Arno Penzias Encountering cosmology

Arno Penzias shared the 1978 Nobel Prize in Physics with Robert W. Wilson for their discovery of the CMBR. He joined the staff of Bell Laboratories after graduate school and remained there until retiring as Vice President of Research and Chief Scientist. He then joined New Enterprise Associates, a Silicon Valley venture capital firm, where he advises emerging companies in the fields of information technology and alternative energy sources.

My first serious brush with cosmology came in 1958, when Charles Townes accepted me as one of the students in his radio astronomy group at Columbia University. My project was to be the first maser-based astronomical study of 21-cm line emission from neutral atomic hydrogen. At that time, the only known source of radio line radiation, neutral atomic hydrogen, had by then been studied by several groups of observers. Since my system would yield an order of magnitude improvement in sensitivity over the best systems available to other radio astronomers, it seemed to me that I could extend trails already blazed in interesting directions. All I had to do was pick the most interesting body of prior work.

The choice of this observing project stemmed from a review of the then current radio astronomy literature, most notably a special (January 1958) issue of the Proceedings of the Institute of Radio Engineers devoted entirely to radio astronomy. From the first, I was most taken by an article (Heeschen and Dieter 1958) that addressed an interesting puzzle: clusters of galaxies appeared to contain more mass (determined from dynamical studies) than could be accounted for by the sum of the masses of their constituent objects. According to the data reported in this article, that discrepancy could be accounted for by the large amounts of neutral atomic hydrogen observed within each of the clusters investigated by the authors. Having selected an H I survey of clusters of galaxies as my target, I proceeded to design the maser preamplifier and other components that I would need, to create a low-noise radiometer for my radio telescope - an 85-ft parabolic antenna owned by the US Naval Research Laboratory (NRL). (In practice, that meant installing my equipment on a mount that could only be reached by a 40-ft scaffold, and servicing it with cryogenic liquids. Small wonder then, that I later became so attracted to the cozier geometry of the Bell Labs' horn-reflector antennas.)

In order to stabilize my system against gain fluctuations, I employed a scheme in which the maser input was switched between the antenna's feed horn and an attenuator immersed in the same helium bath that cooled my maser preamplifier. When finally completed and installed on the radio telescope, the system performed perfectly, yielding scans across the sky with unprecedented sensitivity, limited only by the thermal noise expected from the system. To my dismay, however, my data showed little, if any, trace of the hydrogen the literature promised. I made further searches at longer integration times to improve sensitivity, but found nothing more than traces of continuum radiation from individual galaxies. By then, my time on the telescope had run out, leaving me with enough to qualify for my PhD, but far less data than I had hoped for.

With my degree, and hands-on knowledge of how best to apply cryogenic radiometry to microwave radio astronomy, I applied for a temporary job at Bell Radio Research Laboratory, an organization in which David C. Hogg was then a leading contributor, and began working there in the late spring of 1961. At that time, this group's satellite communications research infrastructure made it the best place to continue my project and bring it to a more satisfactory conclusion. In his essay, Dave Hogg describes that project in the broader context of the work which surrounded it, together with accounts of the first glimpses of the CMBR.

When I arrived at Bell Labs early in May of 1961, the 20-ft horn-reflector was still being used in the last stages of the Echo satellite project (Figure 4.9). In the interim, preparing for my planned project left me with time to complete the write-up of my thesis, and to initiate a search for line emission from interstellar OH radicals, using the same horn-reflector that Dave Hogg and his collaborators (De Grasse et al. 1959) had used in the pioneering 5-cm studies recounted in his section.

During this time, I also helped my engineering colleagues by applying radio astronomy techniques to solve a series of technical problems - starting with devising a way to calibrate the pointing accuracy of satellite receiving systems by tracking radio astronomy sources as they moved across the sky.

Fig. 4.9. The 20-ft "home-made" horn-reflector antenna in the foreground designed by A. B. Crawford (head of the Bell Labs department I joined in 1961) served as the Lab's receiving station for Echo, the world's first communications satellite project. Unlike this horn antenna, whose response pattern is tightly grouped about its main beam, the commercial 60-ft (diameter) parabolic antenna, shown in the background, picks up an appreciable amount of radiation from the ground, via spillover of the (downward-facing) feed horn located at its focus.

