Robert A Stokes Early spectral measurements of the cosmic microwave background radiation

Robert Stokes is President and CEO of Versa Power Systems, a solid-oxide fuel cell development company in the Denver, Colorado area. After completion of his PhD at Princeton in 1968 he received an appointment as an assistant professor at the University of Kentucky where he continued work on the CMBR. Later he managed the engineering physics division at Battelle

Pacific Northwest National Laboratories, served as Deputy Director of the National Renewable Energy Laboratory, and was Senior Vice President at the Gas Technology Institute.

As an undergraduate student at the University of Kentucky in the 1960s, I was part of a generation with a growing interest in space science encouraged by the US educational system's response to the Soviet launch of the Sputnik satellite. At the end of my junior year I was selected to attend one of the first Goddard Institute summer study courses in space science at Columbia University organized by Robert Jastrow. After an intense summer of focusing on planetary astrophysics, our group was treated to a memorable tour of several US space science facilities, traveling aboard a chartered DC6 aircraft in August of 1963. The tour included visits to the NSF astronomical observatory at Kitt Peak, Arizona; the Marshall Space Flight Center in Huntsville, Alabama (the tour conducted by none other than Werner Von Braun); the NASA launch facility at Cape Canaveral; and NASA headquarters in Washington, DC. As a result, my interest in space science was greatly intensified, and when presented with the opportunity to attend graduate school at Princeton, a focus on space science was a foregone conclusion.

After a year of graduate study at Princeton and a couple of stimulating classes taught by John Wheeler, I managed to land a summer appointment working as a student research assistant for Bob Dicke and Mark Goldenberg, taking data on a special ground-based telescope designed to measure the Solar oblateness as a test of the predictions of the Brans-Dicke theory. That summer, Paul Henry, another graduate student, and I traded off making observations while observing practice by the Princeton hammer-throw athletes, hoping all the time that our apparatus would not be damaged by a mis-thrown 16-lb steel ball. By the end of the summer I had become a part of the graduate student cadre associated with the Gravity Group, led by Dicke and Wheeler and including their junior colleagues Peter Roll, Jim Peebles, Dave Wilkinson, Mark Goldenberg, Kip Thorne, and Bruce Partridge.

Jim Peebles had already begun some theoretical work on the implications of a hot fireball model for the early universe and the nature of any remnant radiation. Earlier modeling work by Alpher and Gamow (Alpher, Bethe and Gamow 1948) and measurements of atmospheric radiation by Dicke et al. (1946) had laid the groundwork for the research not pursued in earnest until the mid-1960s. But by 1965, Peter Roll and Dave Wilkinson had already begun operation of a 3-cm radiometer specifically designed to test the blackbody radiation hypothesis.

By the time the Princeton group had connected up with Arno Penzias and Bob Wilson at Bell Labs and published the famous 1965 companion papers (Dicke et al. 1965; Penzias and Wilson 1965a) on the CMBR, I had just completed my PhD qualifying exams and was looking for a dissertation topic. Roll and Wilkinson (1966, 1967) had sent their confirming radiometric measurements to Physical Review Letters in January of 1966 and there was great interest in investigating the spectral nature of the newly discovered isotropic microwave radiation.

Dave Wilkinson agreed to take me on as his first doctoral student at Princeton and set me to work building a 1.58-cm radiometer to make coordinated measurements with two radio telescopes he and Bruce Partridge were constructing for a second series of measurements at 3.2 cm and 8.56 mm. We had made arrangements to conduct several months of measurements at a high-altitude laboratory operated by the University of California at Berkeley in the White Mountains along the California-Nevada border during the summer of 1967 to establish a more precise temperature for the background radiation field.

The hypothesis that the microwave background radiation is, in fact, the primeval fireball rests heavily on the spectrum being that of a blackbody. Measurements completed prior to 1967 had all been consistent with a spectral index a = 2 over a considerable wavelength range in the Rayleigh-Jeans region of a 3 K blackbody.

To be convinced one is seeing true blackbody radiation and not that of a hot graybody, it is necessary to go to short wavelengths and look for the curvature in the spectrum due to quantum statistical effects. In early 1967, Dave Wilkinson, Bruce Partridge, Paul Boynton, and I began a series of experiments aimed at refining the absolute radiometric techniques and extending the wavelength coverage to 3.3 mm, a wavelength sufficiently short to differentiate between a true blackbody and a hot graybody.

Four Dicke radiometers were constructed by the group using similar designs at wavelengths of 3.2 cm, 1.58cm, 8.56 mm, and 3.3 mm and taken to mountaintop-observing sites to reduce atmospheric background. The 3.2-cm and 1.58-cm measurements were repeated to check earlier work and to provide an accurate determination of the spectral index. The 8.56-mm and 3.3-mm points were expected to show deviations from the frequency-squared dependence in the spectrum of a hot graybody, the deviations amounting to 20% at 8 mm and 300% at 3 mm.

