Hubble’s law states that the distance to a galaxy is proportional to its recession velocity. Mathematically, this can be stated as
where Ho is the Hubble constant.
The Hubble’s equation reformed the field of cosmology as it indicated that galaxies beyond our neighbourhood gathering were all moving away from us. This, thus suggested in the past they were nearer together, for example, the universe started in a hot, thick state. The value of Ho gives us a measure of the age of the universe!
Because we can measure the recession velocity of galaxies very accurately indeed we can infer the distance to galaxies that all appear to us as existing on a flat plane – there is no sense of depth in the night sky. We measure this velocity using the red-shift of the light we receive and the uncertainty is very low.
In effect, this law gave us (a) a sense of scale and (b) a sense of time in cosmology.