Galaxies and the Expanding Universe
In 1929 US astronomer Edwin Hubble first noticed that distant galaxies were moving away from us. He also realized that the further away they were, the faster they were moving and that this relationship was constant.
The gradient of this plot is known as the Hubble Parameter, or Hubble Constant is defined in the following equation
Limitations of Hubble’s Law
By using Hubble's equation, it is possible to determine a galaxy's distance from us, once we have measured its velocity of recession. This is calculated by determining by how much the observed light has been shifted into the red part of the spectrum. There are, however, several factors that make this measurement problematic: