Arno Penzias The Nobel Prize in Physics 1978

autobiography

The existence of the CMB radiation was first predicted by George Gamow in 1948, and by Ralph Alpher and Robert Herman in 1950. It was first observed inadvertently in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in Murray Hill, New Jersey. The radiation was acting as a source of excess noise in a radio receiver they were building. Coincidentally, researchers at nearby Princeton University, led by Robert Dicke and including Dave Wilkinson of the WMAP science team, were devising an experiment to find the CMB. When they heard about the Bell Labs result they immediately realized that the CMB had been found. The result was a pair of papers in the Physical Review: one by Penzias and Wilson detailing the observations, and one by Dicke, Peebles, Roll, and Wilkinson giving the cosmological interpretation. Penzias and Wilson shared the 1978 Nobel prize in physics for their discovery.

Today, the CMB radiation is very cold, only 2.725° above absolute zero, thus this radiation shines primarily in the microwave portion of the electromagnetic spectrum, and is invisible to the naked eye. However, it fills the universe and can be detected everywhere we look. In fact, if we could see microwaves, the entire sky would glow with a brightness that was astonishingly uniform in every direction. The picture at left shows a false color depiction of the temperature (brightness) of the CMB over the full sky (projected onto an oval, similar to a map of the Earth). The temperature is uniform to better than one part in a thousand! This uniformity is one compelling reason to interpret the radiation as remnant heat from the Big Bang; it would be very difficult to imagine a local source of radiation that was this uniform. In fact, many scientists have tried to devise alternative explanations for the source of this radiation but none have succeeded. Why study the Cosmic Microwave Background?

Since light travels at a finite speed, astronomers observing distant objects are looking into the past. Most of the stars that are visible to the naked eye in the night sky are 10 to 100 light years away. Thus, we see them as they were 10 to 100 years ago. We observe Andromeda, the nearest big galaxy, as it was about 2.5 million years ago. Astronomers observing distant galaxies with the Hubble Space Telescope can see them as they were only a few billion years after the Big Bang. (Most cosmologists believe that the universe is between 12 and 14 billion years old.)

The CMB radiation was emitted only a few hundred thousand years after the Big Bang, long before stars or galaxies ever existed. Thus, by studying the detailed physical properties of the radiation, we can learn about conditions in the universe on very large scales, since the radiation we see today has traveled over such a large distance, and at very early times. The Origin of the Cosmic Microwave Background

One of the profound observations of the 20th century is that the universe is expanding. This expansion implies the universe was smaller, denser and hotter in the distant past. When the visible universe was half its present size, the density of matter was eight times higher and the cosmic microwave background was twice as hot. When the visible universe was one hundredth of its present size, the cosmic microwave background was a hundred times hotter (273 degrees above absolute zero or 32 degrees Fahrenheit, the temperature at which water freezes to form ice on the Earth's surface). In addition to this cosmic microwave background radiation, the early universe was filled with hot hydrogen gas with a density of about 1000 atoms per cubic centimeter. When the visible universe was only one hundred millionth its present size, its temperature was 273 million degrees above absolute zero and the density of matter was comparable to the density of air at the Earth's surface. At these high temperatures, the hydrogen was completely ionized into free protons and electrons.

Since the universe was so very hot through most of its early history, there were no atoms in the early universe, only free electrons and nuclei. (Nuclei are made of neutrons and protons). The cosmic microwave background photons easily scatter off of electrons. Thus, photons wandered through the early universe, just as optical light wanders through a dense fog. This process of multiple scattering produces what is called a “thermal” or “blackbody” spectrum of photons. According to the Big Bang theory, the frequency spectrum of the CMB should have this blackbody form. This was indeed measured with tremendous accuracy by the FIRAS experiment on NASA's COBE satellite.

This figure shows the prediction of the Big Bang theory for the energy spectrum of the cosmic microwave background radiation compared to the observed energy spectrum. The FIRAS experiment measured the spectrum at 34 equally spaced points along the blackbody curve. The error bars on the data points are so small that they can not be seen under the predicted curve in the figure! There is no alternative theory yet proposed that predicts this energy spectrum. The accurate measurement of its shape was another important test of the Big Bang theory. “Surface of Last Scattering”

Eventually, the universe cooled sufficiently that protons and electrons could combine to form neutral hydrogen. This was thought to occur roughly 400,000 years after the Big Bang when the universe was about one eleven hundredth its present size. Cosmic microwave background photons interact very weakly with neutral hydrogen.

