Zeroing in on the blackbody.

The flux of radio microwaves that pervades the universe and arrives at the earth from all sides has been an object of intense interest to cosmoligists since its discovery two decades ago. Because it is believed to be left over from the earliest moments of the universe's beginning, it may tell something about conditions at that time. Over the years measurements at more and more wavelengths have strengthened the cinviction that the spectrum is that of a perfect thermal radiator, a blackbody.

The clinching evidence should come at short wavelengths, where the curvature that distinguishes a blackbody spectrum from other possibilities should clearly show up. A recent measurement in this range by Jeffrey B. Peterson and Paul Richards of the University of California at Berkeley and Thomas Timusk of McMaster University in Hamilton, Ontario, shows a very close fit to the spectrum of a blackbody at a temperature of 2.78 kelvins, which is what the background should be. The measurement, which is particularly difficult because the atmosphere absorbs radiation in this frequency range, was done from a balloon floating 24 miles above the ground.

About six years ago, Richards and David P. Woody did a similar measurement of th is range of the background spectrum (SN: 7/7/79, p. 4). Although they found a general conformation to the 2.78 K blackbody, there were deviations from exactness. One interpretation of those deviations was that the universe was somehow a more efficient emitter of radiation than a blackbody -- a contadication in terms, as a blackbody is defined as a perfect thermal radiator. The other horn of the dilemna was to suppose that something other than a thermal radiator was contributing and thereby complicating the cosmological evidence. The present measurement lays those questions to rest.

Richards, Peterson and Timusk plan another balloon flight to try to refine the measurement still further. In 1987 NASA plans to launch the Cosmic Background Explorer Satellite (COBE), which will take advantage of the better observing conditions in space to probe this and other questions about the background.