Penzias and Wilson Discover Cosmic Microwave Background Radiation

Astronomers Arno A. Penzias and Robert W. Wilson discovered that the universe is filled with a uniform background radiation, consistent with the view that the universe began with a so-called big bang.

Summary of Event

In 1961, Arno A. Penzias had completed his doctoral thesis on the use of masers Masers (microwave amplification by stimulated emission of radiation) to amplify and then measure the radio signal from intergalactic hydrogen. Penzias noted in his Nobel Prize lecture that the equipment-building went better than the observations. Charles Hard Townes Townes, Charles Hard , who won the 1964 Nobel Prize in Physics for his work on masers and lasers (light amplification by stimulated emission of radiation), had worked for Bell Telephone Laboratories Bell Telephone Laboratories some years earlier and suggested that Penzias consider working there. Penzias wanted to use Bell’s 6-meter horn-shaped radio antenna to continue the observations he began in his dissertation, but the antenna was being used for communications. While waiting for the horn antenna, he pursued more practical projects at Bell. Cosmic microwave background radiation
Microwaves, cosmic
Astronomy;background radiation
[kw]Penzias and Wilson Discover Cosmic Microwave Background Radiation (1963-1965)
[kw]Wilson Discover Cosmic Microwave Background Radiation, Penzias and (1963-1965)
[kw]Cosmic Microwave Background Radiation, Penzias and Wilson Discover (1963-1965)
[kw]Radiation, Penzias and Wilson Discover Cosmic Microwave Background (1963-1965)
Cosmic microwave background radiation
Microwaves, cosmic
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[g]North America;1963-1965: Penzias and Wilson Discover Cosmic Microwave Background Radiation[07500]
[g]United States;1963-1965: Penzias and Wilson Discover Cosmic Microwave Background Radiation[07500]
[c]Astronomy;1963-1965: Penzias and Wilson Discover Cosmic Microwave Background Radiation[07500]
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Penzias, Arno A.
Wilson, Robert W.
Dicke, Robert H.

Robert W. Wilson was also using masers to amplify weak astronomical radio signals. Wilson participated in the making of a map of the radio signals from the Milky Way. He also wanted to use the horn antenna to do pure research in radio astronomy. Bell enabled Penzias and Wilson to spend half of their time engaged in applied research.

The horn antenna Radio astronomy;telescopes
Telescopes;radio was originally designed in 1960 to collect and amplify the weak radio signals bounced off large balloons high in the earth’s atmosphere. Called Echo Echo system , this early telecommunications system was used to send radio signals over very long distances. Telstar, the first telecommunications satellite that amplified incoming signals, marked the end of the usefulness of the Echo system. Penzias and Wilson were eager to begin using the antenna because the electronics designed for the Echo system were very sensitive to weak signals, the same kind of signals that come from space. Other characteristics unique to the antenna made it the world’s most sensitive radio telescope.

In 1963, in preparation for making their delicate observations, Penzias and Wilson began to identify and measure the various sources of “noise” in the antenna. One source of noise was the thermal noise of the antenna itself. The electrons in the atoms of the antenna underwent random thermal motion which generated weak radio signals. Radio astronomers speak of the “temperature” of the antenna when referring to the amount of thermal noise produced by the antenna. The higher the temperature, the greater the thermal motion of the electrons; thus, the greater intensity of the noise produced. Penzias and Wilson adopted the standard language of astronomers in speaking of the temperature of the antenna when referring to the intensity of the radio signals they measured: The greater the intensity of the radio signals, the greater the temperature.

E. A. Ohm Ohm, E. A. , one of the engineers on the Echo project, noted in 1961 an excess noise of 3 Kelvins. Little notice was taken of this observation, since the amount of discrepancy was small enough not to upset the functioning of the Echo project. Identifying and eliminating such excesses was crucial, however, for the kinds of sensitive astronomical observations Penzias and Wilson intended to make.

