1929

Hubble’s Law

Edwin Hubble (1889–1953), Vesto Slipher (1875–1969)

The 1848 discovery of the Doppler Shift of Light provided a tool that astronomers could use to determine the velocity of an astronomical object relative to Earth. All that was required was the ability to detect and measure the shift of a suitable absorption line or lines using spectroscopy. In 1912 the Lowell Observatory astronomer Vesto Slipher obtained the first spectra of spiral nebulae and other objects that were later recognized as other galaxies. Slipher discovered that most spiral nebulae have spectral lines Doppler-shifted toward longer (redder) wavelengths—they are red-shifted and receding from us.

The American astronomer Edwin Hubble was also interested in studying the spiral nebulae, and beginning in 1919 he had the advantage of access to the brand-new 100-inch (254-centimeter) Hooker reflecting telescope at the Mount Wilson Observatory in Southern California—then the largest and most sensitive telescope in the world. Hubble studied Slipher’s galaxy red-shift data and spent a decade painstakingly collecting more data of his own.

In 1929 Hubble published a landmark paper describing his initial results. He had found that, remarkably, the red shift of the galaxies increases the farther away a galaxy is from the Earth. Everything seemed to be moving away from us, and the farthest stuff the fastest. The implication of this observation, which has become known as Hubble’s law, is that the volume of the observable universe is expanding. This amazing result was consistent with earlier theoretical predictions about the expansion of space-time by the Russian cosmologist Alexander Friedmann, based on Albert Einstein’s theory of general relativity.

Edwin Hubble at work on the 100-inch (2,540-millimeter) telescope at Mt. Wilson Observatory.

Hubble’s law means that space was smaller in the past. Today, cosmologists interpret the data to indicate that all of space and time—the universe as we know it—began in an enormous explosion called the Big Bang, some 13.7 billion years ago. Hubble had profoundly changed our understanding of the cosmos.

The Hubble Space Telescope, named in honor of the discoverer of the expanding universe, took this unprecedentedly sensitive photograph during more than 11 days of exposure on the same tiny patch of the sky in 2004. Almost every dot and smudge in this photo is a galaxy!