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Oral History Transcript — Dr. P. J. E. Peebles

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Interview with Dr. P. J. E. Peebles
By Christopher Smeenk
April 4, 2002

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P. J. E. Peebles; April 4, 2002

ABSTRACT: The interview focuses primarily on Peebles' many contributions to physical osmology: research on nucleosynthesis in the early universe in 1965, the theoretical and observational study of large scale structure formation in the 70s, and the development of the cold dark matter model and numerical simulations of structure formation, to mention the most prominent topics. Peebles describes his interactions with colleagues and other influences that shaped his research, as well as describing his own research style and the style of physicists he admires (notably that of his mentor, Bob Dicke).

He discusses in detail his response to and assessment of various other topics in cosmology: the anthropic principle, different types of dark matter,dark energy, inflationary cosmology, MOND, various structure formation scenarios, and quantum gravity. We briefly discuss the institutional support and funding for cosmology and how that has changed over the course of Peebles' career.

Peebles also describes several of the changes that have taken place in the practice of cosmology, from describing the introduction of numerical techniques and increased interaction in particle physics, to the increasing pace of research.

Transcript

Session I | Session II

Brief Note:

This is an interview with Jim Peebles conducted by Chris Smeenk.

Smeenk:

It's April 4th, 2002 and we are in Princeton, New Jersey. I want to start out by asking you some questions about your background. I know you grew up in Manitoba.

Peebles:

Yes.

Smeenk:

And you were born in 1935.

Peebles:

Yes.

Smeenk:

Beyond that I know very little about your background. What was your upbringing like? What did your parents do?

Peebles:

Born and raised in the greater Winnipeg area. My father worked at the Grain Exchange. It was then a set of private businesses that as a group occupied a building in Manitoba in Winnipeg. It dealt with grain and other agricultural products from the west. My mother was a housewife, and she had no job throughout her entire life.

I went to public schools, which is to say publicly financed. I was born in Saint Boniface, which is a small city right next to Winnipeg, nominally French speaking. The province is nominally French speaking. The French part has withered away. I went to, I still remember, King George the Fifth elementary school. Then we moved to Saint Vital, a small suburb further south of Winnipeg, still in the greater area. Spent a very tranquil existence in middle school, in high school, learned very little. Graduated from high school with maybe twelve other people. Very small.

Smeenk:

In your graduating class?

Peebles:

Yes. And about three of us went on to the University of Manitoba. In those days anyone could enter but the failure rate was pretty high. No admission standards.

Smeenk:

Was there anyone in high school that you remember, any teachers that were particularly influential?

Peebles:

Well, no. I enjoyed myself a lot. I learned to skate, square dancing, had a lot of fun, but academia was not very strong there. When I went to the University of Manitoba I was just shocked about all the things I didn't know. I didn't know what trigonometric functions were. So going into the university was quite a jump into cold water. I can't remember any teacher in high school who particularly influenced me, I have to admit. My father liked to build things, and I certainly have always loved to work with my hands - carpentry, machine work, building things. Gardening I also liked a lot, and mechanical things attracted me. I had no particular introduction into things intellectual or mathematical until I got to the University of Manitoba.

Smeenk:

So when you were going to the university did you envision an academic career?

Peebles:

No, no. In fact I'm not sure I was aware there was such a concept. I was aware there were engineers and I liked to build things so I entered the School of Engineering, and stayed there for two years. And I did enjoy it. Maybe I enjoyed more those courses that are no longer given, like engineering drawing. It's all CAD/CAM now I'm sure. But in those days it was fun to do that and projective geometry. I was impressed also by the other courses. I could have been happy, I think, as an engineer. But after two years, I noticed I was making more friends over in science, and in physics in particular, than in engineering and they seemed to be doing more interesting things, so I switched over.

And that was the best thing I think I could ever have done. I said I could have been happy as an engineer, but would have been I think a mediocre engineer. It turns out that what I really love to do is what they were doing over in physics. It was great. We had a small class. One talked about honors physics there, the equivalent of a major in physics here with the difference that once I moved into science and honors physics I didn't take another course in anything but mathematics and physics.

Many of the courses in retrospect were very dull. We plodded through spectroscopy and many other things that were not exciting. We didn't get too far into modern physics either, but what I learned when I took qualifying exams here is that I had learned an awful lot of basic physics. So once I was in the physics department at the University of Manitoba it was very clear to me I wanted to go off and be a physicist.

Smeenk:

Did you take any astronomy courses?

Peebles:

No. I was aware of the sky, of course, and aware of the Northern Lights, for example, but no, I had no interest in astronomy. I couldn't tell you the name of a star aside from a few. I did learn, as part of engineering, surveying. And I learned how to find the North Star and how to find terres- trial latitude and longitude. So I knew a little bit about stars as means of finding your way around, but by no means did I have any interest in how a star works or - I had no idea there was such a thing as a galaxy at that time. I did learn a lot of quantum mechanics and I came to Princeton with the intention of studying particle physics and particle theory, and that's about all I knew about.

Smeenk:

Was there anyone at the University of Manitoba doing particle physics that was?

Peebles:

Yes. Sam Neamton was a very impressive guy. Harold Coish. Two theoretical physicists who were both awfully good and I think very good for me, because they were patient teachers and they taught a lot. Also very influential was an experimentalist Ken Standing. He's still there. I saw him just a few months back when I visited. He was an experimentalist doing nuclear physics then, and now doing mass spectroscopy weighing heavy complex molecules - work that even the National Institutes of Health here value, sorting out complicated molecules. He was a graduate student of Princeton University. He said, "You should go to Princeton." Bernard Whitmore, another impressive theoretical physicist there, said, "You should go to Oxford." And well, I don't know why I made the choice, but I went to Princeton.

Smeenk:

Did you have any kind of senior research project at Manitoba?

Peebles:

No. There was no independent work to speak of. I did under Neamton write an essay on quantum mechanics that thrilled me. I was required to put down in my own words what the subject was about, and that was exciting. That's about as close to independent research as you got there. Lots of problem solving, which is awfully good practice, but thinking on your own, no, that wasn't emphasized there. And of course that was one of the, again, shocks of transition when I came to Princeton, where there were no course requirements. To qualify to write a Ph.D. dissertation you only have to pass qualifying exams.

Smeenk:

I didn't realize that. So there was no standard curriculum; you just had to do what you had to do to pass the qualifying exams?

Peebles:

Yes, and essentially that's still true. There have been a few bells and whistles added. Now students have to do a small experimental project. They have to start on a bit of independent research before writing qualifying exams. Modest changes, but basically it is now as it was then. You are free to do what you want to prepare yourself for qualifying exams and for writing a thesis. It works well for some people. If you have an independent turn of mind you can do what you want and in the most efficient way get ready to do research. It's not good for everyone. It was fine for me. I took all of the courses that seemed interesting anyway. I didn't have to take any of them, but I did.

Smeenk:

And you said that you came in thinking you would be a particle physicist?

Peebles:

Yes. I have written just one paper on particle physics.

Smeenk:

I saw that actually. It was 1962, right?

Peebles:

That is right. It grew out of a course I took with Mary [JP: Marvin; usually known as Marv, but pronounced "Murph"] Goldberger, no longer here but still with us. I saw him recently in La Jolla, UCSD. I talked to the faculty in particle physics. We were negotiating possibilities. I didn't notice great enthusiasm on their part, but then that's kind of traditional. But then I fell into orbit around Bob Dicke.

He had come back from the war [WWII that is] with new technology and new ideas in I guess what could be called quantum optics where he spent a very productive decade after the war. Then about the middle 1950s, he noticed that gravity physics was in a remarkably inactive state. We had a theory of great elegance, general relativity. We had a few tests, almost all of which were remarkably schematic: the deflection of light barely detected, the redshift barely detected, the preces- sion of the perihelion was precise, but that was it. He, just as I arrived, had started on a pretty large program of experimental checks of relativity. Over the next decade that produced an awful lot of results and has stimulated a lot of work elsewhere, I think. It's continuing. He also developed an interest in theories, alternatives to general relativity, if only to serve as foils. So in fact my doctoral dissertation was on the time variability of alpha, the fine structure constant. Recently this is back in the news.

Smeenk:

Have you been following the results on this?

Peebles:

Yes. So I had constraints on the time variability of alpha, in part from - already even then one had spectra, fine structure lines in high redshift galaxies, and that gave you constraints on alpha. Not nearly as tight as now, of course. Radioactive decay ages were not that much worse then than now, and again it gave you constraints on alpha.

Smeenk:

So who else was in Dicke's group at this time?

Peebles:

Well, quite a large number of people.

Smeenk:

I know Roll and Wilkinson were both-

Peebles:

Yes, although they were neither of them graduate students. They came as postdocs.

Smeenk:

Okay. And they came later in the mid-sixties?

Peebles:

Yes, although I would say more early sixties, somewhere in there. They were both on the premises a couple of years before Bob got the idea of looking for the microwave radiation. And they had both done other experiments. Peter Roll in part on the Eötvös effect experiment, the equivalence of - the material independence of gravitational acceleration. David Wilkinson, well he was certainly working on the lunar laser ranging, I think, before he got into the thermal background radiation measurements. But they were both quite young and certainly very flexible in what they were going to do. I think Bob didn't find it hard to persuade the two of them to build an instrument to look for this effect.

Smeenk:

So was there a great air of excitement in Dicke's group, given that you were starting to look at general relativity again in a way that hadn't-?

Peebles:

Yes. Would I call it excitement or simply interest? One didn't have the feeling that we were going to overturn Einstein or that we were going to find something astoundingly new. In part it was new experiments. Let's just pull down the limits on possible departures from predictions in general relativity. So that is in a sense rather pedestrian work. You're not looking for a real effect. You are looking for - you're just pushing down the zero, the null effect. What was exciting though was that one got to look at an enormous range of things, concepts, from the laboratory to the evolution of the stars, to the structures of the planets, to the decay ages of meteorites. You got to cast your thoughts over an enormous range of ideas and phenomenon, and that was a lot of fun.

