Hubble finds proof that the universe is expanding
The two keys to Edwin Hubble's breakthrough discovery were forged by others in the 1910s.
The first key, the period-luminosity scale discovered by Henrietta Leavitt, allowed astronomers to calculate the distance to variable stars from Earth. Hubble had already used this knowledge in his 1924 discovery that the Andromeda nebula, containing a variable star, was more than 900,000 light years from Earth -- way beyond our own galaxy -- a surprise to everyone at the time. With this scale and other tools, Hubble had found and measured 23 other galaxies out to a distance of about 20 million light years.
The second key was the work of Vesto Slipher, who had investigated the spiral nebulae, before Hubble's Andromeda discovery. These bodies emit light which can be split into its component colors on a spectrum. Lines then appear in this spectrum in particular patterns depending on the elements in the light source. Yet if the light source is moving away, the lines are shifted into the red part of the spectrum. Analyzing the light from the nebulae, Slipher found that nearly all of them appeared to be moving away from Earth. Slipher knew that a shift toward red suggested the body was moving rapidly away from the observer. But he had no way to measure the distances to these reddish bodies.
Hubble's brilliant observation was that the red shift of galaxies was directly proportional to the distance of the galaxy from earth. That meant that things farther away from Earth were moving away faster. In other words, the universe must be expanding. He announced his finding in 1929. The ratio of distance to redshift was 170 kilometers/second per light year of distance, now called Hubble's constant. The numbers were not exactly right, and refinements in measuring techniques and technology have changed all of Hubble's early figures. But not the basic principle. He himself kept working on the problem and collecting data throughout his career.
Some view Hubble's discovery as the most important event in astronomy in the century. It made the most basic change in our view of the world since Copernicus 400 years ago. His results showing that the universe was expanding supported a theory that had been proposed by Georges LeMaitre in 1927. A universe expanding, much like the aftereffect of an explosion, must have once been "unexploded," a single mass in time and space.