Discovering the Expanding Universe
Beginning about 10 years earlier, an astronomer at the Lowell Observatory in Flagstaff, Ariz., Vesto M. Slipher, had begun obtaining data that would prove useful to Hubble in his next great discovery. Studying the light from spiral “nebulae”—their true nature as galaxies had not yet been determined by Hubble—Slipher found that in almost every case the light was shifted toward the red end of the spectrum.
This phenomenon, called the red shift, had been used by astronomers since the 1800's to measure the speeds of stars orbiting the center of the Milky Way. The red shift occurs because light waves emitted by a rapidly receding source become stretched out as they move away from the source. The stretching effect increases the wavelength of the light. Longer wavelengths fall toward the red end of the spectrum. In contrast, light waves from a source moving rapidly toward an observer are compressed and shifted toward the short wavelength blue end of the spectrum.
Hubble was intrigued with Slipher's finding that the light from the spiral nebulae (spiral galaxies) was red shifted. This could only mean, he said in a 1929 research paper, that all the other galaxies are rushing away from us at high speed. The universe, as Einstein's theory had predicted, was expanding.