The behavior of CMB photons moving through the early universe is analogous to the propagation of optical light through the Earth's atmosphere. Water droplets in a cloud are very effective at scattering light, while optical light moves freely through clear air. Thus, on a cloudy day, we can look through the air out towards the clouds, but can not see through the opaque clouds. Cosmologists studying the cosmic microwave background radiation can look through much of the universe back to when it was opaque: a view back to 400,000 years after the Big Bang. This “wall of light“ is called the surface of last scattering since it was the last time most of the CMB photons directly scattered off of matter. When we make maps of the temperature of the CMB, we are mapping this surface of last scattering.

As shown above, one of the most striking features about the cosmic microwave background is its uniformity. Only with very sensitive instruments, such as COBE and WMAP, can cosmologists detect fluctuations in the cosmic microwave background temperature. By studying these fluctuations, cosmologists can learn about the origin of galaxies and large scale structures of galaxies and they can measure the basic parameters of the Big Bang theory.

I was born in Munich, Germany, in 1933. I spent the first six years of my life comfortably, as an adored child in a closely-knit middle-class family. Even when my family was rounded up for deportation to Poland it didn't occur to me that anything could happen to us. All I remember is scrambling up and down three tiers of narrow beds attached to the walls of a very large room, and then taking a long train trip. After some days of back and forth on the train, we were returned to Munich. All the grown-ups were happy and relieved, but I began to realize that there were bad things that my parents couldn't completely control, something to do with being Jewish. I learned that everything would be fine if we could only get to "America". In the late spring of 1939, shortly after my sixth birthday, my parents put their two boys on a train for England; we each had a suitcase with our initials painted on it, as well as a bag of candy. They told me to be sure and take care of my younger brother. I remember telling him, "jetzt sind wir allein" as the train pulled out.

My mother received her exit permit about a month later (just a few weeks before the war broke out) and was able to join us in England. My father had arrived in England almost as soon as the two of us, but we hadn't seen him because he was interned in a camp for alien men. The only other noteworthy event in the six or so months we spent in England, awaiting passage to America, occurred one morning in a makeshift schoolroom. At that moment, I suddenly realized that I could read the open page of the (English) school book I had been staring at. We sailed for America toward the end of December 1939 on the Cunard liner Georgic - using tickets that my father had foresightedly bought in Germany a year and half earlier. The ship provided party hats and balloons for the Christmas and New Year's parties, as well as lots of lifeboat drills. The grey three-inch gun on the aft deck was a great attraction for us boys. We arrived in New York in January of 1940. My brother and I started school and my parents looked for work. Soon they became "supers" (superintendents of an apartment building). Our basement apartment was rent free and it meant that our family would have a much-needed second income without my mother having to leave us alone at home. As we got older and things got better, we left our "super" job and my mother got a sewing job in a coat factory; my father's increasing wood-working skills helped him land a job in the carpentry shop of the Metropolitan Museum of Art. As job pressures on him eased, he found time to hold office in a fraternal insurance company as well as to serve as the president of the local organization of his labor union.

It was taken for granted that I would go to college, studying science, presumably chemistry, the only science we knew much about. "College" meant City College of New York, a municipally-supported institution then beginning its second century of moving the children of New York's immigrant poor into the American middle class. I discovered physics in my freshman year and switched my "major" from chemical engineering to physics. Graduation, marriage and two years in the U.S. Army Signal Corps, saw me applying to Columbia University in the Fall of 1956. My army experience helped me get a research assistantship in the Columbia Radiation Laboratory, then heavily involved in microwave physics, under I.I. Rabi, P. Kusch and C.H. Townes. After a painful but largely successful struggle with courses and qualifying exams, I began my thesis work under Professor Townes. I was given the task of building a maser amplifier in a radio-astronomy experiment of my choosing; the equipment-building went better than the observations.

In 1961, with my PhD thesis complete, I went in search of a temporary job at Bell Laboratories, Holmdel, New Jersey. Their unique facilities made it an ideal place to finish the observations I had begun during my thesis work. "Why not take a permanent job? You can always quit," was the advice of Rudi Kompfner, then Director of the Radio Research Laboratory. I took his advice, and remained a Bell Labs employee for the next thirty seven years. Since the large horn antenna I had planned to use for radio-astronomy was still engaged in the ECHO satellite project for which it was originally constructed, I looked for something interesting to do with a smaller fixed antenna. The project I hit upon was a search for line emission from the then still undetected interstellar OH molecule. While the first detection of this molecule was made by another group, I learned quite a bit from the experience.