Also there were so many experiments to be done in initiating these tests of gravity physics, and each experiment was of course fascinating in its own right and each very different in its challenges. Bob during this time had a weekly research group meeting. Typically it was on a Friday evening, of all things. And we all attended. We often would go for beer afterward - Bob didn't attend the beer part - but because the research was interesting and so very varied, he didn't have any trouble getting people to show up on a Friday evening.

Smeenk:

On average, how many people were at these group meetings?

Peebles:

It would have to be a guess, but how many were sitting in that room, I would say a dozen. It would be a mixture of undergraduates, graduate students, postdocs, some senior faculty. There were quite a few visitors, maybe averaging one a year - senior people. Joe Weber, for example, a name you might remember from gravity waves, was a visitor for a while.

Smeenk:

Did anybody from Wheeler's group attend these meetings?

Peebles:

Well, there were certainly students. Students such as Kip Thorne would often sit in. John didn't. I don't think he ever sat in on one of our meetings. John had his own meetings, some of which I attended - not as regular as with Bob. How many people? You know, one measure we have is that on Bob's death I inherited - we've got to do something with them - his theses. [Peebles indicates a small bookshelf filled with theses.] So there they are. You notice the first two red-back volumes are his patents and publications. I don't remember which is which. And then a row of theses. So that's a measure of his production, much of it during the years of gravity physics.

Smeenk:

A lot of the years are late fifties, early sixties.

Peebles:

Yes, and then stretching into the sixties and maybe ending in the seventies. You mentioned independent work as an undergraduate, not a concept at the University of Manitoba. Here even then the notion of a senior thesis was big. Students were required to do a little bit of independent research and write it up carefully. And Bob led many undergraduate senior theses. These students would have been sitting there with the rest of us, in the gravity meetings.

Smeenk:

Okay. When you finished your thesis then you got a postdoc here as well.

Peebles:

Yes.

Smeenk:

How was that arranged? Was it something that happened naturally, or were you looking for positions elsewhere?

Peebles:

I was certainly looking for positions elsewhere. I didn't get any firm- Well no, I remember one firm offer from Henry Hill who was a post- doc and assistant professor here went off to Wesleyan University, the one in Middletown, Connecticut, and offered me a position there which I was seri- ously considering. I had looked for a position at the University of Alberta in Edmonton. You know I'm Canadian, and I was thinking of going back, but Bob said to me, "You might consider staying around for a few years." So I did, and one thing led to another.

Smeenk:

And how would you characterize the research outlook in, say, 1962. What did you picture working on? Had you started to drift into cosmology at that point?

Peebles:

Well yes. Let me remind myself. I arrived here '58, and for the first year and a half I was busy preparing myself for generals. I was talking to the theory faculty about possible theses, but I was also starting to attend Bob Dicke's gravity group meetings. I can't remember when the first was, but it must have been by 1959, and I started to notice that I enjoyed the gravity group meetings of Bob Dicke much more than I did discussions of particle physics. I don't quite know how Bob decided that he should log [JP: did I mean lock onto?] onto me, but I do remember him saying, "You might consider writing a thesis with me." So I did, and I did. In fact I remember, now that I think back, that my first projected thesis with Bob Dicke was an experimental one.

We were interested - I had started to work on constraints on possible time variability of alpha, the fine structure constant. If alpha varies it changes energy levels in atomic nuclei. That changes decay rates. And I found that there was one particularly attractive decay scheme, rhenium-187 - why do we remember such things? - decaying to osmium- 187. The energy level difference wasn't even known, it was so small. It is so small. And- [Brief interruption, as Smeenk closes the door.] So you see a sensitive probe of time variability of alpha is a decay scheme in which the two energy levels are almost exactly equal, because when the strength of the electromagnetic interaction changes they will each move in a different way.

Smeenk:

Oh. Okay.

Peebles:

And so it could even happen that although now rhenium decays to osmium, in the past it could have gone the other way. So we were very interested to know the energy difference, and I cooked up a little experiment to measure it. I still remember - Bob Dicke looking at some notes I had written up and a little sketch and his saying, "You know, you might consider theory." [Both laugh] I didn't say anything. I didn't ask him why he said that. Somehow I guess he could just tell I was born to be a theorist. And I have never looked back. So I can't quite sort out the sequence in which I withdrew from particle theory and moved into- Did you say it was '62 when that one particle theory paper was published?

Smeenk:

Yes. I'm not sure when it was submitted, but it appeared in the Phys. Rev. in '62.

Peebles:

I did have a little trouble getting it by the referees, which is good, because I much improved it on the second rewrite. I bet I had finished that by 1960. And I bet by then – yes. I had already started, heavily started in work on my thesis. Bob, I remember, even encouraged me to write that up. Very sensibly so. I had done the calculation. There was very limited interest, but it was interesting, why not clean it up and publish it. So I did. But rather quickly in my career I got into orbit around Bob and into doing things that were exciting. So it began the research focused on testing gravity physics.

That leads to astrophysics because that's where gravity is dominant, and so I naturally moved over first to the structure of the Sun where I didn't make many contributions, and then to the structure of Jupiter where I did I think make an original contribution. I argued - and I think it's now well established - that Jupiter has a rocky core and that an atmosphere of hydrogen and helium mainly - with heavy elements, of course, but the nonvolatile heavy elements in a core.

Smeenk:

I wasn't aware that you had written on that.

Peebles:

Yes. One has a constraint on how the mass is distributed within Jupiter from the fact that rotation causes it to flatten, and the amount of flattening is a function of how the mass is distributed within it. So I could only fit the observations by postulating that there was a compact core of heavy elements. It was about then that - well, two things had happened. Bob became very interested in the test of general relativity theory represented by the precession of the perihelion of Mercury. Which of course is quite close to what general relativity theory predicts. He wondered if that may not be a coincidence, you might recall, and that the Sun might be more flattened than one thinks due to rapid internal rotation. That would also contribute to the precession of the perihelion of Mercury and how do we know that there wasn't an accidental agreement with Einstein because of-

Smeenk:

Right. That's a very small effect.

Peebles:

It's a very small effect. So he had started to make a measurement of great precision of the shape of the Sun as a measure of its internal rotation and its quadrupole moment. I had done quite a bit of work on the theory of why the surface should be rotating slowly. There is a torque on the surface because it's throwing out plasma in the solar wind and that slows the surface down. And so I was working on that after my thesis, but not long after my thesis Bob came up with the idea of the microwave background. It's a sign I think of a brilliant physicist that can say something as Bob said to me: "Why don't you go think about the theoretical implications?" And I started doing it and haven't stopped. He didn't have anything specific in mind; he simply had the intuitive feeling that if there is anything to this concept it's going to have fascinating theory and someone should start looking at it.

Smeenk:

What did you admire most about Dicke's approach to physics?

Peebles:

Oh well, of course that he is so tightly locked. [JP: I don't know what this last sentence means! He was strongly committed to physics.] He loved theory, but only if it has a pretty immediate experimental application. He's really a very gifted theorist, but not at all inclined to go onto flights of fancy. I remember telling him after I was a postdoc about some ideas about how to quantize gravity physics. He laughed and said, "Go find your Nobel Prize and then come back and do some real physics." To him real physics was certainly theory, it could be very speculative theory, but it had to lead to an experiment that could be done in the near future.

Smeenk:

He seems to be unique in his combination of both great experi- mental work on radio telescopes as well as optics and theoretical insights.

Peebles:

You can count the number of Nobel Prizes he could have received. One that strikes me particularly is he discovered the Mossbauer effect.

Smeenk:

I didn't know that.

Peebles:

In molecular physics rather than in nuclear physics. The effect is now in some circles called the Dicke-Mossbauer effect. A wonderfully subtle business. When I used to teach quantum mechanics, I would always teach them the Dicke effect before the Mossbauer effect. Really wonderfully subtle, and Bob hit upon it. So he certainly was capable of great subtlety in his theoretical insight, but of course it was always focused on the phenomenology, experiments. And of course he was deeply gifted in his experimental concepts. I also appreciated, I think, that he had a somewhat light air to him. He never took himself very seriously and he certainly never took any of the rest of us seriously. Always quick with a light joke - to bring us down to earth more often than anything else. And also I should mention his breadth of interests. And that certainly grabs me a lot. Of course one always in doing research has to dig a narrow trough, a narrow trench. Otherwise you won't get very deep. But it's I think important not to focus entirely on the trench, but to look around, and he was very adept at doing that.

Smeenk:

Another question I had just in terms of the intellectual environ- ment here. There seemed to be a lot of people at Princeton in the sixties like Kurt Gödel who had an interest in relativity theory who might not have been part of any of the working groups.

Peebles:

Yes.

Smeenk:

So did you interact much with Gödel or-?

Peebles:

Well, do you have a lot of tape in that?

Smeenk:

[Laughs] I have a lot of tape.

Peebles:

Well, the charming story that John Wheeler and I both love. First you know John Wheeler and you know he is a man of great enthusiasm. I had a graduate student named Danny Hawley who wrote a semi-observational thesis. One can ask whether the galaxies are aligned in any particular way or are they random. And if two galaxies are close together, do they tend be like this or like this or just random? [JP: That is, do the position angles correlate, or do the ling axes tend to point to a nearby galaxy?] At the time the old particle physics technique of bubble chamber scanning had become obsolete.

There were no electronic methods. [JP: That is, the old technoque was manual, not electronic as it is now.] There were unemployed bubble chamber scanners and their machines. So we initiated a study, Dan Hawley and I, of the statistics of orientations of galaxies to see whether there is any possible departure from random. And we did an elaborate double-blind study in which bubble chamber scanners were shown images of galaxies. They were not informed they were seeing each galaxy image twice at a different orientation. So we had a control on the accuracy with which they could measure orientations [JP: and, more important, on systematic errors].

We had a spectacularly tight bound on departure from random; galaxies just flop all over the place. Well that's only of relevance because when Dan Hawley was in my office - there was a time on the other side of the building - handing me his thesis very - you know, it's a nice moment. John happened to be in my office when Dan came in. John immediately had to know about the thesis, and when he heard about it he said, "Oh. Gödel will be fascinated by this." So Dan Hawley, who is perhaps not the most widely read of individuals, said, "Who is Gödel?" And so John said, "To say that Gödel is the greatest logician since Aristotle is to do Gödel a disservice." He went on in a few short words to build Gödel as this enormously towering intellectual figure. Quite true, but he really laid it on. And then reached and picked up my phone and dialed up Gödel at the Institute and then handed the phone to Hawley, who melted down. It was quite a hilarious moment. You know that Gödel had this solution, a homogeneous anisotropic universe that would tend to rotate. On his death in his office was found a book of images of galaxies. It's the thing that's in fact lying on that pile of books - the long, flat one.

