Who found it first?

Weekly astronomy column from Ken Tapping

  • Sep. 14, 2011 5:00 p.m.

The expanding universe was probably the biggest 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 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 middle 1920’s, when Monseigneur Georges Lemaitre, a senior Jesuit, got to work digging deeply into Einstein’s equations.

He found they described a universe, but not the one that the astronomers of the time thought we lived in. Most of them believed we live in a static, unchanging universe. Lemaitre 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. Lemaitre 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 “Hubbleís 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 tells 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 looks in our sky to determine how far away that star is, and of course how far away its host galaxy is.

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 Hubbleís work, it was generally assumed that our Milky Way galaxy was all there is, and those fuzzy spiral things in the images were new planetary systems in the process of forming. Hubble was instrumental in showing those spirals are 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.

Jupiter rises around 10 p.m. Mars comes up around 2 a.m. The Moon will reach last quarter on the 20th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, B.C.























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