They soon ran into a serious problem that, because it refused to disappear, eventually led to Nobel Prizes Nobel Prize in Physics;Robert W. Wilson[Wilson]
Nobel Prize in Physics;Arno A. Penzias[Penzias] for Penzias and Wilson in 1978. To calibrate their system, they constructed a “cold load.” This device, an artificial source of radio waves cooled by liquid helium, served as a reference source for the amount of noise produced by the electrical circuits. By switching from the signal coming from the antenna to that coming from the cold load, the effect of the noise coming from the electronics was eliminated, since the amount of electrical noise was the same in both cases. The problem was that the signal from the antenna was about 3 Kelvins higher than that from the cold load.

Penzias and Wilson spent much time and energy trying to track down the source of the excess noise. They ruled out human-made sources of noise by pointing the antenna at New York City and noticing no change. They ruled out radiation from the Milky Way and from extraterrestrial radio sources. They evicted a pair of pigeons that had taken up residence in the antenna. No change in the amount of noise was seen, even after the antenna had been cleansed of pigeon droppings. They put metallic tape over the riveted joints of the antenna, yet noticed no change.

By now it was spring, 1965, and more than a year had passed since the first measurement of the excess noise. Two additional sources of noise were ruled out because of the long period of observation. First, any source in the solar system would have exhibited variation as Earth moved in its orbit, yet no variation was seen. Second, if the excess noise came from the remnant of a 1962 above-ground nuclear test, then the noise should have decreased as the radioactivity decreased. No change, however, was seen.

The answer to their problem was that there was no instrument error or random noise. What they had measured was a uniform radio signal in the microwave region of the spectrum coming from all directions. They began to realize that a theoretical explanation for their measurements was possible after a telephone call to Bernard Burke Burke, Bernard at the Massachusetts Institute of Technology in which the unexplained noise was mentioned. Burke said that he recalled hearing of the work of Phillip James Edwin Peebles Peebles, Phillip James Edwin , then working with Robert H. Dicke at Princeton. Penzias and Wilson received a preprint of Peebles’s paper that calculated that the universe should be filled with a background radiation of about 10 Kelvins (later revised downward). This radiation was thought to be the aftermath of the hot and highly condensed first few minutes in the life of the universe—the so-called big bang Big bang theory
Astronomy;cosmology . The model developed by Peebles was of an oscillating universe—periodic expansion and contraction.

Wilson was trained by those who believed in the “steady state” model of the universe. In this model, as the universe expanded, new matter is continually created to keep the separation between galaxies the same. In the steady state model, there is no beginning of the universe. Wilson, in particular, was not convinced that Peebles’s explanation was the only one possible, while the group led by Dicke at Princeton was enthusiastic about the observations. The two groups agreed to publish their results in two articles in the Astrophysical Journal. In their paper, Penzias and Wilson limited themselves to a discussion of their observations. They left a theoretical explanation to the Princeton group.


Penzias and Wilson’s measurement of the cosmic microwave background radiation represented an interesting case study in the history of science. On numerous occasions for at least twenty years before the 1965 measurements, both theoreticians and experimentalists had run across “evidence” for the 3-Kelvin cosmic microwave background radiation. After completing their measurements, Penzias and Wilson learned of the work of George Gamow Gamow, George and others in the late 1940’s, which led to a prediction of 5 Kelvins for the background radiation. Astrophysicists in the Soviet Union and England, working independently of Peebles, performed calculations that also indicated about 5 Kelvins for the background radiation. Steven Weinberg Weinberg, Steven , best known for his work on elementary particles, presented three reasons why the experimentalists and theorists were so far apart: There were some serious problems with the assumptions made in the work done by Gamow; most theorists prior to 1965 were under the mistaken impression that a background temperature of 3 Kelvins could not be measured with existing instruments; and most physicists could not take the theory of the early universe seriously, so no search was made for the remnants of the big bang.

Experimentalists also had measured the cosmic background radiation before 1965, but none appreciated the significance of the observations. Probably the most ironic of these measurements was by Dicke himself in the 1940’s. His measurement of the maximum background cosmic radiation was a by-product of his research on the absorption of radio signals by the earth’s atmosphere. By the 1960’s, he had forgotten about his own measurements made twenty years earlier.