Peebles:

You asked whether I had contacts with Gödel. I did not apart from the Hawley moment. I don't think Bob Dicke had any contacts with Gödel. He surely would have mentioned it if he had. Bob did have some contact with Einstein. John certainly did too, John Wheeler. I did not. Einstein had passed away by the time I arrived. Other people interested in relativity here, well of course Oppenheimer. I never talked to Oppenheimer personally. He was rather an Olympian figure and I was a mere graduate student and we didn't make contact. Tried once. He had lined up a speaker who had some ideas that I thought then, I still do, were pretty off line. I tried to make contact, but it didn't develop. You mentioned other people interested in relativity. I'm not sure there were more. Certainly Lyman Spitzer was interested in relativity over in astronomy, but not deeply so I would say. I think we-

Smeenk:

I think that's about it.

Peebles:

Yes.

Smeenk:

In general how would you characterize the working conditions, and in more particular, how was Dicke's group supported institutionally? Were you always able to get grant money when you had projects that were going on? Was he supported by the department?

Peebles:

Yes. He was certainly strongly supported by the department. The department was mixed on his ideas, for example the Brans-Dicke theory was not welcomed by the particle physics community here. It's a little ironic, because it's still flourishing with us. Well, regarding that a side remark, not long before his death I said to Bob, "I see the Brans-Dicke theory is rearing its ugly head once again," and he said, "I never thought it was so ugly." I had to cover my traces pretty carefully then.

So in the sense that his theoretical ideas were not particularly welcomed, but they were not rejected by any means, and he had I think lots of respect in the department for his experimental work that carried over to all the support I think he needed for his research. That also includes research money from funding agencies. The situation then was very different from now. It was only a decade after the war. The research money that started flowing during the war was still flowing pretty heavily.

The physics community was much smaller, and my impression is that it was a lot easier to get research grants then than now. In any case, our spending needs were small, and mine in particular, aside from travel, but I never had occasion in those days to hesitate about the cost of say a trip to a conference. It was not an issue.

Smeenk:

Was your postdoc funded directly through the university or-?

Peebles:

To begin with, once I had finished my Ph.D., it was funded I think by the Office of Naval Research, of all things. And Bob Dicke] had research support from the Office of Naval Research and the Signal Corps of the U.S. Army. Again, two unlikely sources, but that was the way it was in those days. There was lots of small sources of support for pure research in unlikely looking places, such as the U.S. Army Signal Corps. I was a postdoc for a few years and then became an instructor, so that I was then paid half time by the university for teaching and half time out of a research grant. The one that I was paid out of I couldn't even tell you now. Then all during the time I was an assistant professor I was on half salary from research, half from teaching.

Smeenk:

Was that when you first started teaching, when you were ap- pointed to be instructor?

Peebles:

Indeed, when I was an instructor I started teaching. It's a quite light teaching load, and which is a very good arrangement. The university gets me at a very low cost. Doesn't get very much, but they do get also the research activity. So I would talk to students, do research together.

Smeenk:

I know you've discussed this in your interviews with Harwit and Lightman, the original work on the background radiation, but could you just briefly describe your work and what Dicke's group was doing and the phone call from Penzias and Wilson?

Peebles:

Well, let's see. One general comment which I hadn't thought to Martin Harwit or to Alan Lightman was - maybe I did make it, but - cosmology was an active physical science at that time but it had just a handful of active people. Furthermore I wasn't aware that there was much research going on. I wasn't even aware of the small amount of research that was going on - cosmology in say 1964 and 1965. I had read about cosmology in textbooks, in particular the great Classical Theory of Fields by Landau and Lifshitz has a section on cosmology, but it is pure mathematics.

Smeenk:

It's very condensed and concise.

Peebles:

It's very condensed and it has nothing about phenomenology. Well almost nothing. It has one very perceptive comment I later discovered. From such books I got no impression that there was ongoing research in physics. So when I started thinking about implications of this search for microwave background or a possible detection or, of course you had to prepare for it, for the possibility they just put another limit on the temperature. I was starting, so to speak, from scratch. I knew very little astronomy, but of course I had the resources of the astronomy department here.

They were awfully good people, and particular Martin Schwarzschild was very helpful, as was Harry Hess over in geology, another broadly knowledgeable physical scientist. But I was not aware even of the limited amount of research that was going on when I started out. That of course led us into problems, because I rediscovered, I reinvented the wheel a few times, and only over a year or so did we get it straight just what was new and what was already invented. That was around the time 1965. By the time I wrote my first little book on physical cosmology, which would be around 1970-?

Smeenk:

I think it was published in 1971.

Peebles:

Yes. So I would have- I had realized that this is an active physical science and I had started to study carefully the literature. So when I began, as I say, I didn't know that people had worked in this field at all.

Smeenk:

Just to go back, had you come across Tolman's book?

Peebles:

I knew Tolman. Yes. It's not here. [Referring to shelf behind his desk. It's over there, as a matter of fact, in connection with the paper I'm now writing. I've pulled those books out and I have got to put them back. I knew Tolman [the book, not the man]. I don't remember reading his derivation of the theorem that in an expanding universe thermal radiation will remain thermal.

Smeenk:

Right. And that just the temperature-

Peebles:

The temperature drops. That's right. That's key to this whole experiment of course, and I think I learned it from Bob Dicke, who char- acteristically gave a very simple argument, which I can describe. Take the expanding universe and place in it reflecting walls that make a container that expands with the universe. You have of course upset the universe; you have changed it by putting in these walls, but not statistically because now a photon bounces off. But before statistically the photon would have gone this way and the other this way. You really haven't disturbed the universe appreciably. But now we all know what happens in a cavity. One has stand- ing modes and one knows about the thermodynamic occupation number and all of that.

And it's just a couple of lines for you to see that as the cavity expands it cools automatically. It's Tolman's result without any of the mathematics Tolman used, and it was from Bob Dicke and it was the basis for this whole experiment, this whole phenomenon. I might have been aware of Tolman, because I remember in my first colloquium here on this subject Eugene Wigner asked me about oscillating universes and what happens to this accumulation of radiation. And I do remember being able to tell him that if the universe oscillated a hundred times the starlight we see would build up to the thermal background radiation we see. That notion of buildup of entropy during successful oscillations is in Tolman. You might have noticed the bouncing universe model in Tolman.

Smeenk:

Yes. He discusses that. I haven't looked at it all that carefully.

Peebles:

Yes. The idea of an oscillating universe then was in the air back then.

Smeenk:

And did Dicke favor that?

Peebles:

Yes he did. Now another interesting question that we can no longer ask, at least in this world, is what Bob knew at the time. He had, he remembers, attended a lecture by Gamow and possible seeds were sown. He doesn't have a very firm recollection of what Gamow talked about or that Gamow - had no recollection that Gamow was talking about thermal radiation, which could, of course, be entirely true. I've gotten the feeling in the last decades that there is though a sort of low-level, low-fidelity trans- mission of information through remarks exchanged in the hallway, random comments. The ideas diffuse around. And I don't know whether it's only idle speculation, but the thought that when people simultaneously or near simultaneously make a discovery, it's not really a coincidence necessarily. It could be that the idea was percolating in this low-level way.

I particularly find this picture appealing when I review the history of ideas on lambda and its connection to energy - which, as I remarked earlier, goes so far back. And the reappearance of ideas seems so roughly similar although different it fits, to me, in this idea of information diffusing around with low fidelity. And it's entirely possible that that was the case with Bob. He did like an oscillating universe, and that goes back at least to Lemaitre in the 1930s, who liked an oscillating universe. Whether there is a connection, who knows, but it could well be.

And of course it was the oscillating universe thought that led Bob to the radiation - not through light element production as with Gamow, but rather the entropy argument essentially. You'll have starlight, it will be thermalized during the bounce, and it will be useful too, because the hot thermalized radiation will be a good way to photo-dissociate complex nuclei and bring it back to hydrogen, which we need for the next oscillation. I think from the beginning Bob was motivated by the notion of an oscillating universe that would contain thermal radiation from the last phase.

Smeenk:

And you were not motivated as much by the oscillating universe idea?

Peebles:

No, no. We remarked that Bob was a very well-rounded physicist within theory and phenomenology. Also very willing to speculate if it could lead to a meaningful experiment. I tend to be more pedestrian, I think more Plebeian. I don't know. I get less excited about grand ideas and the notion about a universe that bounced, if we could make something of it I could get excited, but as a concept I shouldn't say it leaves me cold; it leaves me unmoved, because I don't know what to do with it. So no, I was never grabbed by the notion of a bounce, but I was grabbed by the notion of a warm universe. It has all sorts of interesting properties, and possibly observationally testable consequences, if you know what I mean. We just go - oh, we were talking about Tolman. So yes, I had read Tolman as a graduate student. I still have the volume. I hadn't read very far into it. I don't know whether I had read about the thermal radiation or about the bounce, or whether that came much later.

Smeenk:

And do you recall what your immediate response was when Dicke picked up the phone said, "Well boys, we've been scooped"?

Peebles:

Well, I wish I could remember. I suspect - perhaps I am even remembering it - a mixture of incredulity and pleasure, because it seemed to me - and I suspect to the others - that this was a longshot experiment. And to think that there might actually be something there to go after was exciting rather than disappointing. Of course in the longer, somewhat longer term, it would have been exciting to see the discovery made here - but not that much more exciting I think. For a few years after Penzias and Wilson and - well, let's remind ourselves. I remember a remark by Phil Morrison shortly after discovery during conversation in a very crowded room, and he said, "You measure the level of noise in this room and convert that into a thermal temperature and you'll get an absurd answer. How do you know you aren't doing the same thing here? Of course we didn't. And it wasn't until in fact COBE that we knew for sure that this was really thermal radiation. That was about 1990, wasn't it?

