James Peebles Interview: Conversations with History; Institute of International Studies, UC Berkeley

A Cosmologist’s Intellectual Journey: Conversations with James E. Peebles, Professor Emeritus of Cosmology, Princeton University; October 12, 2006 by Harry Kreisler

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Cosmology

You are a cosmologist. What is it that this field of the physical sciences studies?

The easy answer is the universe, but of course, it's also the wrong answer. The universe is everything. I don't study you.

[laughs] Oh, I'm shocked!

So, when I say I'm studying -- [laughs] yeah, "I'm shocked." I'll try to work you in! But when I say I'm studying the universe I really mean I'm studying what the universe looks like on the largest observable scale.

It's a little surprising that I could make such a statement. How do I know there's a largest observable scale? To be a little more explicit then, I should say we observe that matter is arranged in a hierarchy of structures. You and I are a little concentration of material here in the sprawl of the Bay Area on the planet Earth that's in the solar system that's in a galaxy of stars that's in a supercluster of stars, and so on. Layer upon layer of structure. It's kind of a neat arrangement and of course, it goes down in size too.

What is striking is that as you look on successively larger scales, you reach a point where there is no more structure. The distribution of material objects becomes uniform: no center that we can see, no edge. It's that wonderfully simple arrangement that is the basis for what we study when we're studying the large scale structure of the universe. What is the nature of that large-scale distribution and what is its behavior? It's been speculated for a long time, fairly recently demonstrated, that this uniform distribution is nearly uniformly expanding. It contains fossils from the early universe, in particular thermal radiation. The radiation is cooling.

It is observed that distant galaxies look younger, and that makes sense, because we're seeing them as they were in the past because of the limited light travel time. We're seen evolution. We're seeing the growth of departures from this large-scale uniformity. Matter is growing together in clumps and the clumps are growing together in clumps of clumps. We are studying the growth of the big levels in this hierarchy of structure.

And is the concern only peripherally how it all started?

Oh, of course you'd love to answer that question. You make it peripheral if you can't give it a good answer, and it's debated whether we have a good answer. To me, as an old-fashioned theory-and-observation type guy, we don't have yet a definitive answer. We have some very good ideas that people are working hard on, and remarkably enough they're developing tests, perhaps clues too, to what the universe was doing before it was expanding. That's the next great generation of experiments in this subject.

There are ideas, as I said, about what the universe was doing before it was expanding; they go under the name of inflation. Inflation is not a theory because there are so many options within it, so many hypotheses to be made and considered. So far, we have very few firm predictions out of inflation, and unfortunately, most of these predictions were, in effect, "post-dictions," the theory constructed to fit ideas that are now confirmed.

But that could change, things will show up. [There is a] search for gravitational waves that have been generated by processes thought to have happened during inflation. Gravity wave detectors are being designed, and not only will they see interesting things produced in the universe as it is now, maybe they'll see some fossils from the universe when it was not describable by an expanding world model.

In your lecture yesterday you talked about certain things that we know -- I want to be sure to use the right term -- laws or principles: the uniformity, no center, no edges. These were the ideas that motivated the research and have been proven in a way. You just mentioned that the universe is expanding but not evolving --

But evolving. And evolving.

And evolving.

That's key.

You know, there was a steady state model. I like it as an example of an elegant idea that has been ruled out. In formulating ideas about how the universe around us, the world around us, may be operating we do rely on our notions of elegance. Of course, our notions of elegance are strongly informed by what works. The steady-state cosmology is an example of an elegant idea that was very influential, drove a lot of research (in part because it so infuriated some people), advanced the subject, but then was shown to be totally wrong. So, goodbye elegant idea, let's look for another elegant idea. I interrupted you a bit there. You were ...

But I'm glad.

Talk a little about some of the concepts that have emerged. In your lecture you were talking about thermal radiation, dark matter, dark energy, and in your lecture you conveyed the sense of how our understanding of these terms emerged over time through generations, as scientists had an idea which was either confirmed or disconfirmed by satellite observations, and so on. Give us a feel for that, so people can have a little taste of that lecture.

Okay. Let us choose an example. First, let's notice that you and I are made of atoms that go under the general name of baryonic, "bary" being heavy -- a historical term. We are made of material that is to us visible. The striking evidence that we have from studying the property of the universe in the large is that you and I, and baryons in general, make up only 5% or so of the mass of the universe. A little shocking, but we'll have to live with it. [There is] pretty convincing evidence that there is dark matter that is not baryonic and whose properties are mysterious. Such stuff was known already in the 1930s.

