

Recessional velocity is the rate at which a celestial object’s distance from an observer increases. It may reflect cosmic expansion, motion through space, or both.

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Hubble's law: A relation stating that, on large scales, a galaxy’s recession speed is proportional to its distance. It connects recession speed to distance in the expanding universe.
Distance: A measure of spatial separation between two points or objects. Recessional velocity is defined through the changing separation between object and observer.
Hubble constant: The present-day proportionality between cosmic expansion rate and distance in the local universe. It sets the nearby scale relating distance to expansion-driven recession speed.
Observable universe: The region of the universe from which light has had time to reach an observer. Expansion and recession help determine which regions can ever be observed.
Cosmological redshift: The stretching of light to longer wavelengths as the universe expands while the light travels. Its wavelength shift is a primary way to infer distant objects’ recession.
Velocity: The rate of change of position with time, including both speed and direction. The term’s velocity component follows the ordinary rate-of-position-change definition.
Cosmic distance ladder: A sequence of calibrated methods for measuring astronomical distances beyond the reach of direct parallax. Its distance estimates combine with redshifts to determine recession rates and expansion.
Cosmic horizon: A boundary in cosmology beyond which events cannot currently affect an observer through signals. Rapid recession can place distant regions beyond causal or observational reach.
Doppler effect: The change in a wave’s observed frequency caused by relative motion between its source and observer. It describes the motion-related component of spectral shifts, distinct from expansion.
Scale factor (cosmology): A measure of how the universe's size changes over cosmic time in cosmological models. Its growth drives the expansion contribution to recession over cosmological distances.