Smeenk:

Mm-hm [affirmative]

Peebles:

In 1965, twenty-five years earlier, this was a longshot. There were quite quickly a few indicators that it might be right, the cyanogen measurements. The Princeton measurement here. But they were by no means convincing. And so even I think as late at 1970 when I wrote the little book on cosmology I was pretty cautious about the conclusion that this really is thermal radiation left over from the Big Bang. So you have to factor that into the reaction of the phone call. First pleasure that there was actually something there to look for, a meaningful and positive measurement. Then incredulity - "Do they really have it?" Well, that took a visit (which I didn't take part in) to Bell Labs by Bob Dicke, Dave Wilkinson, Peter Roll. I don't know if anyone else went. They came back impressed.

Smeenk:

I could imagine they wanted to check the instrumentation to make sure it wasn't an instrument effect.

Peebles:

They wanted to talk very carefully, and they did, yes. That was right. What were Bob's feelings, I don't know. He didn't sound particularly distressed when he got the phone call. Dave Wilkinson remembered his saying, "Well, boys, I think we've been scooped." So maybe Bob was a little chagrined. I don't know what Dave's reaction at that point was. I've never thought to ask him.

Smeenk:

So you, as you said, I think you were very cautious in the way you presented these results in your book, and at what point-? ... Definitely COBE pinned down the fact that this was a black body spectrum.

Peebles:

Yes.

Smeenk:

But it seems that very quickly a wide section of the community was willing to assume that it was thermal radiation, so I was just wondering what your assessment was of the reaction.

Peebles:

I was a little uneasy. I hope I have this right in retrospect. Because we didn't - we had the power law part of the spectrum pretty well, but power laws often occur. How do you know that this isn't a power law of some other effect? Well, it has the right power law index. That's very encouraging. But you didn't know it broke over to the Wien exponential on the other side of the spectrum, and furthermore there were at least two serious measurements that suggested the break was not there. I remember first Martin Harwit, one of the first balloon borne - or rather, rocket borne - experiments to measure the short wavelength part of spectrum indicated an anomaly.

Then Berkley-Nagoya a decade later, an even more elaborate experiment. It seemed to be pretty strong evidence for an anomaly. And in fact when COBE flew - and we shouldn't forget our colleagues at the University of British Columbia, it's not so well known that almost at the same time an almost equally good spectrum was obtained by Herb Gush [and colleagues: Ed Wishnow and Mark Halperin] from a rocket experiment, flown in White Sands, New Mexico by a group from the University of British Columbia. They had results within a month and a half of each other.

Smeenk:

Oh. And COBE got most of the news.

Peebles:

COBE got most of the news. But again, it's curious that the big news from COBE was the detection of the angular fluctuations, the anisotropy. I think was the biggest news was that gorgeous measurement of the spectrum, or equally good.

Smeenk:

Yes, that's just a beautiful spectrum.

Peebles:

It didn't get the publicity, but it really had a profound effect on my feelings. Here it is: it's thermal spectrum, the subject is closed. Before that it's true, as you say, that many people took it as given that this is a thermal radiation from the Big Bang, and I felt a little uneasy about that because of the reasons I've explained. There is a tendency to adopt a working hypothesis and then forget that it's a hypothesis. Something that makes me uneasy about- I think we're seeing history replayed in this sense in the cosmological constant or dark energy. Many people accept it as real and established. The case is strong, but I don't think it's convincing. [JP: But post WMAP I think the case for detection of Lambda, or something that acts like it, is close to compelling.] It think it's not dissimilar to the case just before COBE with the thermal spectrum. You had indications, pretty good indications, that it's thermal. But you had worries. There is the difference that we don't have a measurement that contradicts apparently the assumption of lambda in the same way that the Berkley-Nagoya rocket measurement seemed to contradict the thermal spectrum. Although there are some technical issues. Lensing rate, the rate at which a background quasar is split into a multiple image by a foreground galaxy, most neatly fits an Einstein-de Sitter universe. We tend to think there's something wrong with that measurement, but that's a bit of wishful thinking. I'm sure - this is not a serious objection to lambda, but it is a worry. There is a tendency to put to one side the negative indicators and emphasize the positive. It's adaptive I think - adopt an optimistic point of view, you'll be a happier person. But the community does tend to swing too strongly I think to firm positions.

Smeenk:

Can I go back to your question from around when you were writing your textbook? There was another set of debates in observational cosmology at that time with Sandage and others about whether the universe was flat and what the value of the Hubble constant is.

Peebles:

Yes.

Smeenk:

And so I was wondering what your- I mean, your textbook seems to be very cautious in this regard and you don't seem to enter into the fray. I was wondering if you preferred a particular standpoint or just thought that the observational case wasn't strong enough then?

Peebles:

There are several issues here, to which I would add lambda, the cosmological constant. In the book I had only a few comments about lambda.

Smeenk:

I think you mention it in your introductory remarks and say, "We'll set it to zero."

Peebles:

Yes. And I probably said it because there wasn't a good measure- ment yet that can tell us whether or not it's there. At that time already Bob Dicke and I had this coincidence argument that later we published saying that it's very unlikely on the face of it that lambda should be an appreciable part of the stress-energy tensor. I didn't put it in the book because it didn't grab me one way or the other very strongly, and I didn't have strong opin- ions on lambda one way or the other. At the time some did. Some thought you absolutely must include it as an important unknown.

Others that it's absolutely absurd to think about lambda. Much the same remarks apply to space curvatures, space open, flat, closed. It didn't grab me because I didn't at the time know of any good interesting physical arguments that would say one thing or the other. Of course with inflation one got arguments, but then not. The value of Hubble's constant is rather different. I named that book Physical Cosmology for a very specific reason: I didn't want to tread on the toes of people like Alan Sandage, who to my way of thinking were doing as- tronomical cosmology. And I wanted to separate this. I was entirely happy to leave H0 in his book.

Smeenk:

Right.

Peebles:

And instead to go after the physical processes that were operat- ing, one thinks, in this evolving universe.

Smeenk:

That's interesting. I would have always guessed that you were also distancing yourself from say the steady state theory and the precursors which were more, I don't know, philosophically oriented.

Peebles:

Right. Well, it depended on the particular precursor. I think in the book I was fairly - I was not too negative about the steady state universe. I think I was a little more negative about the universe that Hannes Alfven favored in those days - an empty universe into which there is an explosion, plasma expanding. I had some pretty harsh words to say about it because I thought there was no way you could understand the very close to perfect isotropy of the thermal radiation in such a universe. The thing is, you have material rushing away from a center, and the only way the 3-degree radiation - of course we didn't know it was thermal then, but we knew it had about that effect of temperature, and we know it was isotropic to much better than part in a hundred. And that's very difficult to arrange in Alfven universe unless you have some sort of scattering centers that scatter the radiation. And with a short mean free path. But if there are lots of scatterers, we are not going to see high redshift radio galaxies.

Smeenk:

Because they'll be scattered.

Peebles:

They'll be scattered [that is, obscured] too, and so there is no way to make it fit. I know I annoyed him a great deal. And there was someone else, no it was Alfven I guess. There were others whose names I forgot who liked the Alfven universe for philosophic or other reasons who complained to me, but the simple interpretation of the observations seemed to me to be pretty convincing to rule it out. But there is a case that I guess illustrates what I like. I admire the steady state universe because, well first it was simple and elegant, and second it made definite predictions that could be checked. And I wasn't convinced entirely that it was ruled out because I wasn't convinced that this thermal, this background radiation really was thermal. On the other hand the Alfven universe - well, I never thought it was particularly attractive, but really what grabbed me was that it just didn't look right. There were other universes under discussion at the time, but not seriously so and I don't remember paying much attention to them. The old Milne universe, which you know fits the supernova redshift magnitude relation with really impressive accuracy.

Smeenk:

I didn't know that.

Peebles:

If that was the only measurement we had, Milne would be very happy.

Smeenk:

I also wanted to ask you: it seems in the late sixties and then throughout the seventies one of the major focuses of your research was on structure formation, and I'm just wondering what you see as the major de- velopments in that theory leading up to your book in 1980, and then the observational work about the correlation function.

Peebles:

Yes. The book was 1983 approximately?

Smeenk:

I thought it was 1980, but I could be wrong. Approximately. You know the book. Yes, 1980 actually.

Peebles:

Yes. Parts of the field have moved forward dramatically. The big successful story of course is the detailed measurement now of the temperature fluctuations of the thermal radiation across the sky. And the wonderfully good fit to simple theory. I was at the time fascinated by the distribution of galaxies, simply because it hadn't been studied much previously - in part I think because until we had high-speed computers it was such a laborious job. So the work that was summarized in that book was the result of a situation that I particularly like. It was an open field with lots to do and not many people doing it. Some aspects of that work have not advanced much.

I spent a lot of time on the small scale characteristics of the distribution of galaxies. I have been interested of late to notice a little work is being done on that, and of that work how much of it is - well, my guess is reinventing the wheel. There are some very elaborate pictures these days for how galaxies cluster on very small scales under the notion of - I guess the label is the "halo paradigm," in which one has these dark mass halos within which you put galaxies.

Smeenk:

Is that a fairly recent development?

Peebles:

This is fairly recent, and I can't even tell you the names of the people involved. I did write a commentary on it a year ago.

Smeenk:

Okay.

Peebles:

In fact if we remember, I'd be glad to give you a copy of the commentary. It is a case where I think people haven't remembered the old lessons and are reinventing the wheel. That is referring to small scale struc- ture, which arguably isn't so interesting anyway, because small scale structure is complex. And no matter what the ultimate truth of the phenomenology and what it's telling us, it's going to be a complicated thing to interpret. The big advances of late have been on large scale structure, which have been exquisite. When I wrote the book they were not adequate to describe the large scale fluctuations in the distribution of galaxies. The data are not much improved.

The situation is much better. There are now measurements where I had only crude upper bounds to the autocorrelation function of the power spectrum. The theory of these large scale fluctuations is working wonderfully well. It is based on a paper that I wrote actually in 1982 approximately, the cold dark matter model. That paper was a natural - I think I wrote it at about the time that book was published, plus or minus a year, it must have - you're right, it must be in about 1980, because the book was published, I was using results from it to write down this CDM model which is now the standard paradigm. I wrote it down because the measurements of the angu- lar fluctuations of the background radiation were improving, and the upper bounds were getting smaller and smaller ...