Fig. 4.9. The 20-ft "home-made" horn-reflector antenna in the foreground designed by A. B. Crawford (head of the Bell Labs department I joined in 1961) served as the Lab's receiving station for Echo, the world's first communications satellite project. Unlike this horn antenna, whose response pattern is tightly grouped about its main beam, the commercial 60-ft (diameter) parabolic antenna, shown in the background, picks up an appreciable amount of radiation from the ground, via spillover of the (downward-facing) feed horn located at its focus.

In the pointing project, I made use of the fact that Bell Labs experimental satellite receiving systems were designed to function as radiometers as well as receivers - so as to provide a convenient means of measuring each system's sensitivity (normally expressed in units of equivalent noise temperature), as well as a way of monitoring atmospheric attenuation. As a result of this work, most early commercial satellite receiving systems were also configured to operate in a radiometric mode. In that way, operators could use celestial radio sources as reference objects for antenna pointing as well as measuring overall sensitivity. This practical work allowed me to stay connected to the work going on around me, even though the majority of my time continued to be spent on radio astronomy.

In the meantime, the 13-cm Echo receiver was removed from the 20-ft antenna and replaced by a 7-cm receiver - the wavelength employed by Tel-star, the follow-on satellite project to Echo - thereby delaying my access to that antenna until shortly after Telstar's successful launch in July of 1962. At that point, the Holmdel horn, and its new ultra low-noise 7-cm traveling wave maser, became available for radio astronomy - subject only to the concurrence of local management: Rudi Kompfner, the director of our Laboratory. All I had to do was give a seminar-like talk outlining the research topics that seemed most interesting. Reasons to use the 7-cm system before moving to 21cm seemed almost self-evident. Two-wavelength measurements of astronomical objects (most notably our own Galaxy) with the same instrument would yield valuable spectral information. 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. In addition to finishing our separate projects, we set to working together early in 1963.

At that time, Bob was also working with Dave Hogg, who had come up with a novel way of measuring the effective collecting area of the Andover antenna (AT&T's primary satellite ground station). The idea was to measure our 20-ft horn by means of a helicopter-borne source, use that calibration to measure the absolute flux of strong "radio stars," and then use the antenna temperature obtained with the Andover antenna with those sources, to determine the collecting area of that antenna (Hogg and Wilson 1965).

This addition to our program appears to have left an indelible mark on the folklore of cosmology. Once the relatively elaborate helicopter data had been collected, we were unable to modify the antenna in any way, until the related flux measurements of discrete radio astronomy sources (intended as intermediate flux standards) had been completed. As a result, while we were able to evict a pair of band-tailed pigeons from their preferred resting place in the throat of our antenna, removing all signs of their prior presence had to be deferred for several weeks after the start of our observations. Once we had measured the flux densities of Cassiopeia A (Cas A) and the other discrete radio sources whose absolute fluxes we wished to establish as calibration objects for future use, we cleaned the throat of bird droppings and found, as expected, no measurable increase in antenna efficiency, and only a minor diminution in antenna temperature.

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, including the 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. In particular, we needed to resolve the uncertainty surrounding the seeming extraneous sources of system noise encountered by several of our Bell Labs colleagues, and described in Dave Hogg's section (beginning on page 70).

This multi-year endeavor, which resulted in our discovery of the CMBR, is described in detail in Bob Wilson's (1979) article on the subject. Briefly, we spent most of 1963 converting the horn to radio astronomy. A mechanically based coordinate converter which allowed us to move the antenna in right ascension and declination, the cold load, a carefully built switch, and back end electronics were the main items that we added.

Since we planned to depend on our "cold load" as a noise standard (Penzias 1965), I decided to first design the microwave device I wanted, and then worry about how I might cool it. Clearly, I would use an absorber immersed in liquid helium, and connected to its (room temperature) output flange by a waveguide. Instead of the plated stainless steel generally used in cryogenic microwave spectroscopy, I opted for a meter-long section made of the well-behaved high-copper brass alloy used in AT&T's microwave radio towers because of its low attenuation. In addition to thinning the walls of the waveguide by machining away material from its outside surfaces, to reduce its thermal conductivity, I added a series of gas baffles to allow evaporating liquid helium to cool the transmission line as this gas flowed upward toward the vacuum pump connector. Calibrated thermistor diodes, attached to each of the baffles as well as other key points along the waveguide, allowed us to monitor its temperature profile - thereby allowing us to calculate the noise temperature at its output flange to greater accuracy.

