Why do we need the formula #v=H_oD# to measure the recessional velocity of a galaxy when instead we can just do #v=(dD)/(dT)#?
Hubble's formula is a correlation formula and is not in mathematical exactitude. The velocity of a galaxy is estimated by the Doppler effect. Galaxies appear to be receding from one another.
Velocity as rate of change of distance with respect to time is applicable to continuous variation. It stops working at discontinuities. Doppler effect: Light emitted by a source is shifted in wavelength, by the motion of light source..