Peebles:

[You asked about] that book.

Smeenk:

Actually I was thinking of first the developing views leading up to the book. Do you remember what the standard views were in the seventies?

Peebles:

Well, in the sixties a standard view was that people were aware that the galaxies are not uniformly distributed. There are clusters, there are groups. There were analyses of the evolution of this irregular distribution and the expanding relativistic cosmology. There were some mistaken arguments that gravity would not much affect this distribution.

I got into this game in part because I worked through again the theory of the evolution of departures from homogeneity, convinced myself that the universe is gravitationally unstable, so convinced myself that the fact that the universe now is pretty close to smooth means that it had to be very close to smooth in the early universe, and furthermore that that meant that one could follow the development of structure from near homogeneity to increasing clumpiness, and by measuring the clumpiness now get a handle on what initial conditions were. This was not a popular line of approach in the late 1960s, because as I say people had somewhat mistakenly convinced themselves that gravity is not an important factor in galaxy formation and formation of clusters of galaxies.

Smeenk:

What did they think were important factors?

Peebles:

Well, it was left open. One didn't know why galaxies existed and why galaxies tended to be clustered. There were phenomenological dis- cussions. There were debates you might recall on whether galaxies formed and then moved together to form clusters or whether clusters formed and fragmented to make galaxies.

Smeenk:

So whether it is bottom-up or top-to-bottom.

Peebles:

Yes. And whether for example we could have been ejected from the Virgo cluster. It was not a very coherent discussion at that time. People had different opinions but didn't analyze them very carefully. At the same time in the 1970s there were lying fallow some catalogs of galaxies that could be analyzed, their distribution measured by simple statistics and compared to the theory that I believe was correct, that is correct for the evolution of these fluctuations and how that would be manifest in the distance, these n-point correlation functions that are at the center of that book.

So it was, this was - when I starting studying these statistics there were a few older papers by people who had measured these statistics in a very simple way, simple because the data were limited and the data handling capability even more so, by Nelson Limber at the University of Chicago, Vera Rubin at the Carnegie Institution in Washington.

It's notable as I reflect on this that I was able to meet and talk to all the principle players in this subject when I decided to get into it. There were only a handful. Try to do that today in this field, it doesn't happen. I could see a good line of approach that involved both the measurement of these statistics and the interpretation of the results within theory. That's what led up to the book.

Smeenk:

And did you work with a number of graduate students on this?

Peebles:

Yes, yes, quite a few. I tend not to have many graduate students. I tend to like to sit in the corner and scribble away. But for that project we needed graduate students, both on the analysis side and the theoretical side. It's an interesting sociological phenomenon I believe, a real one, that when I needed graduate students they appeared. Graduate students talk to each other and they know what's interesting and who is doing something that might be interesting to a fellow graduate student.

Smeenk:

Who were some of the graduate students you have had?

Peebles:

Well, let me see. Mike Seldner who is now with Lucent, at least the last I heard; Ray Soneira who started his own computer company, still in New Jersey; Jim Fry who is on the faculty at the University of Florida in Gainesville; Bernie Siebers who went off to work at CDC, the big supercom- puter company - I'm not sure where he is now. It's not such a large number, but I think that might be everybody.

Smeenk:

Were you Margaret Geller's thesis advisor?

Peebles:

Margaret Geller was earlier, yes. In fact, the analysis of the the- ory of how galaxies cluster and clustering develops originated with Margaret Geller, not in her thesis but in a separate project ahead of her thesis. We wrote the first analysis of the relation between the clustering of galaxies and their velocities, developed into a now standard tool to estimate the mean mass density. So sure, she should be counted as part of that pack, although she was a precursor well ahead of the rest. There were a few postdocs. Marc Davis played an important role in the theory side. He is now at the University of California in Berkeley, along with Jim Fry on the theory side. ]JP: That is, on the theory side, not at UCB.]

Smeenk:

Again a general question. During this time period was there a really pronounced increase in the number of graduate students willing to work on cosmology, and the support for this type of research?

Peebles:

Oh yes, both here and elsewhere the field started growing. The field was at a very low level of activity in the early 1950s. Almost dead - not quite, but close to it. The activity had already picked up by the time of the discovery of the 3 degree radiation and the radiation certainly helped increase the activity. Support was not a problem. Remember as I said, in those days getting a research grant didn't seem to be as difficult as it is now.

A reasonably active person would get a grant. It was more that people increasingly decided to work in this field. There was not a rush of activity immediately after discovering thermal radiation, but rather a slow buildup in activity that continued into the 1970s. Actually I think in the 1970s the activity reached somewhat of a lull. I was very active in the 1970s with the analysis of the distributions of galaxies and the theories of what the measurements indicated, but I was not at all pressed by competitors, people who were getting into this field. The big burst of activity I think came in the late 1980s. Now of course the level of activity is just amazing, orders of magnitude greater. More people are working in the field than that. [JP: I can't interpret this last sentence.]

Smeenk:

Right. There did seem to be a brief burst of activity focusing on approaches to the initial singularity in say Misner's chaotic cosmology program.

Peebles:

Yes.

Smeenk:

And I was wondering what your reaction was to that line of research, which seems to have petered out sometime in the seventies.

Peebles:

Yes, yes. Well I was always skeptical of it, but Misner's Mix- master Universe was wonderfully elegant and he identified the very serious problem with horizons. But I was always skeptical of the result because I had already convinced myself the universe is gravitationally unstable - meaning that absent highly special initial conditions if the universe is chaotic to begin with it will just get worse.

Smeenk:

Right.

Peebles:

And so I could not get too excited with the Mixmaster Universe or any of the ideas that [were] then floating about on how to understand the initial singularity. It was important work. I was not nearly as supportive of it as I probably should have been, but it just seemed to me to be not promising - at least not to my taste.

Smeenk:

Right. It's interesting that, as far as I know, they did focus on homogeneous and anisotropic solutions.

Peebles:

Yes.

Smeenk:

So they weren't even the most general solutions. And you could try to show that that isotropized, but it was still not inhomogeneous solutions. It still wasn't completely chaotic.

Peebles:

You've dropped only one shoe. So that is exactly the point of why I was not attracted by that approach. Inflation was really quite a shock to us all. The problem was serious; we just had no idea what to do about it.

Smeenk:

I wanted to ask you a little bit more detail about-you have a '79 paper which a lot of people refer to with Dicke, on the enigmas and nostrums, the paper in Einstein's centenary volume. It's not clear to me how seriously you take these fine tuning problems based on that paper, how one should take these. And in the paper you present them as, well if you look at what the initial state has to be it's very odd, but I was wondering how you thought you should deal with that, or whether you would be happy stipulating a special initial state at the end of the day.

Peebles:

Of course one isn't happy to stipulate anything. You want to derive it from fundamental principles. The tone was lighthearted in that article. That's Bob Dicke's style, I hope mine. We were maybe a little fey, but I think we were discussing serious physics, but it seemed to us that the coincidence argument that was presented there wasn't a theorem. It wasn't new to us. I think I remember that argument floating around here when I was a graduate student. I don't know where it originated. I have been interested to try to discover other references to the idea.

There are a few in the literature, but scattered here and there. I suspect more often it was discussed but not published as an argument you don't know what to make of. What do you do with this argument? And that was one of our problems in writing the paper you refer to. There are these puzzles, but you don't know whether they are deep, telling us something important, or only curiosities. It is the difficulty of presenting a problem without being able also to present a possible solution.

I think the problems were pretty well known and recognized in the community but more treated as dirty laundry, if that's an appropriate expression, because you didn't know what to do about them. We certainly spent a lot of time puzzling about the initial conditions and going in circles.

Smeenk:

And with the work on large-scale structure you could trace back to an initial spectrum of density perturbations.

Peebles:

It was at least a view into what the early universe was like, and that view you value. You don't know what you're going to learn from the view, but it surely can't hurt to have it.

Smeenk:

So then when inflation came along, as you said, it was a surprise to the community. But what was your initial reaction?

Peebles:

Skepticism of course. I tend to be very conservative, I think reactionary, for [someone with] such bleeding-heart liberal political views I tend to take a long time to change my mind about physics. But I eventually came to see that it is a savior. It also made me feel uneasy, you know, because one is presented with a solution to a problem that was well posed - not well advertised, but the problems were known. And so some bright young person came up with a solution. How do we know the bright young person wasn't simply crafting a fairy tale to fit the required problem? To fit the problem? How do we know? We still don't know.

That is why I got pretty excited by Paul Steinhardt's ekpyrotic universe and these other notions of brane cosmologies - not because I think they are any more persuasive than inflation but because they are an alternative. And for a long time it seemed that we were going to have to accept inflation by default. We have these deep puzzles, inflation offers a solution, we have no other solution, so by default we accept it. That's not the way I like to do physics. I like to have an experiment that forces us to accept it.

Smeenk:

Right. So you like to have some understanding of what the alternative theories are.

Peebles:

Or else, if there is only one theory it should offer us a smoking gun - a prediction that is so distinctive that it compels acceptance.

Smeenk:

Right. And in the case of inflation you don't think there is-?

Peebles:

Have you heard of a smoking gun?

Smeenk:

Well, one other question I wanted to ask you, a lot of times people characterize inflation like Mike Turner, who calls it a paradigm without a theory. And a smoking gun, one of the problems with trying to identify a smoking gun is that you can invent models of inflation which are so different that they don't have the same features.

Peebles:

There is the frustrating thing. Let me repeat that phrase "it's a paradigm without a fundamental theory." Well, it's a catchy description. I'm wondering what it means. A paradigm is a set of operations of description of how to behave, how to act. I guess it's a fair description. There it is. As you say, because inflation is not a fixed theory, instead it is very difficult to find a distinctive prediction of inflation. So how do we know it really happened? Maybe we'll never know. But I think fundamentally important is to know whether there is an alternative paradigm. That would get me excited. And if people who are very clear and intelligent spend a lot of time and come back and report to me that there just isn't another paradigm in sight I'll have to be impressed, but also disappointed. We'll be left with a race with only one horse. It's not very exciting.

