R Bruce Partridge Early days of the primeval fireball

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Bruce Partridge is a cosmologist turned radio astronomer who has taught at Haverford College for 38 years. He spent five years, 1965-1970, in the fabled Gravity Group at Princeton working on the "Primeval Fireball" (the CMBR) and primeval galaxies. He also served six years as the Education Officer of the American Astronomical Society, is president-elect of the Astronomical Society of the Pacific, and even survived eight years as an academic administrator at Haverford.

I will start as I propose to continue, in a quite personal and even anecdotal tone. I'll begin with my interest in astronomy, awakened in my teen years by building two reflecting telescopes with my father, and end with studies of the spectrum and anisotropy of the cosmic microwave radiation begun at Princeton in 1965.

In my college years, I bounced back and forth between history, physics, and astronomy. In retrospect, I can see that these were pointing toward my eventual fascination with the evolution of the universe and how we can determine it. Physics ended up as my major, but I got some grounding in astronomy as an undergraduate. In the early 1960s, Princeton University was just developing an undergraduate astronomy track. To gain admission, one had to take an elementary astronomy course designed primarily for the dimmest of undergraduates. We used a text coauthored by the professor, a text that mentioned the word "universe" only twice, both times misiden-tifying it with the Milky Way Galaxy. Fortunately, my subsequent courses were with George Field, a master teacher as well as a visionary astronomer. In 1960 I took from him a course that dealt in part with cosmology; that section of the course was based on Hermann Bondi's (1960a) thin book, Cosmology. Bondi's book was a fair representation of the state of cosmology at the time: attention was focused on cosmological models and possible observational tests of them. The largest scientific question in the field was whether the steady state model fit the (meager) data better than what we now call "big bang models."

It is perhaps a mark of how small a dent cosmology made on me that I elected to do research with George Field in the areas of interstellar grains and radio astronomy instead. But my main focus in my last year at Princeton and thereafter at Oxford was in quantum physics (my Oxford DPhil was on optical pumping in helium gas). Nevertheless, fascination with large-scale questions in astronomy was ticking away in the background. I recall attending, in 1964, a meeting of the Royal Astronomical Society (RAS) to hear about the newly discovered phenomenon of quasars. It was at that meeting, incidentally, that I first encountered Dennis Sciama, and noted both his wonderful ability to explain scientific principles clearly and his collegial treatment of a very young Stephen Hawking.

So, when it came time to apply for postdoctoral positions, I looked to groups in both Britain and the USA that were bringing techniques of physics to bear on astronomical or cosmological questions. My Princeton background led me to send an application to Bob Dicke. Bob's invitation to join the fabled "Gravity Group" was the crucial event in my scientific career.

As a 25-year-old with a scant knowledge of cosmology, I walked into Bob Dicke's office in the late summer of 1965. I knew of Bob's ongoing work on the Eotvos experiment, but his enthusiasm in 1965 was more firmly directed toward either explorations of Solar oblateness (as a test of relativity and the scalar-tensor variant) or the newly discovered microwave background radiation. Generous as always, he offered me a free choice, and then took me to see the two experimental setups. We went first to the Solar oblateness experiment, housed in a small wooden hut down by the Princeton Observatory. The hut was crowded with complicated electronics, many of them lock-in amplifiers, a Dicke invention I came to love and rely on. But the assembly of electronics was rather daunting. In contrast, the microwave background apparatus looked comfortingly simpler and even familiar - I

had used microwave techniques in my thesis research. And I thought Dave Wilkinson would be a fine person to work with. Boy, was I right!

With great good fortune, I chose as my first effort in the Gravity Group to work with Dave on designing and running what became the first specifically planned CMBR anisotropy experiment. The way that experiment was planned and carried out provides some useful lessons on how one should -and should not - design an experiment.

Dave and his colleague Peter Roll (whose contribution commences on page 213) had a year or so earlier designed an experiment to detect the radiation left over from the big bang. This instrument, shown in Figure 4.23, was specifically designed to make an absolute measurement of temperature or intensity of the CMBR.

To measure or put limits on the anisotropy of the radiation requires a quite different approach. On the one hand, anisotropy measurements are easier, since they can be made comparatively (is this part of the sky hotter than that part?). On the other hand, Dave and I recognized that to be meaningful, such an experiment needed to be much more sensitive, and to produce temperature measurements accurate to a few parts in 1000. Penzias and Wilson (1965a) in their discovery paper had already noted that the "excess noise" they picked up is approximately isotropic, with any variations in intensity below about 10%. We aimed to improve this limit by nearly two orders of magnitude. The plan was to scan a circle in the sky at constant

Fig. 4.23. The former pigeon coop atop the Geology Building that housed the Roll-Wilkinson (1966) CMBR spectrum measurement and the 1965-1967 Princeton "isotropometer." Photo: Robert Matthews.

declination over a long enough period so that any diurnal variations would cancel out. A dipole distribution in the CMBR temperature would then produce a 24-h variation (in sidereal time), and a quadrupole distribution would produce a variation at 12-h period.

As anyone who has lived in New Jersey knows, however, the atmosphere over Princeton is not exactly stable. To cancel out the atmosphere to first order, we needed to make calibration observations of a stable, unmoving region of the sky through a comparable air mass. We thus elected to switch the beam (observing direction) between the north celestial pole (the fixed point) and a point an equal angular distance away from the zenith to the south. We thus ended up scanning a circle at declination 5 = —8°. There were two levels of beam switching. First, we switched at about 1000Hz back and forth between our main horn antenna and a much smaller antenna pointed toward the zenith. As a further control, we switched the beam of the primary antenna itself every few minutes by raising a reflecting sheet to divert the beam to the north celestial pole. This was the Princeton "isotropometer" housed in an unused pigeon coop on a tower of Guyot Hall (Figures 4.23 and 4.24).

The kilohertz signal was phase-sensitively detected, and plotted out using a pen and ink chart recorder. (Mentioning a pen and ink chart recorder to scientists today must be the functional equivalent of telling my children that I walked 3 miles each day to catch the school bus. Both are true.) Dave and

Fig. 4.24. Schematic of the "isotropometer," showing the moving reflector used to zero the instrument (Partridge and Wilkinson 1967). ©1967 American Physical Society.

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