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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.
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:
where is the Hubble constant, is the proper distance, and is the recessional velocity. The recessional velocity of a galaxy (or any cosmological object) at a particular distance is also termed as Hubble velocity.
Several factors can influence the calculated distance including peculiar velocities and extinction based reddening.
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