A great deal of experience was gained from the Roll and Wilkinson radiometer that was operated atop a building on the Princeton University campus, and Dave Wilkinson in particular was able to build on his exacting electron g-2 PhD work at Michigan (Wilkinson 1962) to design an approach that dealt with systematic errors in the experiments. A number of other articles in this volume provide a good bit of detail and photos of the experimental apparatus used by the Princeton group (including the article by Bruce Partridge starting on p. 221), so I will not repeat the details here.

The 3.2-cm, 1.58-cm, and 8.56-mm experiments (Stokes, Partridge and Wilkinson 1967; Wilkinson 1967) were performed at an altitude of 12,470ft at the Barcroft facility of the White Mountain Research Station, Bishop, California, during July and August of 1967. The 3.3-mm result (Boynton, Stokes and Wilkinson 1968) was obtained in March 1968 from an altitude of 11,300 ft at the NCAR High Altitude Observatory, Climax, Colorado. Figure 4.40 shows a photo of the author with the 3.3-mm radiometer at the Climax site.

Paul Boynton had completed a PhD in nuclear physics at Princeton (Boynton 1967) and decided to stay on as a postdoc in the Gravity Group. At about the time that Dave Wilkinson, Bruce Partridge, and I departed for Barcroft, California, via Yuma Arizona, Boynton started designing and procuring parts for the new 3.3-mm Dicke radiometer that was to be employed for the follow-up series of measurements.

Fig. 4.40. Robert Stokes with a 3.3-mm radiometer - Colorado 1968.

Whereas the microwave radiometer components for the longer-wavelength radiometers were mostly commercially available items, the microwave mixers for the 3.3-mm superheterodyne receivers were still very much a development-stage component in 1967. After my return from California, Boynton and I spent several months attempting to procure or develop an acceptable microwave mixer that would work at 90 GHz (3.3 mm). We visited several military development labs and received considerable help and loaned components from the Army lab staff in pursuit of a working radiometer in late 1967. At the beginning of 1968 we made the decision in consultation with Dave Wilkinson to give up on locating a reliable microwave mixer and put together a portable laboratory to transport to Colorado so that we could construct the detectors ourselves from GaAs wafers and gold-alloy sharpened cat whiskers. It seemed a lot like the early days of radio experimentation, but it worked! However, for the Colorado observations, we typically needed to change out the mixer once or twice during each of the all-night runs.

The choice of wavelengths for the radiometers was dictated by the location of atmospheric windows in the millimeter band. This absorption and subsequent reemission are the result of closely spaced pressure-broadened resonance lines that occur in the water molecule near 1.3- and 0.27-cm wavelength, and in the oxygen molecule near 0.5- and 0.26-cm wavelength. To further minimize atmospheric effects, measurements were performed at high altitudes during times of low absolute humidity. In addition to the usual problems with absolute measurements, the CMBR spectral measurements were made more difficult by the impossibility of modulating the signal due to its isotropy. Since the microwave background signal is the residue after one has accounted for everything else, control of the systematic effects and careful calibration were crucial.

The results of the four radiometer measurements made by the Princeton group using these techniques were all consistent with a 2.7 K blackbody. A graybody spectrum fitted to the measurements at 3.2 cm and 1.58 cm would have predicted a value five standard deviations above the result at 3.3 mm; thus these were the first direct radiometric measurements indicating spectral curvature.

Paul Boynton and I used an improved version of the original 3.3-mm radiometer carried to an altitude of 14.9 km in the NASA Ames Research Center Learjet to get the first direct radiometer measurement in which the atmospheric contribution was less than the cosmic background. The radiometer was not calibrated using a primary calibration source during the air-borne measurements. It was calibrated before and after flight. This experiment was the result of follow-on work by Paul Boynton and me after we left Princeton. Paul had taken an assistant professorship at the University of Washington, and I had an appointment as an assistant professor at the University of Kentucky. Much of the final preparations for the air-borne experiment were facilitated by my spending the summer of 1971 at Battelle Pacific Northwest National Laboratories in Washington State.

A typical air-borne experiment involved a flight to an altitude of 55,000 ft in order to perform the measurements above the tropopause of Earth's atmosphere. Even though the Learjet was pressurized, we were required to wear oxygen masks in case of a failure of the modified safety hatch that carried the radiometer antenna. On our last flight, after we announced to the exNavy pilot that the experimental results looked good, the pilot treated us to a perfect 1-g barrel roll without losing a drop of liquid helium from the calibration reference Dewar flask. Figure 4.41 is a photo of Paul Boynton and the pilot (left) just before takeoff on one of the flights.

By the early 1970s there was an excellent demonstration of the thermal spectrum as is beautifully illustrated in Figure 1 in Boynton and Stokes (1974). But bolometer measurements at submillimeter wavelengths indicated anomalies that many took seriously until COBE in 1990.

My subsequent career choices have taken me away from space science and cosmology to a focus on energy technology; however, I continue to follow developments in cosmology and space science as a highly interested individual.

Fig. 4.41. Paul Boynton (third from left) at NASA Ames in 1971.
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