Although the intensity of the CMBR is extremely uniform in all directions,with fluctuations measured at only 1 part in 10⁵, the local distribution of galaxies is extremely irregular, withfluctuations in the density of galaxies per volume of space beingwell in excess of 100 percent. Maps of the distribution of galaxiesin space reveal a remarkable pattern of thin, filamentary structuresconnecting small and large central concentrations of galaxies, punctuatedby large, quasi-spherical voids. The example of the map shown inFigure 2 (p. 4) is the result of several years of painstaking spectroscopicobservations with modest-size optical telescopes. The far-flung distributionof galaxies in the universe, the complex assemblage of clusters,filaments, and voids, is referred to as large-scale structure.

It is not surprising that galaxies are clustered. As explained above,the early universe contained small density irregularities, as measuredby fluctuations in the CMBR, and the amplitude of these small bumpsgrew via their self-gravity to make the structure seen today. Thiscondition of gravitational instability can amplify the initial densityfluctuations of seeds on all scales. Galaxies and large-scale structureare all part of the same process; both are relics of the Big Bang.

Finding clumps of galaxies was thus expected, but their huge extentcaught astronomers by surprise. Typical voids are 200 million light-yearsacross, and one enormous curtain-like structure—the Great Wall—is draped acrossthe universe in a span half a billion light-years across. Even thislarge size, however, is less than a tenth the scale measured by theCOBE satellite discussed above. Altogether, the distances involvedin the study of large-scale structure range over a factor of a million,from the size of galaxies to the CMBR anisotropy measured by theCOBE satellite. This combination of observations gives us a powerfulprobe of Big Bang density. fluctuations over a wide range of scales.The extent of early density fluctuations on different size scalesand their subsequent growth under gravity are critical clues to thenature and amount of dark matter in the universe, as explained below.

Making maps of galaxies in three dimensions requires knowing howfar away each galaxy is from Earth. One way to get this distanceis to use Hubble's law for the expansion of the universe. Hubblediscovered that the velocity at which two galaxies recede from eachother is proportional to the distance between them. Inverting thisrelation yields an estimate of distance from observed velocity. Thevelocity with which a galaxy is receding from us is obtained by measuringthe shift to redder colors of spectral features in its spectrum,a “redshift” analogous to the familiar Doppler shift in the frequency of soundwaves from a receding source. The greater the redshift, the largerthe velocity, and, by Hubble's law, the larger the distance.

We are truly living in the age of mapping the universe. The lastdecade has seen a revolution in the technology of light detectorsthat has made it possible to measure redshifts rapidly, even withmodest-size telescopes. In 1976, there were only 2,700 galaxies withmeasured redshifts—now there are 100,000. By the year 2000 astronomersexpect 1 million! This field of astronomy is still on a steep discoverycurve.

The first step in making a redshift survey is compiling a catalogof galaxy positions and brightnesses on the sky. Traditionally suchcatalogs have been based on photographic surveys taken in visiblelight. We are learning, though, that even small biases in the listof target galaxies may have a big effect on the final maps. Hencethere is strong interest in new and