Earthsky Interviews
Edwin Hubble and the expanding universe
03-15-2005 - Space
A hundred years ago, most astronomers believed that the whole universe consisted of just one galaxy, our own Milky Way.
That was until astronomers were able to pick out individual stars in distant galaxies – and watch how they behave.
In the 1920s, Edwin Hubble observed stars that vary in brightness in a patch of light known at the time as the Andromeda nebula. He knew that these stars changed in brightness in a way that depended on their true brightness. He then saw how bright they looked – to find the distance to the Andromeda nebula. He showed that this patch of light was really a separate galaxy beyond the Milky Way.
Soon other nebulae were revealed as separate galaxies, and the known universe got much bigger.
But another mystery involved the light of galaxies as a whole. Their light was shifted toward the red end of the light spectrum. This red shift was interpreted as a sign that the galaxies are moving away from us. Hubble and his colleagues compared the distance estimates to galaxies with their red shifts. And – on March 15, 1929 – Hubble published his observation that the farthest galaxies are moving away faster than the closest ones.
This insight became known as Hubble’s Law. It was the first recognition that the galaxies are moving away from each other – that our universe is expanding.
Hubble was a multi-talented man. Although he majored in science as an undergraduate at the University of Chicago, a promise to his dying father caused him to take up a study of the law. He was also an amateur heavyweight boxer, and reportedly turned down the chance to fight professionally. He returned to science as a graduate student at Yerkes Observatory in Wisconsin. In 1919, he accepted a position at the prestigious Mount Wilson Observatory.
But his story really begins earlier. In 1908, an astronomer named Henrietta Leavitt had discovered a relationship between the period and luminosity of a class of pulsating stars called Cepheid variables. By timing its period one could work out the true luminosity of a Cepheid – and by comparing the true luminosity with the observed brightness one could work out its distance. This worked fine for judging distances inside the Milky Way, but it wasn’t until the 1920s that telescopes existed that were powerful enough to observe Cepheids in other galaxies. Hubble spotted his first Cepheid in the Andromeda ’spiral nebula’ in 1924.
It was Vesto Slipher of Lowell Observatory whose study of ’spiral nebulae’ showed that these objects exhibit red shifts. Hubble and Humason observed the Cepheids in 18 of Slipher’s objects – now known to be distant galaxies – using the new 100-inch Hooker Telescope at Mt. Wilson in California.
It’s said that Albert Einstein was elated to hear of Hubble’s work. Einstein’s Theory of Relativity implied that the universe must either be expanding or contracting. But Einstein himself rejected this notion in favor of the accepted idea that the universe was stationary and had always existed. When Hubble presented his evidence of the expansion of the universe, Einstein embraced the idea. He called his adherence to the old idea ‘my greatest blunder.’