P James E Peebles How I learned physical cosmology

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Jim Peebles has been at Princeton University since 1958 and is now Albert Einstein Professor of Science Emeritus.

I arrived in Princeton in 1958 from the University of Manitoba as a graduate student intending to study particle physics. At Princeton Bob Dicke somehow saw that I was much better suited to work on his new research interest, gravity physics.

Dicke had recently changed directions from research in quantum optics and precision measurements in atomic physics to the study of the physics of gravity. At the time we had an elegant theory, general relativity, but very limited tests. Dicke set out to improve the situation. By the time I arrived a considerable number of people were working with him, including undergraduate and graduate students, postdocs, and junior faculty. Bob Moore, who had been two years ahead of me at the University of Manitoba and was one of Dicke's graduate students, brought me to a meeting of Dicke's Gravity Group. I was fascinated by the variety of topics under discussion, and intimidated by how much everyone knew. Dicke, in particular, seemed to have a ready and well-informed assessment of every issue that arose. But he was drawing from a deeper well of understanding of the physics of the real world than anyone else I have encountered.

Bob Dicke encouraged me to join the group. I wrote a doctoral dissertation under his direction, on constraints on the time-variability of the strength of the electromagnetic interaction (Peebles 1962). Bob's motivation was his fascination with Mach's principle, which might be read to say that as the universe evolves so do the laws of physics. I was fascinated by all the evidence one could bring to bear, from the laboratory to geology and astronomy. My evident lack of interest in Mach didn't seem to bother Bob: I stayed on as his postdoc and evolved into a member of the faculty.

I learned about the general relativity theory solution for a homogeneous and isotropic expanding universe as part of preparation for the physics department graduate general examinations. I remember feeling a little surprised that people might consider this a serious model for the real world rather than one of the over-simplified problems you solve in exams, along with the acceleration of a frictionless elephant on an inclined plane. My textbooks on general relativity and cosmology, Landau and Lifshitz (1951) Classical Theory of Fields and Tolman (1934) Relativity Thermodynamics and Cosmology, present beautiful theoretical physics but little phenomenology. When we were graduate students Ken Turner introduced me to a book that has more phenomenology, Bondi's (1960a) Cosmology. I don't remember what I thought about this book at the time, apart from being shocked by the steady state cosmology: they just made this up. But I felt much the same about the relativistic big bang cosmology. Bob Dicke led me to see that cosmology then was a real physical science, with meaningful - if sparse -connections of theory to experimental physics and observational astronomy. By the end of the 1960s I had learned that there are many good things to say about the physical science of cosmology, including the steady state model, and I wrote a book about it, Physical Cosmology (Peebles 1971).

I don't remember much about the Gravity Group meeting at which Dicke explained why we might want to look for a sea of blackbody radiation that nearly uniformly fills space. But I think it was at this meeting that he gave an explanation that sticks in my mind for why the radiation would cool as the universe expands. He invited us to imagine placing a box with perfectly reflecting walls in the sea of radiation, with the same radiation temperature inside and out. The walls are expanding with the general expansion of the universe. They have no effect on the radiation (at wavelengths small compared to the box size) because for every photon that approaches the box from outside and is reflected there is on average an interior photon that bounces off the wall to replace it. I think I remember his concluding remark: we all know that radiation is cooled by the adiabatic expansion of the cavity. It was obvious to Bob that the spectrum remains thermal as the radiation cools. I don't remember whether he explained that. I convinced myself of it by a variant of the argument that is presented in the glossary under the CMBR energy spectrum.

Bob invited Peter Roll and David Wilkinson to build a Dicke radiometer to look for this radiation. His casual remark that I might look into the theoretical implications of the outcome of the experiment set the direction for my career. Great people can do things like that.

What was Bob thinking? I know he liked the idea of an expanding universe, and I remember his inviting us on more than one occasion to consider what the universe might have been doing before it was expanding. The answer he liked was that the universe was collapsing following a previous cycle of expansion. He instructed us on the production of entropy - largely in a sea of blackbody radiation - during the bounce, and on the role of the radiation in the thermal dissociation of the heavier elements produced in stars in the previous cycle. (The argument is reviewed in Section 3.3.) I believe his proposal to Roll and Wilkinson was meant to test this idea; I don't think the possibility of distinguishing between the big bang and the steady state cosmologies was a serious consideration. My scarcity of recollections of Bob's comments about the steady state philosophy, apart from his dislike of the passionate debates about it, leads me to suspect that this line of thought simply did not interest him. In those days the very limited fund of empirical evidence allowed us a lot more freedom in following our instincts in the search for clues to the nature of the universe.

I have some notes about what I was doing following Bob's invitation to think about the physical implications of the search for the fossil radiation. But I rarely put dates on the notes, so I can only say for sure that by the fall of 1964 I was making progress on two ideas. One was that thermonuclear reactions during the early rapid expansion of a hot universe, when the radiation temperature was T~109 K, could produce appreciable amounts of helium and deuterium. The other was that when the temperature was greater than about 3000 K matter would have been thermally ionized and radiation drag on the plasma would have strongly affected the growth of the clustering of mass we observe now in galaxies and concentrations of galaxies. In 1965 I learned that much of the first idea had already been worked out. I think my first clue was Dicke's instruction to look up a paper by Hoyle and Tayler (1964). I don't know how Bob knew this paper. In that same year my second colloquium on what I was doing led to the connection between the Princeton search for a sea of microwave radiation and the problem of unexpected noise in a Bell Laboratories microwave radiometer.

I presented my first colloquium on this subject at Wesleyan University in Connecticut on December 2, 1964. Henry Hill, a former member of Dicke's Gravity Group, invited me. He wanted to explore the possibility of my moving to Wesleyan. I was impressed by the faculty, and particularly remember Thornton Page for his instructions about astronomy. But I don't remember any feedback about cosmology, and nothing came of the job idea.

In the colloquium I showed the two graphs in Figure 4.21. The curves in the panel on the left are examples of thermal spectra. The hotter one would have about the energy density of the Einstein-de Sitter cosmological model. (The mass in this model is such that the universe in effect is expanding just at escape velocity, with no cosmological constant.) The symbols show measurements or upper bounds on the cosmic radiation energy density across a broad range of wavelengths. It was known then that space is filled with a near uniform sea of X-ray to 7-ray radiation. The amount of energy in this form is much less than the equivalent of the observed mass in stars in galaxies. There was an upper bound on the cosmic mean energy density at optical wavelengths. Now we have a measurement of the accumulated amount

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