Owing to the large thermal mass and size of the Dewar flask which contained the cold load, each day's fill consumed the contents of a 25-l helium container. Since each such fill lasted through a full day and night of observation, we were almost always ready to quit working well before our helium ran out. Remarkably, our local carpenter shop - headed by Carl Clausen, a long-time employee who had built the antenna that Karl Jansky had used in the 1930s - managed to build the 20-ft horn for a mere $20,000. On our part, Bob and I almost certainly spent more than that amount on liquid helium during the years we used that antenna for our observations.

Those observations began in late May of 1964 - with us working to collect data, while also tracing possible sources of the excess antenna temperature which proved to be the CMBR. By then, it seemed unlikely that the excess temperature was due to measurement errors, since three independent measurements had yielded similar results. Was it then due to the receiver, the antenna, or something outside the maser systems themselves? Our first observation exonerated the receiver. Figure 4.10 contains readings from each of the cold load's 11 thermistors, together with a temperature reading from a thermometer attached to a variable attenuator which connected the cold load to one of the two input ports of our waveguide switch.

Fig. 4.10. Record of our first 7-cm observation. Data were recorded on rolls of paper by means of a Leeds and Northrop chart recorder, with the output voltage of a detector located at the output of our radiometer plotted as a function of time. Because of the inherent stability of our system, the phase-sensitive detection used in a Dicke radiometer was not used. The switch was turned manually between the antenna feed and the reference arm (denoted as "cold load," along with the setting on the variable attenuator described in the text).

Fig. 4.10. Record of our first 7-cm observation. Data were recorded on rolls of paper by means of a Leeds and Northrop chart recorder, with the output voltage of a detector located at the output of our radiometer plotted as a function of time. Because of the inherent stability of our system, the phase-sensitive detection used in a Dicke radiometer was not used. The switch was turned manually between the antenna feed and the reference arm (denoted as "cold load," along with the setting on the variable attenuator described in the text).

The attenuator - a standard Western Electric component with its resistive absorber replaced by a much less lossy material - had a range of 0.12 dB, or about 10 K when used at room temperature.

As can be seen from the chart, the antenna temperature at 90° elevation was observed to be ~3K (~0.04dB) hotter than the noise temperature of our cold load. We knew from our prior calibration that our cold load had an output temperature of about 5 K with the attenuator set at zero - with its precise temperature determined by the physical temperature distribution of the connecting waveguide, and calculated from the thermistor readings. Since the atmosphere's contribution to the antenna temperature at zenith was about two degrees less than the physical temperature of the liquid helium bath in which the cold load's absorber was immersed - together with the fact that the antenna throat was expected to introduce roughly the same small amount of noise as that due to waveguide in the reference arm -we knew immediately that the excess noise temperature must be coming from outside our apparatus.

By the late fall of 1964, we had made all the absolute flux measurements we needed, and had exhausted an extensive list of possible terrestrial noise sources, as well as known astronomical sources. What to do? We wanted to publish our result, but were hesitant about writing a stand-alone paper. In those days, a considerable fraction of the radio astronomy literature was taken up with spurious results, and we didn't want to run the risk of having our first joint publication to be cited as totally wrong. We therefore decided to include our detection of excess temperature as a section in one of the other papers then in preparation - but fate intervened.

In December of 1964, Bernie Burke and I met at an American Astronomical Society meeting in Montreal, exchanging accounts of our work and promising to keep in touch. He called me a few weeks later (late February, as I remember it) to tell me of a talk he had heard about (from Ken Turner) - saying that there was "a guy from Princeton" with a theory predicting "ten degrees at X-band" (radio engineering jargon for the microwave band around 3-cm wavelength). Bernie's mimeographed copy of Jim Peebles' preprint arrived in my office a few days later. Sure enough, the abstract contained a prediction of 10-K radiation, confirming what Bernie had told me over the telephone. I was happy to find a theoretical explanation for our puzzling phenomenon, even though I wasn't sure that the general model described in the paper was necessarily the right one. I don't remember paying much attention to the details of the cosmological theory, other than that it mentioned a cyclical universe model, apparently proposed by Bob Dicke, who had organized an experimental search for this phenomenon at 3-cm wavelength.

