By 1929, enough galaxy redshifts had been measured for the cosmologist
Edwin Hubble to make a study of whether a galaxy’s redshift depends on
its distance from us. Although measuring a galaxy’s redshift is relatively
easy, and can be done with high precision, measuring its distance is difficult.
Hubble knew z for nearly 50 galaxies, but had estimated distances for only
20 of them. Nevertheless, from a plot of redshift (z) versus distance (r),