Immediately after the publication of their results, Penzias and Wilson’s mood was one of cautious optimism; they were not willing to accept immediately the theoretical interpretation put forth for their measurements nor were they completely convinced of the accuracy of their own measurements. One reason for skepticism was that they had measured the temperature at only one wavelength. By mid-1966, other experimentalists had measured the intensity of the microwave background radiation at a variety of other wavelengths and all results were close to 3 Kelvins. Another issue was the uniformity of the observed radiation. The early phase of the big bang that created the radiation now observed was exceedingly uniform, so if the radiation was truly the remnant of the big bang, then measurements of the equivalent temperature made in many different directions must be the same. By the early 1970’s, no differences could be found in the equivalent temperature when measured in different directions.

Penzias and Wilson’s measurement of the cosmic microwave background radiation has been called one of the most important scientific discoveries of the twentieth century. While it may be true that the importance of the measurements has been overstated by some proponents, the demonstration of the cosmic microwave background radiation, combined with the earlier demonstration by Edwin Powell Hubble that the galaxies are receding, provided very strong evidence for the big bang model of the universe. By the mid-1970’s, a new name had been coined for the big bang model—astronomers simply referred to it as the “standard model.”

Weinberg has noted that “in the 1950s, the study of the early universe was widely regarded as not the sort of thing to which a respectable scientist would devote his time.” It is not that the steady state model or any other model held sway in the astrophysical community, it is merely that there was insufficient experimental evidence or theoretical justification for the notion of the early universe. In the decades after Penzias and Wilson’s measurement, the big bang model was fleshed out by the work of many other physicists. The early universe now had become a respectable field in which to work. Cosmic microwave background radiation
Microwaves, cosmic
Astronomy;background radiation

Further Reading

  • Bernstein, Jeremy. Three Degrees Above Zero: Bell Labs in the Information Age. New York: Charles Scribner’s Sons, 1984. Presents a dual profile of Penzias and Wilson in the last section of the book. Written for general readers, this book contains very little technical language. The first three parts discuss Bell Labs’ research on computers, solid-state physics, and telephony.
  • Burke, Bernard F., and Francis Graham-Smith. An Introduction to Radio Astronomy. 2d ed. New York: Cambridge University Press, 2002. A survey of the history, methodology, and discoveries of radio astronomy.
  • Davies, Paul. The Edge of Infinity: Where the Universe Came from and How It Will End. New York: Simon & Schuster, 1981. Presents the accepted ideas of modern physics and cosmology. Requires no detailed background knowledge of either physics or mathematics, though many of the ideas require some mind-bending imagination. Bases most of the exposition on diagrams and pictures.
  • Gribbin, John. In Search of the Big Bang: Quantum Physics and Cosmology. New York: Bantam Books, 1986. Wide ranging, this book is a readable account of the modern view of cosmology. Although Penzias and Wilson take up only part of chapter 5, the rest of the book puts their work and the big bang theory into perspective.
  • Silk, Joseph. The Big Bang. Rev. ed. New York: W. H. Freeman, 2001. Using moderately technical language (but no mathematics), this book describes the big bang theory of cosmic evolution. Numerous diagrams. Updated from a 1980 edition.
  • Singh, Simon. Big Bang: The Origin of the Universe. New York: Fourth Estate, 2004. Comprehensive history of the big bang theory, beginning with an overview of cosmological theories from antiquity to 1900, delving into the science behind the big bang, and concluding with a discussion of unresolved issues. Includes descriptions of the competing claims of big bang and solid-state theories of cosmology.
  • Weinberg, Steven. The First Three Minutes: A Modern View of the Origin of the Universe. 1977. Updated ed. New York: Basic Books, 1993. Written for general readers. The chapter “A Historical Diversion” deals with all the missed opportunities involving the discovery of the microwave cosmic background radiation. Discusses Penzias and Wilson in several sections.
  • Wilson, Robert. “The Cosmic Microwave Background Radiation.” In Les Prix Nobel, 1978. Stockholm: Almqvist and Wiksell International, 1978. Clear and concise account of how the cosmic microwave background radiation was discovered and the events that happened after that discovery. One of the few Nobel Prize lectures in physics that is written using nontechnical language.

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