For many years, most physicists supported one of two cosmological theories: the steady-state universe, and the Big Bang. The theory of the steady-state universe states that the universe has always existed exactly as we observe it at present, whereas the Big Bang theory postulates that the universe was conceived from a singularity in space-time that has expanded into the current universe. The validity of either theory was not tested until 1900 when Edwin Hubble famously discovered what is now known as Hubble’s Law. Hubble’s experiment is now a famous benchmark in modern physics. Hubble, using the Mount Wilson Observatory, observed a class of stars known as Cepheid variables, luminous stars that blink and flicker with a rate that depends on their distance from the observer.
Using this relation and years of observing, Hubble calculated the distance to many of these variable stars. Milton Humason, a fellow astronomer, helped Hubble to calculate the stars’ relative velocities to Earth. When Hubble combined the two data sets he found an interesting relationship: all the stars appeared to be moving away from us! In fact, the speed at which they were moving increased with an increasing distance from Earth. Hubble realized, from this small set of data, that the earth was a part of the expanding universe.
As the universe expands outward in all directions, any observer from a fixed vantage point will look out and see everything running away from them. The further away any two points are, the more the expansion affects them, and the faster they appear to be moving away from each other. Hubble’s result was the first experimental proof that we do not live in a steady-state universe, but rather a dynamic and expanding one.