Cosmology is one of the great success stories of contemporary physics. A few investigators began theorizing about the history of the universe in the 1940s, but there was precious little observational evidence to work with. The one big experimental fact was that the distant galaxies are receding, as inferred from Doppler shifts in their spectral lines. Because there was so little opportunity to test theories against experimental evidence, cosmology was considered by many physicists to be too speculative to be taken seriously.

Three atomic spectral lines of hydrogen, as measured in the lab. The dashed lines show how these lines in the light of a distant galaxy are shifted toward the red end of the spectrum. (image courtesy of HyperPhysics, by Ron Nave, Georgia State University)

Arno Penzias and Robert Wilson, in front of their microwave antenna at Bell Labs. They won a Nobel Prize for the discovery of the microwave background (photo courtesy of Lucent Technologies, Inc.)

All that changed in the mid-1960s when two radio astronomers from Bell Labs observed a persistent background radio noise from all directions in the sky, which they could not explain. They soon learned that cosmologists (See Catch a Cosmic Microwave) had predicted just this kind of noise as a remnant of the Big Bang fireball, hypothesized to be the beginning of the universe. Subsequent investigations over a wide range of frequencies confirmed this interpretation and firmly established the Big Bang theory as a model well-supported by experiment.

But a serious problem remained, for the observed background appeared to be uniform in all directions. Since the background is the relic of the fireball radiation, then that would have been uniform too, and this uniformity would have prevented matter from clumping together through gravitation to form stars and galaxies. Theorists became uneasy, since they had expected that the increasing precision of these measurements would reveal irregularities in the background between neighboring patches of sky.

More sensitive detectors, placed on artificial satellites, came to the rescue. In 1992 the Cosmic Background Explorer (COBE) satellite began taking data from space, above most of the disturbances of the atmosphere, and revealed the long-awaited granularity in the microwave background, as shown in the image. The results enabled cosmologists to explain the observed distribution of matter in the universe using the concept of “inflation,” the extreme expansion of space immediately after the Big Bang explosion.

The microwave sky as imaged by COBE. The variations in color correspond to temperature differences in the microwave background between neighboring patches of sky. (image courtesy of NASA).