Ken Tapping, September 14, 2011

The expanding universe was probably the largest astronomical discovery of the 20th Century. Many books and articles give the credit to Edwin Hubble, the famous astronomer after whom the Hubble Space Telescope is named. However, this is not entirely correct. What he did was combine other data with his observations to describe that expansion better than anyone had done before.

The story starts with Albert Einstein, who came up with a revolutionary new description of time and space, called the General Theory of Relativity. One of the things buried in his equations was a description of the universe. However, finding this description and applying it to observations had to wait until the mid 1920's, when Monseigneur Georges Lemaître, a senior Jesuit, got to work digging deeply into Einstein’s equations.

Lemaître discovered that the equations described a universe, but not the one that the astronomers of the time understood. Most of them believed that we lived in a static, unchanging universe. Lemaître found that such a universe would be catastrophically unstable; the slightest disturbance would make it expand or collapse. When he looked at observational data, he concluded that the universe was expanding, and suggested that it started billions of years ago as a small, dense, hot lump, which then started to grow. Lemaître found that the further galaxies are from us, the faster they are being carried away by the expanding universe. He went on to estimate the relationship between the distance and the speed of the expansion. Strangely, this equation is now known as “Hubble’s Law” and the parameter describing the relationship between speed and distance as “Hubble’s Constant”. 

Edwin Hubble's arrival on the astronomical scene coincided with the opening of the 100-inch (2.5-m) telescope on Mount Wilson, California. He used this telescope to detect Cepheid variable stars in distant galaxies. These stars vary cyclically in brightness, and the duration of a cycle indicates how luminous the star is. It is relatively easy to measure the duration of the cycles and estimate the luminosity of the star. One more simple operation is to use how luminous the star is, and how bright it appears in the night sky, to determine how far away it is, as well as the distance of its host galaxy.

Vesto Slipher and Milton Humason had been determining the speeds with which distant galaxies are moving away from us. Edwin Hubble compared their data with his distance measurements, and got the relationship Lemaitre had already suggested, but much more accurately. 

Prior to Hubble’s work, it was generally assumed that our Milky Way Galaxy was all there was, and those fuzzy spiral things in the images were new planetary systems in the process of forming. Hubble was instrumental in showing that those spirals are actually a very long way away and are galaxies in their own right. Surprisingly, even though Hubble helped quantify the expansion of the universe, he did not really believe in it. He instead demonstrated how a scientist should let his results speak more loudly than any opinion or personal preference.