The theory requires the relation v = H D to hold at all times, where D is the proper distance, v is the recessional velocity, and v , H , and D vary as the universe expands (hence we write H 0 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 v . For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one.