Cosmic Background Radiation

Chapte:r 7 Astrophysics and Magnetospheric Physics Cosmic Background Radiation The interest of Bell Labs scientists in both astrophysics and the physics of the magnetosphere stems from a desire to understand the factors affecting electromagnetic propagation in the earth's atmosphere. The contributions to astrophysics began in the early 1930s, soon after sensitive antennas were built to gather data on radio propagation. Serious studies of the earth's magneto sphere did not begin until three decades later when Bell Labs launched the first Telstar communications satellite. K. G. Jansky's investigation of the source of noise affecting transatlantic radio propagation gave rise to the birth of the astrophysics field of radio astronomy. The research of A. A. Penzias and R. W. Wilson in the 1960s on microwave background radiation had strong impact on modern cosmology. Other research contributions by Bell Labs scientists to astrophysics include the development and use of highly sen sitive detectors, which led to the discovery of many molecules in the inter galactic space as well as to gamma-ray astronomy and the search for gravity waves. The studies of the magnetosphere dealt with the earth's radiation environ ment produced by energetic electrons and protons trapped in distinct belts around the earth and how these are affected by solar flares. Satellite investi gations are complemented by magnetic data on ultra-low-frequency variations in the earth's magnetic field, obtained from magnetometers placed in experi mental stations in Antartica and at high northern latitudes. I. ASTROPHYSICS Studies in astrophysics at Bell Laboratories were motivated by the need to understand sources of interference in radio communication. In the 1920s and early 1930s interest was focused in the 105 to 108 Principal authors: L. J. Lanzerotti, R. A. Linke, and R. W. Wilson 269 270 Engineering and Scienc~~ in the Bell System hertz (Hz) region of the electromagnetic spectrum, covering the range of AM and FM radio-frequency bands. With the development of radio-relay microwave applications to cross-country communication in the 1940s and the beginning of exploration of electromagnetic propa gation in the circular waveguide, interest shifted to the study of pos sible sources of interference in the microwave and millimeter wavelength (109 Hz to 1012 Hz) regions. The two major fields of astrophysics in which Bell Labs scientists played leading roles-radio astronomy and the experimental confirmation of the "Big Bang" theory of the origin of the universe-can be traced directly to the Bell System's involvement in communications in these two broad regions of the electromagnetic spectrum. 1.1 Radio Astronomy- Early Observation of Galactic Radio Noise The science of radio astronomy was initiated in 1932 by K. G. Jan sky, who had been studying the sources of noise on high-frequency-20 m•~gahertz (MHz)-transatlantic radio circuits. He had built a large, highly directive antenna on a turntable that rotated three times per hour, in order to resolve the angle of arrival of the noise. [Fig. 7-1] The received signal strength was recorded on a chart recorder. Jansky also listened to determine the character of the noise, and discov~!red that there were three major sources of noise: the familiar crackling noise from nearby thunderstorms, distant thunderstorms, and a continuous hiss-like noise whose intensity varied in a pattern that almost exactly repeated from one day to the next.1 In further investigation of the continuous hiss component, Jan sky discovered that its maximum occurred four minutes earlier each day and after a year had shifted by a full twenty-four hours. A. M. Skellett of Bell Laboratories, a Ph.D. astronomy student at Princeton University, showed him that this is what would be expected from a source associated with the fixed stars.2 In a later paper, Jansky showed that the hiss-type static came from the plane of the Milky Way and that the maximum came from the direction of its center.3 This first radio-astronomical observation was published and publi cized, but astronomers of the day did not fully appreciate its signific:ance. Part of the problem was that the astmnomers did not understand that the sltrength of the hiss that Jansky reported in microvolts/meter actually corresponded to a thermal temperature many times greater than the value of lO,OOOK that they would have expected from ionized hydrogen regions.4 It was only after World War II, when the radar developers took an interest, that the science of radio astronomy began to blossom. Astrophysics and Magnetospheric Physics 271 Fig. 7-1. K. G. Ja nsky in front of the a ntenna he used to discover radio waves coming from space. 