You can easily demonstrate that an observer in galaxy 2 or galaxy 3 will
find the same linear relation between observed recession speed and distance,
with ˙a/a playing the role of the Hubble constant. Since this argument can
be applied to any trio of galaxies, it implies that in any universe where the
distribution of galaxies is undergoing homogeneous, isotropic expansion, the
velocity – distance relation takes the linear form v = Hr, with H = ˙a/a.