Cosmic Background Radiation

Fossil radiation

Cosmic background radiation (usually abbreviated CMB, but also CBR, and sometimes called relic radiation) is a long wavelength type of electromagnetic radiation that is believed by cosmologists to show evidence of left over energy from the big bang, a theory for the possible explosion that began the universe. CMB impinges onto the Earth, and other celestial bodies, uniformly in all directions.

In 1965, German-born American physicist Arno Allan Penzias (1933–) and American physicist Robert Woodrow Wilson (1936–) announced the discovery of microwave radiation that uniformly filled the sky and had a blackbody temperature of a little less than three degrees Kelvin (3K). The pair had been testing a new radioamplifier 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 American physicist and inventor Robert Henry Dicke (1916–1997) at nearby Princeton University (New Jersey) 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 American physicist Ralph Asher Alpher (1921–), German-born American physicist Hans Bethe (1906–2005), and Ukrainian-born American physicist and cosmologist George 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 American astronomer Edwin Hubble (1889–1953) 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 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. Scientists know that for hydrogen, this occurs when the gas reaches a temperature of between 5,000K to 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 away. 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 later 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 2.725K. 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 Hoyle.

Fossil radiation

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

KEY TERMS

Blackbody— A blackbody (not to be confused with a black hole) is any object that absorbs all radiant energy that 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 ft (1 cm to 1 m).

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. There will be a similar shift toward longer wavelengths for light coming from the direction that the Earth is receding. This effect has been observed indicating a combined peculiar motion of the 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 of galaxies and clusters of galaxies. Such fluctuations have been found by the COBE (COsmic Background Explorer) satellite, launched by NASA in 1989. COBE detected these fluctuations at about 1 part in 105 that was 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. Cosmologists 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