1.2 Microwave Background Noise Studies- Impact on Cosmology The cosmic microwave background radiation, considered a relic of the explosive beginning of the universe, was discovered in 1965 by A. A. Penzias and R. W. Wilson [Fig. 7-2] at Bell Labs' Crawford Hill, 5 6 New Jersey, site. • Penzias and Wilson received the Nobel Prize in physics in 1978 for their work in this area. The discovery was made with a uniquely sensitive microwave receiving system, originally built for receiving signals bounced from the Echo balloon-the world's first communications satellite. Two developments formed the key components in the Echo receiving system. The first was a traveling-wave maser amplifier with an intrinsic noise temperature of only a few degrees kelvin? The other was the 20-foot horn reflector antenna,8 whose most important property in this application was its very effective rejection of radiation from the backward direction, which reduced the pickup of ground radiation to negligible levels (approximately 0.1K). [Fig. 7-3] In 1963, when the Echo receiving system was no longer needed for satellite work, preparations were made to use the 20-foot horn reflector for radio astronomy. At that time a maser amplifier from project Telstar, operating at a microwave wavelength of 7.35 centime- 272 Engineering and Science in the Bell System Fig. 7-2. R. W. Wilson Cleft) and A. A. Penzias, discoverer· of cosmic background radiation, direct evidence for the "Big Bang" theory of cosmology. Fig. 7-3. The 20-foot horn-reflector antenna used in the di scovery of microwave background radiation. Astrophysics and Magnetospheric Physics 273 ters (em), was installed and an accurate measurement of its sensitivity was underway.9 A radio astronomy receiver (radiometer) incorpora ting the maser was designed to make accurate measurements. It con tained two additional unique features: a liquid-helium-cooled :refer ence noise source whose equivalent noise temperature (a convenient measure of its output power) was SK and could be accurately calcu lated;10 and a waveguide switch that allowed very accurate compari sons of the intensity of the radiation from the antenna and the refer ence noise source. The first radio astronomy project used the mea sured gain of the 20-foot horn reflector to make precise measurements of the intensities of several bright radio sources that had traditionally been used as radio-astronomical calibrators.11,12 The results oJf this work also provided the data for a convenient and widely used method of measuring the sensitivity of satellite earth stations. When the radiometer was put into operation, the first measure ments showed that the equivalent antenna temperature was about 7K-hotter than the reference noise source by about 2K. The antenna temperature should have been only 3.3K-the sum of the atmospheric contribution (2.3K) and the radiation from the walls of the antenna and ground (1K). This would have been 1.7K less than the cold load. During the period when accurate source measurements were being made (almost a year), the excess antenna temperature was found to be constant, independent of direction in the sky, polarization, and sea son. At the end of that period an intensive effort to find the physical origin of excess temperature was undertaken. Success came in an unexpected way when contact was made with R. H. Dicke and his group at Princeton University. Although not the first to do so, Dicke predicted the existence of such all-pervasive radi ation as a result of a postulated "Big Bang" origin of the universe. (A review of early theories of the "Big Bang"-type universe appears in a paper by Penzias.13) Dicke also suggested that the radiation would be observable, and his group set out to make a measurement. However, measurements by Penzias and Wilson gave a value of the temperature of the microwave background and suggested that the expected radia tion from the early universe was present. Subsequent measurements have borne this out.14 The discovery of cosmic microwave background radiation has had a dramatic effect on the science of cosmology. Not only has it provided direct evidence for a "Big Bang" type of cosmology, it serves as a probe of conditions in very early stages of the universe. In 1966, a second well-calibrated radiometer was built for the 20- foot horn reflector. The radiometer was capable of receiving the 21- cm microwave line of atomic hydrogen, the more important of the two then-known radio frequency lines. A background temperature measurement was made with this radiometer,l5 adding to the 274 Engineering and Science in the Bell System confirmation of the thermal spectrum of the background; it was then used for several additional measurements.16 A new lower limit to the amount of intergalactic neutral hydrogen was obtained, confirming that neutral hydrogen

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