A Valuable Mapping Tool—the Red Shift
Fortunately, astronomers have a tool at their disposal to produce a three-dimensional map of the universe. It is the red shift, the fact that light emitted by an object moving away from an observer is shifted toward the red end of the spectrum.
The red shift has played an important role in astronomy throughout the 1900's. In 1929, U.S. astronomer Edwin Hubble, after learning that the light from other galaxies is shifted toward the red end of the spectrum, concluded that the universe is expanding. Analyzing the red shift of galaxies, Hubble noted further that there is a consistent, proportional relationship between a galaxy's distance and the speed of its movement away from the Milky Way. The farther away a galaxy is, he discovered, the faster it is receding from us. (The Milky Way is also moving, of course. An observer in another galaxy would see our galaxy receding at high speed.)
The relationship between a galaxy's velocity and its distance, which became known as Hubble's law, made it possible to figure the distance to any galaxy, no matter how far away, just by measuring its red shift. For example, a galaxy with a red shift indicating a velocity of 30,000 kilometers [18,600 miles] per second is about 2 billion light years away. A galaxy receding at twice that speed is twice as far away.
This discovery has been invaluable to astronomers, but analyzing the light from distant galaxies to obtain their red shifts is time consuming work. Therefore, making three-dimensional maps of the universe–or red shift surveys, as they are usually called—has proceeded at a much slower pace than earlier mapping efforts. By 1993, astronomers had measured the red shifts of about 40,000 galaxies.
The largest red shift survey has been carried out by a research team led by astronomers Margaret Geller and John Huchra at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. By 1993, they had mapped the volume of space within about 500 million light-years of the Milky Way—only about 3 percent of the distance we can see.
Also in 1993, scientists at several U.S. institutions—the University of Chicago; the Fermilab accelerator laboratory in Batavia, Ill.; Johns Hopkins University in Baltimore; Princeton University in Princeton, N.J.; and the Institute for Advanced Study, also in Princeton—were working on an ambitious project to map the positions of about 1 million galaxies. Known as the Sloan Digital Sky Survey, the project will map 25 percent of the sky to a depth of 2.5 billion light-years from the Milky Way. The survey, which is expected to last 10 years, will be carried out with a specially designed 2.5-meter (100-inch) telescope being constructed in New Mexico. The telescope will be linked to an array of electronic recording instruments that will measure 600 red shifts at once.
