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Fig. 4.31. The number of CMBR-related papers published each year (Partridge 1995). Note the lack of activity in the years 1969-1977.

The emergence and success of these groups is linked to one of the reasons I see for the cooling of ardor at Princeton. By the late 1960s we had, in effect, done all the easy experiments. The experiments Dave and I did were, as I liked to say, "one-Cadillac scale," costing of the order of $10,000 to 20,000 each. They were constructed almost entirely from commercially available components. To improve these experiments, new detectors and optics were needed, as were instruments designed specifically for the detection of CMBR anisotropies or the precision measurement of the CMBR spectrum. In addition to new technology, better observing strategies were needed (recall my remarks above on problems created by the atmosphere). The same strictures applied to the use of existing radio telescopes: observers had pushed them to their technological limits as well.

New groups brought new techniques and technologies to bear. I want to mention specifically the introduction of bolometric detectors into the field, and to praise the foresight of people like Paul Richards and Francesco Melchiorri, and of Rai Weiss, who has written for this volume. Francesco was a pioneer in the field, who unfortunately passed away in 2005.

So there was a pause while new technologies and techniques were brought to bear. Along with new groups joining the field, Dave Wilkinson wisely moved in the direction of balloon experiments. I got interested in the use of radio-frequency interferometry to probe yet smaller angular scales. The introduction or exploration of these new techniques took time, and that is in part responsible for the drop in activity in the CMBR field in the early 1970s.

Another factor, at least in the case of Princeton's Gravity Group, was the explosion of other interesting things to do in astrophysics, ranging from pulsar timing to searches for "primeval galaxies." The experimentalists of the Gravity Group found lots of other intriguing things to do while we waited to sort out new CMBR technologies and techniques. Dave, for instance, began to explore limits on extragalactic optical backgrounds and oversaw Marc Davis's pioneering search for primeval galaxies. I mounted a separate search for primeval galaxies, and got interested in observational tests of the Wheeler-Feynman (1945) absorber theory (Partridge 1973) and searches for bursts of radio-frequency emission. Both Dave and I, joined by Ed Groth and Paul Boynton, spent a lot of time from the spring of 1969 on making precision timing measurements of the optical pulses of the Crab Nebula Pulsar. In an ironic twist, we felt we had discovered evidence that the Crab Nebula Pulsar is slowing down due to the loss of energy by gravitational radiation; it turned out that nature had thrown us a curve ball in the form of a glitch in the pulsar period. But another, cleaner pulsar system would reveal energy loss by gravitational radiation and win the Nobel Prize for Russell Hulse and Joe Taylor.

I will end by floating an idea that may be strongly colored by retrospective wisdom. Could the lull in CMBR activities have been in part influenced by the fact that we were beginning to pay some attention to theoretical predictions as to the properties of CMBR anisotropies and spectral distortions? That is, instead of blindly trying to set better and better limits on both anisotropy and spectral distortion at a range of wavelengths and scales, were we, I wonder, beginning to recognize (a) how hard it would be to see meaningful spectral distortions and (b) that the amplitude of anisotropies would in general be very small except on certain angular scales? Frankly, my recollection of my mood in the late 1960s and early 1970s is now a little too hazy for me to say for sure. What I can say is that the five years, 1965 to 1970, were not only the years that truly established physical cosmology, but were a hell of a lot of fun!

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