Hubble's law

Hubble's law is the name for the observation in physical cosmology that:
 * 1) Objects observed in deep space - extragalactic space, 10 megaparsecs (Mpc) or more - are found to have a red shift, interpreted as a relative velocity away from Earth;
 * 2) This Doppler shift-measured velocity, of various galaxies receding from the Earth, is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.

General formula

 * v = H0D

H0 is the constant of proportionality, the Hubble constant that is between the "proper distance" D to a galaxy. It can change over time, unlike the comoving distance and its velocity v, i.e. the derivative of proper distance with respect to cosmological time coordinate. See Uses of the proper distance for some discussion of the subtleties of this definition of 'velocity'. The SI unit of H0 is s−1, but is more frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away. The reciprocal of H0 is Hubble time.