I immediately picked up the phone, and was soon speaking with Bob Dicke - catching him in the middle of a meeting with Jim Peebles, Peter Roll, and Dave Wilkinson. Rather than saying that he would call me back after his meeting, as I thought he would, he and I began a conversation that lasted for some considerable time as I told him about our discovery, and the additional work that we had done in this connection in the months that had followed it. At the end of our conversation, I invited him to come and have a first-hand look at our apparatus and data, resulting in a visit by Dicke, accompanied by Peter Roll and Dave Wilkinson, to Crawford Hill shortly thereafter.

As soon as the group arrived, Bob Wilson and I brought them to the horn antenna where all five of us managed to squeeze into our control cab in order to give our visitors a first-hand look at our equipment. Bob Dicke looked over what we had, asked a few questions, nodded, and agreed that we had a real result. From there, we moved to a conference room in our main building, where I gave a presentation explaining the motivation behind this portion of our work in the context of our galactic continuum project. Apparently, I assumed more understanding of radio astronomy than the group possessed at that time, because Peter Roll remembers me talking about M31 (the nearby galaxy in Andromeda) and him thinking that we were interested in the sky background in order to aid in our measurements of that galaxy's emission. At that time, I understood that they knew more about their areas of expertise than we did, but it didn't occur to me that the inverse of my assumption (that they knew less about radio astronomy techniques than we did) could be true as well.

This latter situation became clearer when Bob and I paid a return visit to Peter and Dave's lab a short while later. Thanks to phase-sensitive detection, they had effectively eliminated the effects of random noise in their measurement. But they had done less well with systematic uncertainties -especially with their cryogenic noise standard. In particular, I remember the waveguide being covered with frost and condensed water where it emerged from a metal flange atop their liquid helium bath. To me, they seemed to be making many of the mistakes that I had made in my first encounter with such problems in my thesis experiment, and had solved in designing the "cold load" and related apparatus for our 7-cm system. On the other hand, they might just have underestimated the precision they would need, expecting a more intense level of background radiation than the level we had detected. Either way, I went through some of the ways in which they might improve their design details - an area that we hadn't touched upon during their visit to our facility.

Earlier on, toward the end of his visit to Crawford Hill, I remember discussing publication with Bob Dicke and suggesting a joint paper. For his part, Dicke refused immediately, leading me to then propose a pair of back-to-back papers in The Astrophysical Journal -the same place that our Cas A and galactic continuum papers were soon to go.

Our paper (Penzias and Wilson 1965a) consisted of a bare-bones account of our measurement - together with a list of the possible sources of interference we had eliminated - along the lines of what I would have included, had this result been a section of our paper on the 7-cm galactic continuum (Penzias and Wilson 1966). As a result, we submitted the write-up without a single mention of astronomy. We only added a sentence - stating this phenomenon could not be accounted for in terms of sources known to exist in the present universe - some days after we had sent the original version off to the journal. By the time our correction arrived however, the editor had already accepted the original version for publication. Not wishing to withdraw the paper, and replace it with a revised copy, we accepted the editor's offer of including that sentence as a "note in proof."

Notwithstanding the rapid acceptance of our paper, actual publication of the pair of CMBR papers was held up until July 1, with the issues themselves mailed out in the early fall. In the interim, another form of publication took over on May 21st, with a front-page New York Times article headlined: "Signals Imply a 'Big Bang' Universe." Walter Sullivan (1965), then the dean of American science writers, apparently had a "mole" in The Astrophysical Journal editorial office. At that time, Sullivan was hoping to get an early look at an expected submission by Allan Sandage, whom he thought was then about to report observations of particular cosmological significance.

The article reported our discovery and the prediction at Princeton, noting that: "It is clear that Dr. Dicke, and others would like to see an oscillating universe come out triumphant. The idea of a universe born 'from nothing' in a single explosion raises philosophical and well as scientific problems." At the time, however, the likelihood of resolving such cosmological issues seemed remote to me. My first reading of Jim Peebles' preprint had linked it to the cyclic model in my mind (and only later made the Gamow connection) even though Jim didn't have a strong connection to oscillation. Jim seemed to favor a cold early universe - more like the one I heard about from David Layzer later that same year. In those days, each of the principal cosmological theories seemed to be as much about personal preferences as it was about data, at least as far as those of us outside the field could tell. After all, it had taken until the mid-1950s for the Hubble age of the universe to catch up with the age of the oldest stars.

