Recessional velocity

Recessional velocity is the rate at which an astronomical object is moving away, typically from Earth. It can be measured by shifts in spectral lines or estimated by general reddening of a galactic spectra.

Galaxies outside our galaxy often have their recessional velocity calculated relative to the cosmic microwave background.

Application to cosmology

Recessional velocity is most pertinent to distant galaxies, which (due to Hubble's Law) redshift proportionally to their distance from the Earth. The redshift is usually interpreted as due to recessional velocity, which can be calculated according to the formula:

$v=H_{0}D\$

where $H_{0}$ is the Hubble constant, $D$ is the intervening distance in Mpc, and $v$ is the recessional velocity, generally measured in km/s.

The recessional velocity of a galaxy is usually calculated from the redshift observed in its emitted electromagnetic radiation. The distance to the galaxy is then estimated using Hubble's Law.