The redshift "z" is often described as a "redshift velocity", which is the recessional velocity that would produce the same redshift "if" it were caused by a linear Doppler effect (which, however, is not the case, as the shift is caused in part by a cosmological expansion of space, and because the velocities involved are too large to use a non-relativistic formula for Doppler shift). This redshift velocity can easily exceed the speed of light. In other words, to determine the redshift velocity "vrs", the relation: