The theory requires the relation formula_9 to hold at all times, where formula_7 is the comoving distance, "v" is the recessional velocity, and formula_6, formula_12, and formula_7 vary as the universe expands (hence we write formula_8 to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity formula_6. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected.