Hubble’s law is the name for the observation in physical cosmology that: (1) objects observed in deep space (extragalactic space, ~10 megaparsecs or more) are found to have a Doppler shift interpretable as relative velocity away from the Earth; and (2) that 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.This is normally interpreted as a direct, physical observation of the expansion of the spatial volume of the observable universe.
The motion of astronomical objects due solely to this expansion is known as the Hubble flow.Hubble’s law is considered the first observational basis for the expanding space paradigm and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.
|v||recessional velocity (km/s)|
|H0||Hubble's constant (km/s/Mpc)|
|D||proper distance (which can change over time, unlike the comoving distance which is constant) from the galaxy to the observer (Mpc)|