II. THE COSMIC MICROWAVE BACKGROUND RADIATION

What Is the Cosmic Microwave Background Radiation?

The cosmic microwave background radiation (CMBR), discovered in 1964,is a telltale remnant of the early universe. Its very existence iscompelling evidence that the universe has evolved from an extraordinarilyhot, compact beginning. To have produced radiation with the characteristicsof the CMBR, the universe must at one time have been entirely differentfrom what astronomers see today. No galaxies, stars, or planets existed:the universe was filled with elementary particles and radiation atextremely high energies.

The universe is between 8 billion and 15 billion years old. For allof that time, it has been expanding and the CMBR has been cooling.Currently, the radiation temperature is 2.73 K, which means thatmost of the CMBR exists now as radio energy in the microwave band.Man-made microwaves of the same sort link communication satellitesto stations on Earth. But there are two major differences betweensatellite microwaves and the CMBR: First, the CMBR comes from alldirections rather than from only one spot in the sky. Second, theCMBR has its power distributed over a wide range of microwave frequenciesrather than concentrated at a single frequency, as is the case fora radio transmitter. To get accurate information about the earlyuniverse, cosmologists must measure the CMBR over a wide range offrequencies and across most of the sky.

From such measurements, cosmologists believe that the CMBR has beenlargely unchanged, except for cooling down, during the entire historyof the universe. The complex evolution of matter in the universe—such as the formation of stars, galaxies, and large-scale structure—did not affect the CMBR. This radiation is a pristine cosmic remnant.It gives us a wonderful opportunity to look far back in time to studyeven fine details of the early universe. As cosmologists try to understandthe origin and evolution of structure in the universe today, it isessential to know about physical conditions that existed long ago.

What Do We Learn by Measuring the Properties of the CMBR?

The spectrum

Since the discovery of the CMBR, cosmologists have made measurementsof its intensity at different wavelengths—its spectrum. The Big Bangtheory predicts that the remnant radiation will have a special kindof spectrum, a thermal spectrum. The thermal spectrum has a characteristicshape, and the wavelength corresponding to the “peak” depends on the temperatureof the emitting body. The CMBR (at a temperature of 2.73 K) peaksat 2-mm wavelength; the Sun's thermal spectrum (6,000 K) peaks ata visible wavelength. Years of ground-based and balloon-based observationstraced out a crude spectrum that tended to support the Big Bang theory.However, it became clear in the mid-1970s that truly decisive measurementsof the CMBR needed to be done from space, above Earth's obscuringand bright (at these wavelengths) atmosphere. NASA's Cosmic BackgroundExplorer (COBE) satellite, which was launched in November 1989, wasspecifically designed to make accurate measurements of the CMBR.The first scientific result from the COBE satellite was an exquisitelyaccurate measurement of the CMBR spectrum. The spectrum matched thethermal shape, just as the Big Bang theory had predicted. The dataand the prediction are shown in Figure 1 (p. 1). This result provides strong support for the Big Bang theory.

The shape of the spectrum seen in Figure 1 has a distinguished history in physics for reasons not related tocosmology. Early in this century, Max Planck and others reluctantlyintroduced quantum physics to explain this same spectrum, emittedby all cavities at uniform temperature, regardless of the kind ofmaterial used to make the cavity. This same thermal spectrum nowturns out to match the



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