Smeenk:

I also wanted to ask you in terms of a general point about assessing scientific theories, would you say that it's much more important for a theory to make a novel prediction that's completely unexpected than to just be able to accommodate observations that were known beforehand.

Peebles:

Right. Of course a theory may be an excellent approximation to reality and only accommodate observations already known, and how can we complain if that's the situation? It's much more exciting if the theory will make a unique prediction and the prediction can be verified. In the real world I think well established theories have a mix of both, and the mixture depending on the particular theory. What's characteristic of course of the- ories that are really believable is that they manage within the compass of a few parameters, a few adjustable constants and the like to fit a broad range of phenomenology.

The broader the range, the more persuasive the case that you have an approximation to reality. So I believe - I think the word is I am an idealist. I believe that something really happened out there and that we can discover it, and we'll know when we have it because of a dense network of different observations, very different observations, independently drawn aspects of the theory all fit the same theory. So we have that in, say, quantum mechanics. It's just daunting to reflect on how many different applications of quantum mechanics are consistently contained in this simply described theory.

We're starting to get it in cosmology. I am deeply impressed with the successes of the recent passes in cosmological tests. I'm much less con- vinced than perhaps Michael Turner is that we have found ultimate reality, but that's secondary to the excitement both of us feel about the number of different measurements that have come along and will come along that seem to fit together quite well.

Smeenk:

Let me ask you a few more particular questions about inflation. So I'm going to mispronounce his name, but your collaborator Ratra?

Peebles:

Bharat Ratra.

Smeenk:

So you worked on some open models of inflation with him.

Peebles:

Yes, I did.

Smeenk:

Do you still find those to be appealing or-? The reason for asking this question is a lot of other people in inflation think of open models of inflation as somehow inherently contrived.

Peebles:

Yes. It's an interesting question. Am I being cynical when I offer the opinion that if the observations suggested to us that the universe is open, we might decide these open inflation or open theories of the early universe have some pretty attractive features too. Don't we tend to attune our opinion of what's elegant to what works? It must be true to some extent. I don't know how you can decide that open inflation is more or less elegant than the standard inflation. They both have lots of bells and whistles that evoke strictly the same phenomena. So no, I don't - I can't get at all excited about suggestions of superior elegance. We're all impressed with how elegant is fundamental physics, but do we say it's elegant because it works so well?

Smeenk:

Right.

Peebles:

It must be true to some extent, but-

Smeenk:

And when it comes down to comparing particular ideas in terms of elegance that just seems so hard to do.

Peebles:

It is hard to do. It is hard to do.

Smeenk:

What sort of results would make you much more confident that inflation is on the right track or is the ultimate theory?

Peebles:

I guess the burden is going to fall on the particle physicists to come up with tightly constrained, perhaps super-string theory, that will be so well nailed down that it will unambiguously produce for us physics that would drive inflation. That could certainly happen, and it could happen that they find themselves led to a theory that is sufficiently coherent and compelling in its internal consistency that they will have to believe it's true but won't be able to check it because it will be beyond all of the energies accessible in accelerators, but it will have this one wonderful feature that it implies properties of the early universe that the cosmologists can check.

That would be wonderful. Within cosmological tests I don't know of a critical measurement that would ever convince us inflation really happened. [JP: Though I forgot to consider that the effect of gravitational waves on the 3k anisotropy might be pretty close to a smoking gun.]

Smeenk:

So one way to derive inflation would be for it to fall out of a theory of particle physics-

Peebles:

Yes.

Smeenk:

But a lot of people would say that the opposite inference could also work.

Peebles:

Yes.

Smeenk:

That you might have cosmological tests that would force you to include a fundamental scalar field in particle physics.

Peebles:

Yes.

Smeenk:

How do you feel about that second one?

Peebles:

Well, we could think about this. It could happen that we have a tightly checked set of cosmological tests that imply certain properties of the physics of the early universe that then get applied to particle physics and are seen to fit into a series of works. Would we find this a convincing demonstration? Well, we would have to worry that perhaps everyone is being very clever and making a "just so" story. How will we know? But you know I think that Newton could have had this question if he were civil with Hooke. How will we know that this inverse square law really works, say, on large scales?

We'll never know. We'll never be able to extend the test past the Moon. But of course people did. So I think one can be a Pollyanna and say, "Well, it will all turn out all right," but the record is on that side time and again. Science has advanced, and maybe it will happen again.

Smeenk:

That's true.

Peebles:

An example that people are excited about that certainly is excit- ing is the notion of small dimensions that could have experimental laboratory consequences. And if that ever could be detected it would be wonderful indeed.

Smeenk:

Are you referring to the experiments looking for a deviation from the inverse square law at short distances?

Peebles:

Yes, yes, and the possible connection to these Randall-Sundrum and other brane-world cosmologies. Such a connection would be such a won- derful gift from nature and could happen.

Smeenk:

Let me ask you a few more general questions again. There has been a very active research group, very many research groups in the Soviet Union working on cosmology throughout the time period we've been talking about.

Peebles:

Yes.

Smeenk:

And I was wondering, did you self-consciously keep track of the Soviet literature? Was that something that you would typically check, to see what say Zeldovich was doing?

Peebles:

In fact no, not as much as I might have, probably should have, and I have never made a pilgrimage to the Soviet Union to visit Zeldovich and his group. I met him only once. I think only once, in a conference in Hungary toward the end of his career. We were certainly in contact by mail and exchanged opinions. We were certainly in-

Smeenk:

In what language were you communicating?

Peebles:

Oh, English. Yes, yes. I'm so embarrassed. You know I grew up in a province that is officially bilingual, French and English, but when I go to France I have to stumble along in English.

Smeenk:

Well, I don't speak French either.

Peebles:

I earlier commented on the importance to me of TeX as being the universal language in my small field, and it's so very convenient that English is the universal language across the entire world in science, physical science. It was Latin, it was then French, in some fields German. Now it's English. So I was very aware of what Zel'dovich was doing, and we did things often in parallel. Important calculations were done independently in parallel. At that time and I guess still the U.S. translated the main Soviet journals, so they were available in English. I did not haunt the library to see what the latest result was. You have to remember that the rate of production of research then was a lot slower than now. Now if I wanted to I could log onto Los Alamos or the - I don't know, do we say archives these days, Ginsparg's operation?

Smeenk:

Yeah, I have heard it referred to as "LANL", just because of the web address [the Los Alamos E-Print archive; [ http://arxiv.org/].

Peebles:

Yes. One can log onto that each morning and get literally dozens of new papers in cosmology. It was nothing like that in say the seventies. There was a paper a week or so that might be interesting.

Smeenk:

So did you, in order to keep up with the literature, did you hear about these things from personal contacts more than, say, just checking the journals?

Peebles:

Yes. In those days it was very easy to be lazy and not bother with the journals - instead to talk to people you either met here or met at conferences. In fact I do still tend to operate that way. I don't log onto LANL every morning. I find it depressing. Too much stuff. I could not possibly keep up with it. Instead when typically a younger person says to me, "Why aren't you looking at so-and-so?" then I'll go look it up.

So I have still been pretty intellectually lazy and not following every detail as it comes along but instead waiting for people to mention it. And the community does work very well that way. There is an awful lot of exchange of information. And I don't think physical scientists are particularly sociable people. They tend to be a little bit - what, eccentric?

We're not as sociable as the population at large. Maybe I'm wrong, but we are sociable in a sense. We get together and we drop hints at each other. Messages appear that affect me a lot. I find I'm back from a recent conference] and I've got to look at this paper and that paper and I've got to think about this and that remark. That certainly was the way I operated in the seventies. I waited until someone gave me a nudge.

Smeenk:

And clearly your relationship with Dicke was one of the really important relationships in terms of shaping your career.

Peebles:

Oh yes.

Smeenk:

Were there any other people who had sort of comparable influence or maybe people that you influenced in a similar way, graduate students?

Peebles:

I don't know, I don't know.

Smeenk:

Or just collaborators who might interact a bit with you?

Peebles:

You might have noticed in my publication list I'm not a heavy collaborator.

Smeenk:

Yes, I did notice that. That was something that struck me.

Peebles:

Yes. And I remark I love to sit here in my little office - actually big office - and scribble, think. I tend not to interact well because I'm either lagging behind or racing ahead. Other people who have influenced me? Well, quite a few, but no one else stands out the way Bob Dicke does. David Wilkinson one office over has been very important to me. He's an experimentalist, of course, and he keeps me very much aligned with what's experimentally possible. There have been other people through the years.

The few students I've had have certainly been influential and others I've worked with. Margaret Geller started me on this game of the distribution of galaxies, the dynamics. Marc Davis further down that road. Jim Fry on higher order correlation functions. Bharat Ratra. Very few of these people did I seek out. In fact I'm not sure I've ever sought out someone to work with me. They tend to show up, and if they persist I start working with them. I think in each of these cases that was so. How much have I influenced them? You'll have to ask them.

Smeenk:

Okay. And one thing that's really interesting for me is just the relationship between theoretical work and observational work, so in Dicke's generation it was still possible, although he was ahead of the curve in being able to be doing both.

Peebles:

Yes.

Smeenk:

But how about in your own career? Is it mainly through personal contacts with people like Wilkinson that you are able to keep in touch with progress in experimental work and then combine that with your theoretical work or-?

Peebles:

Well, I would have to reflect on that. One comment first. Much of my work has been almost observational. I have not gone to a telescope, but I have done heavy analysis of data, and I have done it because I have been keenly interested in the results. And no one else was analyzing them, so I did. In that sense I guess I've had a pretty good exposure to the way observations act. I also tend to like observa-

Peebles:

...I think a personal choice by and large. Not entirely so. You can't follow in detail both [observational and theoretical work], and modern observations and experiments are getting so complicated that more and more you do need a guide to the literature, to the experiments. That was not the case in the seventies, for example. I don't think I missed out on understanding of phenomena because I wasn't next to a big telescope and couldn't talk to the people who were just coming down off the mountain.

The situation was simple enough and moving along slowly enough that I could digest what was happening and get a fairly clear picture. I'm not sure that that's the case now. It was my prejudice that when the observational situation is confused it doesn't pay to pay a lot of attention to it. It will settle down, and when it does people will start to make coherent remarks and will start to agree with each other. And then it's time to start paying attention.

