# Hubble’s Law Explained

Hubble’s law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.

The motion of astronomical objects due solely to this expansion is known as the Hubble flow. . It is often expressed by the equation v = H0D, with H0 the constant of proportionalityâ€”Hubble constantâ€”between the “proper distance” D to a galaxy, which 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’. )