Hubble's law
The law is often expressed by the equation , with H0 the constant of proportionality (the Hubble constant) between the "proper distance" D to a galaxy and its velocity v (see Uses of the proper distance). H0 is usually quoted in (km/s)/Mpc, which gives the speed in km/s of a galaxy  away. The reciprocal of H0 is the Hubble time.