But then, the link between theory and data began to strengthen markedly once the Times article appeared. Most importantly, unexpected confirmation appeared from an unexpected direction, in the form of a trio of independent analyses by George Field and John Hitchcock (1966), Pat Thaddeus and Paul Clauser (1966), and Iosef Shklovsky (1966) - each inferring a 3-K

temperature of the CMBR at millimeter wavelengths, and all making use of published optical spectra which indicated an otherwise puzzling excitation of interstellar radicals.

Ironically, George Field and I had discussed the optical CN data and its possible connection to radio astronomy, albeit in an entirely different context. In writing up my thesis, I had found myself faced with puzzling theoretical issues I couldn't figure out on my own, so I sought help from George, who was still at Princeton in those days. Some time later, I sought George's help again in connection with a search for line emission from interstellar OH radicals. In both cases, excitation of the emitting gas came up as an issue, and I recall discussing McKellar's CN observations with him, although our memories differ a bit. I recall George mentioning it during our OH discussion, while George remembers it taking place in connection with intergalactic hydrogen. Nonetheless, George made that connection for me with respect to my spectral studies, and later connected the CN excitation phenomenon to the CMBR. For my part, I didn't. While I adopted an estimate of 2 K as the lower limit of radiative excitation for OH radicals (Penzias 1964), I assumed that this "radiative excitation" was due to starlight, that is, confined to wavelength regions much shorter than the one associated with the 17-cm and 21-cm lines studied in my observations.

I realized my oversight a short while after The New York Times article appeared, when I visited Pat Thaddeus in his office. As Pat greeted me with "There's another way of measuring the ...," I glanced down and saw Herzberg's book on the table in front of him. The pieces of the puzzle were coming together faster than I could have imagined just a few weeks earlier.

As for another piece of the puzzle, the connection to the prior work done at Bell Labs, I remember being astonished to learn about Doroshkevich and Novikov's (1964) linking of Ohm's (1961) report of his 13-cm noise measurements to the CMBR implications stemming from the "Gamow Theory." At a time when being "plugged in" usually meant being on key colleagues' preprint lists, keeping up in astronomy generally depended on participating in the informal exchanges that marked life in academic departments -something that Bell Labs couldn't be expected to provide for its radio astronomers.

Fortunately, I soon found such a connection, when Lyman Spitzer invited me to give a colloquium sponsored by Princeton's astronomy department. From that time on, I became an increasingly active participant in the science and teaching of that department - a relationship that lasted well into the l980s.

Other than the single pair of March 1965 visits already touched upon, Bob and I had little direct contact with the members of Dicke's group during the remainder of that year. In the meantime, Bob and I made the two additional 7-cm CMBR measurements described in his section, confirming our original result in both cases. By the time Peter Roll and Dave Wilkinson reported the results of the 3-cm measurements made with their reworked system, the following January (Roll and Wilkinson 1966), they had evidently solved the problems we had noticed in their earlier attempt, judging from the fact that their result produced "the right answer" - matching our 7-cm values, the earlier Bell Labs results at 5 cm and 11 cm, and the work done (on what was by then being called the "3-K radiation") at 2.3 mm from the CN results.

In the meantime, the connection with Gamow's earlier work, and the predictions that stemmed from it, gained increasing attention in the scientific community - in my case, via a personal letter from George Gamow himself. This letter (in Figure 4.11, misdated 1963, for some reason) begins by thanking me for sending him my "paper." Since this could only have referred to a

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preprint of our 1965 article in The Astrophysical Journal, which included a sentence connecting our findings to the accompanying article by Dicke et al-(1965), I assumed that someone else had sent him a mimeographed copy. In those pre-computer days, a dedicated organization in Bell Labs distributed copies of papers submitted for outside publication by means of the same system used for internal technical memos, a kind of paper-based "Google" that employees could search by looking through an index based on authors and topics, and then "downloading" content for delivery via our company's internal mail service. In addition to the copies that Bob and I sent to colleagues, some of our colleagues in the Physical Sciences Division likely sent copies to some of their friends as well.

As I noted earlier, the connection to the predictions cited in Gamow's letter had been made by others even before the events recounted above. As time went on, and the agreement between theory and data grew stronger, many of us began to wonder why the measurements involved hadn't been attempted earlier. In my case (Penzias 1979a), I went so far as to attribute Ralph Alpher's apparent endorsement of Gamow's position (that bolometric measurements of the relict radiation would be confused with other sources of radiant energy, as outlined in a 1948 letter to Alpher and Robert Herman) as a demonstration of his having overlooked the possibility of microwave measurements. As I learned from Bernie Burke only recently, however, Alpher had indeed queried at least one radio astronomy group about the possibility of making microwave measurements like those recounted in the present volume, but was told that it couldn't be done (p. 182).

