The law is often expressed by the equation , with `` H `` 0 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 ' ) .
The law is often expressed by the equation , with `` H `` 0 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 '' ) .