ABSTRACT
The key observation giving rise to the Big Bang theory was made by Edwin Hubble in the 1920s. This is that the universe is expanding. The basic building block of the universe is the galaxy cluster, and clusters of galaxies are observed to be receding at a rate proportional to their distance-the further away they are, the faster they are moving. Although the details are somewhat technical, the principle by which this law is obtained is easy to explain. Quite simply, the distance of an object is determined from its brightness-the further away, the fainter it will be. In addition the light from an object moving away from us will be shifted to the red, and this redshift enables us to determine its speed of recession.