I mentioned Fritz Zwicky, one of my heroes. He was the first to point out that we have a puzzle. There's mass out there that is not baryonic. There are great searches going on, including one beautiful project headed up here in Berkeley by Bernard Sadoulet looking for this dark matter through laboratory searches -- brilliant stuff. Then, fascinating to me particularly is the third major component of our universe, dark energy. The name is really meaningless, it was ad hoc, made up, but it fits with "dark matter." It had its origins already back in 1917 when Einstein had invented his new theory of gravity, general relativity, and found that when he applied it to the universe it gave an absurd answer, the universe can't be stable. "Of course the universe is forever," and so he adjusted this theory by postulating a new term that we now call dark energy. He called it the cosmic constant.

It was a Belgian, Georges Lemaître, in the late 1920s, early 1930s, who recognized that Einstein's cosmic constant acts like a zero level of energy. In physics, energy is a quantity whose differences we care about, transactions, but we don't ever set a zero of energy. He became convinced that the zero level of energy is set by Einstein's cosmic constant. I showed in my lecture a fascinating exchange of letters between Lemaître and Einstein just after the Second World War in which Lemaître tried to convince Einstein of the value of this concept. Einstein would have nothing of it. He grew to hate the cosmic constant because he did think it was needed, and if not needed then away with it. To Einstein simplicity is all.

I did remark that others of the great scientists of that era, Wolfgang Pauli, brilliant person, would have nothing to do with lambda, or Landau and Lifschitz, from the Soviet Union, a magnificent series of textbooks in theoretical physics -- all of us learned so much from them -- they would have nothing to do with lambda.

And lambda is ... ?

Oh, sorry, lambda is dark energy, it is the matrix of dark energy, it is Einstein's cosmic constant. To these people it was an ugly appendix. They had a reason in part for thinking that, because we have the horrible problem in quantum physics that zero-point energy is real and important, and is there, and it exists not only in material objects but in fields, such as electromagnetic field, but a naïve sum of the zero point energy belonging to the electromagnetic field and other fields of nature gives you an absurd energy density with all of the properties of dark energy, Einstein's cosmic constant, except one: its numerical value is ridiculous. "Forget about it, there must be something wrong with zero point energy" for fields. I know of nothing wrong with it. The argument is clean, except the answer is absurd. It's an illustration of how we have, on the one hand, a secure, well-tested theory, but on the other hand, it's only an approximation, there are holes to be filled by the next generation.

What is now charming is on the one hand, people recognize there ought to be a zero of energy that would behave like Einstein's cosmic constant, Lemaître's dark energy. It ought to exist, and through a series of experiments that began again in the 1930s by Edwin Hubble, who drove the development of a big telescope to make the measurements, a 200-inch telescope on Mount Palomar, by great scientists in the sixties, when that telescope was at last finished, in particular [experiments by] Allan Sandage, driving forth the observations that might check for this dark energy, then just in our generation, here at Berkeley, Saul Perlmutter leading a group to carry on those efforts, and at last succeeding and showing us, of all things, that dark energy exists. The stuff that Einstein introduced, then discarded, that Lemaître fought for, that many serious scientists discarded again, is there. What a neat story!

So over time -- not in the sense of the scale of time as one looks at the whole universe but over time and over generations -- there's a movement forward to come up with a concept that is confirmed. In what sense in this process does dark energy matter? I don't mean in terms of its utility for practical things, but relate that to this ongoing conversation of understanding the universe.

Here it is. The dark energy exists. We should first pause to say that ideas don't always have the longevity of dark energy. Many ideas are thrown out and go away forever, but some curiously survive often in a slightly different form. But now, for us at this very moment, the key point is that we have the evidence for the existence of a new phenomenon that does not fit within our ideas of basic physics. Something's wrong and we're going to have to remedy it.

And that's dark energy.

And that's dark energy. It's value is just wrong. Now of course, there's no crisis here. There's opportunity, of course, but there are lots of ideas about how to remedy this. Down the road at Stanford, Lenny Susskind knows in his heart the solution to this problem. Others of us wonder, but that's part of the game.

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