So I've not ever felt isolated from the phenomena even though I don't always go to the mountain to talk to people. I do attend quite a few conferences and I do enjoy very much the observational papers and I get a lot out of hearing people speak about the observations and then talking with them in private in the corridors. I've tended to have forgotten what the drift of your question was.

Smeenk:

It was just a very general question about theory and observation and how things have changed.

Peebles:

Yes. How has it changed? Well, I think it is notable the observa- tions and experiments are getting more complex and so the barrier between theory and observation is growing. I think it's approaching the situation of particle physics where the barrier can be pretty high. Understanding what's coming out of an experiment can be a pretty messy operation. But in both I think it's still the case that you will end up with a few rather condensed results that you tend to believe you would observe, a theorist, because sev- eral different observers will tell you, "Yes, I agree with that. I have tried my darnedest to find a mistake. That's the way it is," and these condensed results are still accessible to both sides. There is of course a lot of theory in cosmology that is quite far removed from any experiment that can now be done - brane worlds. It's brave work, very exciting, but it isn't the sort of research I love because it doesn't make that wonderful connection.

Smeenk:

Do you ever wonder if the high-energy physics, such as string theory, brane worlds or physics of the early universe, is going to just not have that same kind of connection back with experimental reality?

Peebles:

I wonder and worry. It would be a very sad situation if we had a wonderfully constructed theory that had no connection to reality - or at best had a connection in its design. No guarantee that that won't happen.

Smeenk:

That's true.

Peebles:

But we've never had a guarantee we would get as far as we did, and who knows how much further we'll get. Yes, the wonderful possibility, the horrible possibility of tightly constructed totally internally consistent theory that you can't check.

Smeenk:

What do you do at that point?

Peebles:

What do you do with it. Some people will say, "Well, that's it. We've solved the problem. This is the way the world is." I know that people feel that the universe has demonstrated that it is comprehensible by us. That's a wonderful thing. It's quite astounding that the universe has decided to operate by a way we can understand. Why isn't magic part of the world? It just doesn't seem to be. Because the mathematical method in physical science has been so spectacularly successful we tend to think mathematics will tell us what reality is, but of course it isn't quite that simple.

There's a lot more mathematics than there is physical reality. And so far we've been fortunate in being able to make experiments, observations that tell us which parts of mathematics area reality. Of course, I'm assuming there is a reality, and just as one assumes that mathematics will tell us what reality is, it's an assumption. Maybe it's another assumption that there is a reality. But no, physics has worked so well, there is a reality. We are not making this up. The deconstructionists would be entitled to step in, wouldn't they, to this theory where supposedly fits everything well by construction and to say, "Why do you pay any attention to that?" They miss totally the point of present physics. You pay attention because of the spectacular coordination of phenomenology that the theory gives you.

Smeenk:

I think they often just don't realize how tightly over-constrained theories are.

Peebles:

Yes. That's the wonderful thing about physics. Yes.

Smeenk:

There are a few other topics I want to discuss. We could either continue now or save them until tomorrow. I don't know how you're feeling.

Peebles:

We have a tea at this point and cookies. Would you like to stop and then we can decide what to do next?

Smeenk:

Okay, sure. [tape turned off, then back on...] I wanted to ask you about several recent observational results, but one of them I think in the Lightman interview you said it was like having a curtain pulled away from something; that was the results of Geller and her collaborators regarding the large scale voids and structure in galaxy distribution.

Peebles:

Yes.

Smeenk:

I was just wondering if you could comment on your assessment of those results and how they've been enriched by more recent surveys.

Peebles:

I am pausing to think. It's part of the charm of physics that you, through new observations, get new views that enhance your appreciation of the way the world is. The Geller measurement was particularly dramatic because the trend changed from what we had - angular distributions - to three dimensions, and so clarified the nature of the distribution that it was quite a dramatic advance. Perhaps that's in part just psychological; one is impressed to see the richness of detail in three dimensions that was only implicit in two dimensions, but also it's an advance in science, genuine and true. I guess you could say that a similar thing is happening with the theories for structure formation tracing back to the early universe, the CDM model in particular.

When I wrote it down twenty years ago, I didn't take it at all seriously. It was written down - I wrote it down because it was simple and it could fit the observations. It was rather in the nature of the theories we discussed that are self-consistent and elegant in fit with observations but they were designed to fit the observations. And that was certainly the case with CDM. I had a couple of constraints and I showed I could get them, but that didn't mean the theory was right and it certainly didn't mean it to me at the time. I took it as one simple possibility. Deeply impressive to me is how successful that theory has been.

Smeenk:

Just a quick clarificatory remark. Did your original model include biasing?

Peebles:

Oh no. Oh no. And in fact by an irony it got about the right temperature fluctuations on large scale because I assumed no biasing and I assumed an Einstein-de Sitter universe. Which I knew at the time wasn't very likely. Mass density is so high it's hard to understand the low peculiar velocities of galaxies. But I made that assumption for simplicity and so I had a consistent story. It's still the case that so far as the anisotropy of the thermal radiation goes you can fit to an Einstein-de Sitter universe with no lambda and no space curvature with cold dark matter.

[JP: Though WMAP changes that!] It's the other observations that force you away from that theory, not the CBR anisotropy alone. No, I didn't think of biasing. I assumed galaxies trace mass and didn't think about it. Biasing arose a few years later, most notably with DEFW — Davis, Efstathiou, Frank and White. I was never a fan of biasing for a simple reason that traces back to an experience I had with Marc Davis. He was the leader of the team that got the first really substantial systematic redshift survey in which one could look in three dimensions - not at the depth of Geller, so we didn't see the richness of detail, but at least we saw the beginnings of what things look like.

And while I was visiting Marc - he was then at Harvard - he showed me two maps: the giant galaxies and the dwarfs, side by side. He hadn't put on a label which was which and we stared and them and we said, "Now which is which?" And in fact it was very hard to see any difference in the two maps. Giants and dwarfs are distributed in a very similar way. It made a deep impression on me. I still remember the event. I still remember him and me sitting there and saying, "Well, this is crazy. Which is which?" The biasing picture, I am convinced, very naturally predicts that giants are more tightly clustered than dwarfs. It falls in the whole picture of the masses more smoothly distributed, dwarfs more tightly clustered and giants still more tightly clustered.

And that's not what we saw when looking at that map, and that very much influenced my reaction to biasing. I thought it was crazy. That caused me to abandon CDM and to go off in search of other models for structure information - which I think was a good thing in many ways, because we had to cast our net a little more broadly to see what the options are. There was a lot of net casting during the eighties and early nineties. Most of the ideas have fallen by the wayside, at least temporarily, and biasing has fallen by the wayside. Though it's still heavily mentioned.

Smeenk:

Are these falling by the wayside mostly because of conflicts with new observations or-?

Peebles:

Well, it's interesting. In some cases definitely yes; in other cases, well, let's just put it aside because it's a little hard to work out and this other theory is looking awfully good. There was certainly some of that, just abandon ideas because it's ... The CDM is awfully convenient for numerical simulations. That certainly must have been an important motivation for the early work on it. But it also was seen to work very well, aside from this biasing business. I speak with some emotion because many people still emphasize the notion of biasing and say that it's an inevitable consequence of physics. And yet when you look at the situation, boil it down to the simple ideas that the observers are willing to endorse, you see very little evidence of it.

Smeenk:

In a recent review article you mentioned a problem of void galaxies related to biasing. Could you explain that?

Peebles:

Hah. Well, I started with this episode - Marc Davis and me in his lab at Harvard - looking at the two maps, seeing that the giants and the dwarfs have very similar distributions. That phenomenon has now been explored in exquisite detail and the simple situation is that large and small galaxies cluster together. There are large voids in the distribution of galaxies. These voids are respected by or avoided by equally giants and dwarfs. In short, the voids are just empty, very little in them. It's quite dramatic. For a wide range of types of galaxies and of luminosities and I think masses of galaxies the voids are empty. There's nothing wrong with that, except when you look at numerical simulations of the CDM model you see that there is wispy mass in the voids and the mass distribution is lumpy. And I would have thought that the lumps will be capable of making stars and that we will then see little galaxies or stars.

Smeenk:

These are lumps of baryonic matter or-?

Peebles:

Well, of course what's easy to simulate is the lumps of dark matter. Baryons are much more messy and complicated. So what seems most clearly seen are the lumps of dark matter. Heavily debated whether the lumps will hold on to the baryons and some to form stars and make the thing visible. But my opinion - which is not widely shared, which is great, we should have debate - is that the voids are suspiciously empty and that there is something a little wrong with the CDM model.

It can't be deeply wrong because the CDM model is looking awfully good, but I'm convinced that there's something wrong. It's an example of how a theory may be a good approximation, but after all it's part of our philosophy, isn't it, that we never show that a theory is identically right; we show that it's a good approximation. The CDM model is a pretty good approximation in some aspects. It's deeply impressive how successful it is, so I believe it's an approximation to what really happened.

But I think the approximation is still pretty crude and I think the voids are an example of something its missing. Maybe you noticed another article on redshift formation of elliptical galaxies. Again, the subject is not heavily debated, but I think there is a disconnect, that elliptical galaxies look old. And simple interpretations of the CDM model say that many of them formed with redshifts less than one. And that just doesn't quite connect. The CDM model after all was invented to be the simplest of all possibilities that would fit the observations. The world is allowed to be complicated. The world is complicated. And it won't be at all a shattering surprise if we, as I suspect, have to add some complications to the CDM model. Much less clear in my mind is what the complications will be.

Smeenk:

Right. This might reveal how little I know about the area. I'm interested in how the constraints on dark matter from velocities in galaxies compare to the constraints from initial structure formation and then what the different candidates are for those two cases. So could you perhaps discuss

Peebles:

Well, perhaps you have in mind the classical hot, warm and cold dark matter, which describes simply an amount of velocity dispersion among these imaginary hypothetical particles ... Sorry?

Smeenk:

Whether they're relativistic ...

Peebles:

Right. Well, of course we don't observe relativistic particles now, but neutrinos would have been relativistic at fairly modest redshifts. Moving so fast now that they wouldn't stick in extreme dwarf galaxies. That's one reason why they're strongly disfavored now. Another reason is that the free streaming suppresses the formation even of large galaxies until lower redshift, which is quite unacceptable I think.