With the results of present-day CMBR measurements judged significant enough to be taught even in some high schools, it may be hard for contemporary readers to imagine a circa 1950 radio astronomer turning down such an "opportunity." Nonetheless, a more careful look at the state of radio astronomy in those days makes such a turn-down far more understandable. First of all, there were no idle radio astronomers. The first few radio observatories were just being set up, and almost anything they did would break new ground - at least as long as the rudimentary equipment they used worked well enough to produce useful data. Given such circumstances, together with the amount of effort a CMBR measurement would have required, it is not hard to imagine someone being likely to consider such an undertaking outside the realm of possibility.

Thanks to experience and improved techniques however, CMBR measurements began to look almost easy just a few years after the initial report of our discovery. In 1967, for example, Dave Wilkinson and his coworkers reported a trio of highly consistent CMBR measurements done at three different wavelength regions (Wilkinson 1967; Stokes, Partridge and Wilkinson 1967). Given the speed and precision of this work, it is understandably easy to overlook the fact that the same group's first 3-cm measurement took the better part of two years from start to submission for publication. Moreover, that 3-cm project had far better resources than any that would have been available 15 or so years earlier - along with the additional advantage of experience gained from familiarity with a successfully completed project similar to theirs.

Small wonder then, that a potential CMBR experimenter would have balked at anyone proposing such an undertaking back in the 1950s -especially with no more incentive than what was then a tenuous link between an unproven theory and hypothesized data. Under such circumstances, it is not hard to imagine a radio astronomer of that year saying "it can't be done," nor is it hard to imagine the subsequent frustration felt by George Gamow and his colleagues as the events of 1965 began to unfold.

By early 1966, Bob and I had completed our observations with the 7-cm system, and installed a newly-built 21-cm system in its place. Our CMBR measurements at this new wavelength went smoothly, and we were able to report that result later the same year (Penzias and Wilson 1967). Here, for the first time, we found "company" in the form of a similar measurement made by Howell and Shakeshaft (1966), allowing us to compare the results of two independent measurements at the same wavelength. Since the raw data (the sum of the CMBR and galactic radiation) in the two measurements differed by only 0.2 K, the combined result yielded an accurate determination of our Galaxy's spectral behavior - one of the items on my earlier research agenda. While we continued our 21-cm Hi studies for another year or so, our CMBR studies had come to an end. In its place, we began a long-term effort aimed at following the CMBR's companion thread in cosmology - the origin of the elements - by studying the chemical and isotopic composition of interstellar space.

In this endeavor, Bob and I once again moved to a new wavelength range - this one centered on the atmospheric window which stretches from 75-150 GHz (4-2 mm). In contrast to the small handful of hyperfine lines available to microwave radio astronomers, the then still-unexplored millimeter-wave portion of the astronomical spectrum encompasses a rich variety of molecular rotation lines. Fortunately, several of the key components required for such work had been developed for communications research purposes. With much help from Charles Burrus, one of our Bell Labs colleagues, Bob and I assembled a millimeter-wave receiver. Completed in the spring of 1968, I carried it to a precision radio telescope owned and operated by the National Radio Astronomy Observatory at Kitt Peak, Arizona, for preliminary continuum observations. Until we introduced our receiver, millimeter-wave observations with that telescope had been limited to bolometric measurements. Following the success of our continuum work, and the subsequent installation of an National Radio Astronomical Observatory (NRAO)-built spectrometer "back end," we - together with a number of collaborators from other institutions - discovered and studied a number of interstellar molecular species, thereby revealing the rich and varied chemistry which exists in interstellar space.

Since that time, 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 discovery of the first deuterated molecular species found in interstellar space (Wilson et al. 1973) enabled me to trace the distribution of deuterium in the galaxy. This work (Penzias 1979b) provided the first direct evidence for the cosmological origin of this unique isotope, which by then had earned the nickname "Arno's white whale" among my observing colleagues. 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 CMBR, these measurements provided strong support for the "Big Bang" interpretation of our earlier discovery.

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