Hubble’s Law Tells Us That

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 proportionality—Hubble constant—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’. )