In order to make some reasonable estimate of the excitation of the molecule, I adopted the formalism outlined by George Field in his study of atomic hydrogen. To make sure that I had it right, I took my calculation to him for checking. One of the factors in that calculation was the radiation temperature of space at the line wavelength, 18-cm. I used 2 K, a somewhat larger value than he had used earlier, because I knew that at least two measurements at Bell Laboratories had indications of a sky noise temperature in excess of this amount, and because I had noticed in Gerhard Hertzberg's "Spectra of Diatomic Molecules" that interstellar CN was known to be excited to this temperature. The results of this calculation were used and then forgotten. It was not until Dr. Field reminded me of them in December of 1966 that I had any recollection of my momentary involvement with what was later shown (by Field and others) to be observational astronomy's first encounter with the primordial radiation that permeates our Universe. In the meantime, others at Bell Labs pressed the horn antenna into service for another satellite project. A new Bell System satellite, TELSTAR, was due to be launched in mid-1962. While the primary earth station at Andover, Maine, was more or less on schedule, it was feared that the European partners in the project would not be ready at launch time, leaving Andover with no one to talk to. As it turned out, fitting the Holmdel horn with a 7-cm receiver for TELSTAR proved unnecessary; the Europeans were ready at launch time. This left the Holmdel horn and its beautiful new ultra low-noise 7-cm traveling wave maser available to me for radio astronomy. This stroke of good fortune came at just the right moment. A second radio astronomer, Robert Wilson, came from Caltech on a job interview and was hired. After finishing separate projects, we set to work early in 1963.

In putting our radio astronomy receiving system together we were anxious to make sure that the quality of the components we added were worthy of the superb properties of the horn antenna and maser that we had been given. We began a series of radio astronomical observations, ones that I had proposed so as to make the best use of the careful calibration and extreme sensitivity of our system. Of these projects, the most technically challenging was a measurement of the radiation intensity from our galaxy at high latitudes. This multi-year endeavor, which resulted in our discovery of the cosmic microwave background radiation, is described in Wilson's Nobel lecture. When our 7-cm program was accomplished, we converted the antenna to 21-cm observations, including another microwave background measurement, as well as galactic, and intergalactic, atomic hydrogen studies. During this period, I took on a visiting position in Princeton's Astrophysical Sciences Department, thereby enabling me to propose and supervise graduate student research projects in radio astronomy. Like so many others in similar positions, I feel that I learned far more from my students than I could possibly have taught them.

As time went on, opportunities for front line work that we could do with our facility became rarer. Much larger radio telescopes existed, and they were being fitted with low-noise parametric amplifiers whose sensitivities began to approach that of our maser system. As a result, I began looking for new ways of exploring the radio sky. In those days, the portion of the radio spectrum short-ward of one cm wavelength was not yet available for line radio astronomy owing to equipment limitations. At Bell Laboratories, however, many of the key components required for such work had been developed for communications research purposes. With Keith Jefferts, a Bell Labs atomic physicist, Wilson and I assembled a millimeter-wave receiver which we carried to a precision radio telescope built by the National Radio Astronomy Observatory at Kitt Peak, Arizona, early in 1970. Using this new technique, we discovered and studied a number of interstellar molecular species, thereby revealing the rich and varied chemistry which exists in interstellar space. Millimeter-wave spectral studies have proven to be a particularly fruitful area for radio astronomy, and are the subject of active and growing interest, involving a large number of scientists around the world. The most personally satisfying portion of this work for me was using molecular spectra to explore the isotopic composition of interstellar atoms - thereby tracing the nuclear processes that produced them. Most notably our 1973 discovery of DCN, the first deuterated molecular species found in interstellar space, enabled me to trace the distribution of deuterium in the galaxy. This work provided us with evidence for the cosmological origin of this unique element, which had earned the nickname "Arno's white whale". Of all the nuclear species found in nature, deuterium is the only one whose origin stems exclusively from the explosive origin of the Universe. Because deuterium's cosmic abundance serves as the single most sensitive parameter in the prediction of cosmic background radiation, these measurements provided strong support for the "Big Bang" interpretation of our earlier discovery.

In addition to my astronomical research, I always had made it my business to engage in technology-related work at Bell Labs. It seemed only reasonable to contribute to the pool of technology from which I drew upon. Similarly, Bell Labs has always contributed to, as well as used, the store of basic knowledge - as evidenced by their hiring of a radio astronomer in the first place.