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Cosmic Background Radiation

Fossil radiation

In 1965, Arno Penzias and Robert Wilson announced the discovery of microwave radiation which uniformly filled the sky and had a blackbody temperature of about 3.5K. The pair had been testing a new radio amplifier that was supposed to be exceptionally quiet. What better way to do such a test than to tune the radio so that it should hear nothing at all? After many attempts to account for all extraneous sources of radio noise, they came to the conclusion that there was a general background of radiation at the radio frequency they were using. After discussions with a group led by Robert Dicke at nearby Princeton University it became clear that they had in fact detected remnant radiation from the origin of the universe.

Although neither Dicke's group or Penzias and Wilson realized it at the time they had confirmed a prediction made 17 years earlier by Alpher, Bethe, and Gamow. Although the temperature that characterized the detected radiation was somewhat different than predicted, the difference could be accounted for by changes to the accepted structure of the universe discovered between 1948 and 1965. The detection of this radiation and its subsequent verification at other frequencies was taken as confirmation of a central prediction of a cosmology known as the big bang.

The interpretation of the red-shifts of spectral lines in distant galaxies by Edwin Hubble 40 years earlier suggested a universe that was expanding. One interpretation of that expansion was that the universe had a specific origin in space and time. Such a universe would have a very different early structure from the present one.

It was George Gamow and colleagues who suggested that the early phases of the universe would have been hot and dense enough to sustain nuclear reactions. Following these initial phases, the expanding universe would eventually cool to the point at which the dominant material, hydrogen, would become relatively transparent to light and radio waves. We know that for hydrogen, this occurs when the gas reaches a temperature of between 5,000K–10,000K. From that point on in the evolution of the universe, the light and matter would go their separate ways.

As every point in the universe expands away from every other point, any observer in the universe sees all objects receding from him or her. The faster moving objects will appear at greater distances by virtue of their greater speed. Indeed, their speed will be directly proportional to their distance which is what one expects for material ejected from a particular point in space and time. However, this expansion results from the expansion of space itself and should not be viewed simply as galaxies rushing headlong away from one another through some absolute space. The space itself expands.

As it does, light traveling through it is stretched, becoming redder and appearing cooler. If one samples that radiation at a later date it will be characteristic of radiation from a much cooler source. From the rate of expansion of the universe it is possible to predict what that temperature ought to be. Current values of the expansion rate are completely consistent with the current measured temperature of about 2.7K. The very existence of this radiation is strong evidence supporting the expanding model of the universe championed by Gamow and colleagues and disparagingly named the "big bang" cosmology by Sir Fred Hoyle.

Since its discovery in 1965, the radiation has been carefully studied and found to be a perfect blackbody as expected from theory. Since, this radiation represents fossil radiation from the initial big bang, any additional motion of Earth around the Sun, the Sun around the galactic center, and the galaxy through space should be reflected in a slight asymmetry in the background radiation. The net motion of Earth in some specific direction should be reflected by a slight Doppler shift of the background radiation coming from that direction toward shorter wavelengths.

Doppler shift is the same effect that the police use to ascertain the velocity of approaching vehicles. Of course there will be a similar shift toward longer wavelengths for light coming from the direction from which we are receding. This effect has been observed indicating a combined peculiar motion of Earth, Sun, and galaxy on the order of 600 km/sec.

Finally, small fluctuations in the background radiation are predicted which eventually led to the formation galaxies, clusters of galaxies. Such fluctuations have been found by the CO(smic) B(ackground) E(xplorer) Satellite, launched by NASA in 1989. COBE detected these fluctuations at about 1 part in 105 which was right near the detection limit of the satellite. The details of these fluctuations are crucial to deciding between more refined models of the expanding universe. COBE was decommissioned in 1993, but scientists are still unraveling the information contained in its data.

It is perhaps not too much of an exaggeration to suggest that cosmic background radiation has elevated cosmology from enlightened speculative metaphysics to an actual science. We may expect developments of this emerging science to lead to a definitive description of the evolutionary history of the universe in the near future.

George W. Collins, II


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—A blackbody (not to be confused with a black hole) is any object which absorbs all radiant energy which falls upon it and subsequently re-radiates that energy. The radiated energy can be characterized by a single dependent variable, the temperature. That temperature is known as the blackbody temperature.

Doppler shift

—The change in frequency or wavelength resulting from the relative motion of the source of radiation and the observer. A motion of approach between the two will result in a compression of the waves as they pass the observer and a rise in "pitch" in the frequency of the wave and a shortening of the relative wavelength called a "blue shift." A relative motion of recession leads to a lowering of the "pitch" and a shift to longer "redder" wavelengths.

Microwave radiation

—Electromagnetic radiation that occurs in the wavelength region of about 0.4 in to 3.3 f (1 cm to 1 m).

Additional topics

Science EncyclopediaScience & Philosophy: Cosine to Cyano group