Smeenk:

So people understood this in the seventies? Or was this later that people started to realize that cold dark matter-?

Peebles:

In the seventies, even in the eighties, Zel'dovich's group was strongly supporting the hot dark matter picture. Here's an example where we were quite at disagreement. It seemed to me to be crazy to think that the prediction of the hot dark matter and the massive neutrino model could work - that first you would form clusters that would fragment to form galaxies. This is not in the cards.

The Milky Way is in a pathetic little group that's just now forming. How in the world could the Milky Way form in this picture? It used to drive me non-linear. So there was an example of a perfectly attractive model for the dark matter, massive neutrinos. We know neutrinos exist, [we have] compelling evidence. We don't know their masses but you could easily imagine one of them has the right mass to fit this model - but nature chose otherwise. There has not been a lot of work on dark matter that is not simply modeled as a gas, perhaps with point-like scattering cross sections.

We infer the existence, I think pretty compellingly, of dark matter through its gravity. We assume it has properties that are as simple as possible, as we can get by with. Of course that's the thing you try first, but no guarantee that this dark matter really has simple properties. I think we shouldn't be surprised if in the coming decades we discover that the dark matter is more complicated than we visualize. It wouldn't be a shattering blow; it would be fun to explore.

Smeenk:

What do you think of a more phenomenological approach, like modified Newtonian dynamics?

Peebles:

Well, I think I respect it. I think it's not very successful on large scales. It's remarkably successful on the scale of galaxies and I am very impressed with that. I think that's fun. And I encourage those who follow this line because they are keeping us honest, and who knows, they might come up with something important. I'm just working on a challenge to it on large scales, which gets kind of technical. Shall I spend 5 minutes explaining this?

Smeenk:

Sure.

Peebles:

You might be aware that large-scale density fluctuations have a particular property that as time goes on the peaks grow and the valleys get deeper, but the shape of the distribution is unchanged. It's simply the amplitude grows. That is a result of a very specific property of gravity, the inverse square law, and it goes as follows. If you imagine a universe that's exactly homogeneous aside from one lump and ask what happens, well, this extra gravity will cause material to flow into it.

The velocity will be inversely proportional to the square of the distance, because that's the inverse square law under conventional gravity. A velocity that goes as 1/R˛ has the property that the mass that flows through each sphere is the same because the sphere area is 4piR˛. So you get an R˛ times velocity= which is 1/R˛. So the amount of mass flowing into this sphere is the amount of mass flowing into this sphere [a second, larger concentric sphere] and so on. The result is that [the first sphere], as it pulls mass in, doesn't change the mass density anywhere except where it is.

So that's a neat thing, and it has the consequence that the shape of the mass distribution is preserved through linear growth of fluctuations. That is pretty consistent with what's observed when you compare the fluctuations - statistically of course, you don't prepare the fluctuations one by one - but statistically the fluctuations in the thermal background radiation and in the distribution of galaxies are consistent with this picture of the growth of the amplitude of fluctuations but preservation of the form of them. So I've been deeply grabbed lately by the thought that this follows because of the inverse square law. It's a very specific property of an inverse square law.

So I've been writing down here what would happen if we didn't have quite an inverse square law but rather 1/R(˛+n), as in MOND. If n is very much different from zero, bad things happen. We find it difficult to account for the observations. I just presented these arguments a week ago in a conference in France in the Alps. It was glorious. At a ski resort. So we met mornings and evenings and skied afternoons, for a week. I was just exhausted, but it was really great - the sun, the snow, the mountains.

Smeenk:

That must have been beautiful

Peebles:

It's ridiculous. But I presented these arguments. People accepted them, and one young person got excited, so that was - I mean that's the ulti- mate compliment, right? He wanted to help follow these ideas up. I mention this because first it's so deeply impressive to think that this very indirect consequence of the inverse square law checks out. Physics is working. And it certainly makes me more suspicious of MOND. This is not MOND; MOND is a pretty subtle theory with this acceleration-dependent break toward a 1/R type force law. But this very simple-minded consideration certainly strongly constrained the MOND concept and it right in line with conventional general relativity theory.

By no means is this the end of the story. I'm going to have to do this analysis in more detail. There are going to be careful comparisons with the observations by this young guy Kris [Krzysztof M.] Gorski at Munich at the Max Planck Institut für Astrophysik. But already I think it's clear that this is a dramatic new check of fundamental theory, the inverse square law applying on very large scales. You know, one of the things I emphasized at this conference that also grabs me is we assume general relativity in cosmology - it's natural because the theory is very successful and we don't have an alternative that is well developed, of course. But it is an extrapolation.

The precision tests of general relativity are on the scales of the solar system and smaller, say about 1013 cm. We apply the inverse square law on all the rest of general relativity theory in cosmology on scales of the Hubble length and smaller, but the Hubble length, that's about 10˛8 cm, which is to say fifteen orders of magnitude extrapolation. It's just astounding! And the nice comparison that can make you have a feeling for that fifteen orders of magnitude is that if you go down in length scale from the smallest sampled in the big accelerators you hit the Plank length where the world is entirely different.

So why are we so confident we can extrapolate upward by fifteen orders of magnitude? The simple answer is, we don't have an alternative. And also we're not nervous because we don't see anything vastly wrong. But nonetheless it's a tremendous extrapolation. The exciting thing is it can be checked - at least many aspects of it. That's what the cosmological tests are all about, of course, is checking that plus our picture of the contents of the universe, of course, its structure.

But the cosmological tests are also largely about testing gravity physics, and the tests are getting very searching and so far there is nothing bad showing up. That's not to say we won't be surprised, but I'm starting to think the odds are getting longer on our being surprised by new physics of gravity on the scales of cosmology. By no means have we ruled out MOND and people who do MOND should be encouraged a little more than they are.

Smeenk:

I haven't followed MOND that carefully, but it's based on New- tonian theory, right? And then modify the potential?

Peebles:

Modified Newtonian potential, yes.

Smeenk:

And it's not clear to me that you can actually have a consistent Newtonian cosmology.

Peebles:

No.

Smeenk:

Has there been any emphasis on developing a general relativistic version of MOND?

Peebles:

It's been very frustrating. Of course the originators, Milgrom and those few who work on it, are quite aware of the pressing need to have a fully consistent theory that goes beyond the Newtonian non-relativistic limit to a theory that can be applied to cosmology. They don't have one. They fully admit it and they agree that this is a big gap, big lack in the theory. There it is. They do insist that on the scales of galaxies and smaller where it is intended to apply it works remarkably well, and they're right.

There are just a few people working on this theory. The most active of the young people is Stacy McGaugh at the University of Maryland. If you ever get a chance you might be amused to talk to him. It is an interesting aspect of science. He has chosen to be [in] a somewhat iconoclastic position and it's a bit awkward for him.

I've stressed how easy it had been to get funding - it wasn't that easy, but it was not difficult to get research support in the late sixties and the seventies, but you had to have a reputation as being reasonably solid, and it is much more demanding now that you be [considered] a solid person to even be considered for a research grant. He's chosen a rather difficult road. I don't know his funding situation, but I can imagine it's not entirely positive. And that's not a positive statement.

Smeenk:

Right. Going off in this direction, have you ever been in a posi- tion to be on boards or advisory panels awarding grants? And if so, would you actively encourage a diversity of approaches such as MOND and other approaches?

Peebles:

It's very difficult. Yes, I have been on such committees. I find them kind of depressing because it's so difficult to make rational decisions. I can quite sympathize with the reluctance of the funding agencies to support iconoclastic research. First, the subject of loud complaints from the majority who say, "Why are you funding research that is so speculative, when I have a solid program that I know will make only incremental advances, but I know they'll make advances." That's not an easy call at all to make. But I do feel uneasy about that pressure because it could be we're passing by some opportunities. I don't think all is lost. People like McGaugh will persist. He's tough, and he'll continue. I think the strategy is clear. If you want to do iconoclastic things, do them part time. And that's not so hard.

Smeenk:

Do something else and pursue crazy ideas, or however you want describe them, on the side.

Peebles:

That's right.

Smeenk:

Just in terms of interacting with other people on these boards, have you found that your priorities have been different in trying to encourage this type of research or-?

Peebles:

Oh no. We all make slightly diverse emphasis on how to appor- tion effort between speculative and solid but pedestrian, but I don't find large differences in philosophy. That extends I think across all research. When I go to conferences, I don't encounter wildly different philosophies on how to do science. That's in part surely because we all go to the same conferences and we talk to each other all the time, but I hope it's also a common rationality of discourse. It's not so subtle to decide how to do research. We've had a couple of centuries of examples to follow.

Smeenk:

And did you serve on the decadal survey?

Peebles:

I served on the one before last and the one before that, not on the most recent.

Smeenk:

And do you have any recollections of that experience?

Peebles:

No. I do remember being impressed with the lavish support for the first one I sat in on. And I remember a lobster and champagne dinner in Boston, but that seemed to me a bit lavish.

Peebles:

Decadal surveys in astronomy have been very successful. I haven't had a lot to do with them, in part because the big decisions are on the ob- servational side, what do you build. And in part that's a technical decision regarding what's feasible. I did remark that it strikes me that physics has had a much less successful program of decadal research for a simple reason: there are much bigger operations and the diversity of agendas among particle physics, condensed matter physics, biophysics, nuclear physics makes it much more difficult to reach consensus - in fact, I guess about impossible. Whereas astronomy is not so big that astronomers can't get together and decide on a more or less reasonable common set of goals.

Smeenk:

And you don't recall any particular disputes while you were on the boards of the survey?

Peebles:

No, in fact I don't. I don't recall bitter debates that this should be included and that should not be. In part that's because the decadal surveys have been led by pretty strong-minded people who are able pretty well to press their own ... I think it's fair to say that it's not an entirely democratic operation. Opinions are solicited earnestly, but filtered, and the final decisions I think are in a fairly small group of people who are willing to take on the job and are willing also to stand up to the minor quibbles that are inevitably going to happen. I have not encountered any bitterness on decisions of the decadal committees once made. There of course have been debates about strategies, but no lasting bitterness that I've ever noticed.

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