Joseph Silk

Lightman:

I wanted to start with your childhood. Do you remember any particularly influential experiences that you had as a child that may have related to your going into science?

Silk:

As I look back, I would say that one of my most formative experiences was spending most of my youth in the Boy Scouts. We did a lot of camping and spent a certain amount of time outdoors, at night, looking at the sky. That, somehow, must have instilled in me some curiosity about what was up in the heavens. It took a while for that to really come to fruition, because when I went on to university in England, I studied mathematics but eventually turned to astronomy.

Lightman:

Tell me a little about what your parents did.

Silk:

I was the first person in my family to ever aspire to go to college, so it was a real breakaway from tradition. My grandparents were immigrants from Eastern Europe. My parents grew up in poverty in the London slums. Basically, my father left school at the age of 14, and that was typical of the education level of the family. It was quite a breakthrough for me to make it through school, high school, and eventually go to Cambridge as I did.

Lightman:

Did your parents encourage you in academics, or in science in particular?

Silk:

They were very supportive. They were very proud to have a son who was able to do something which they couldn’t understand, but which they admired and was very different from anything they were used to. I suppose I rather naturally was top in my class, or very close to the top, with sufficient motivation to keep on going. As long as I performed well, they encouraged that. It must have been a sacrifice to them, to some extent, because, it was expected in my childhood that the school-leaving age was 15. It was assumed that I would leave school at 15 and go to work to try to support the rest of one’s family — one had younger sisters or brothers or whatever, or aging grandparents. That was what society expected, where we lived in London. Anyway, they supported me, and eventually I got a scholarship to Cambridge University.

Lightman:

Before you went to Cambridge University, do you remember whether you read any popular books about science?

Silk:

No, I don’t think so. The English school system was very narrow. One specialized at a rather young age. From 15 years onward, I was doing only physics and mathematics. Everything else was dropped. There was no time, really, for any wider horizons.

Lightman:

What about before 15 years of age?

Silk:

Before 15, everything was being crammed into us. I was taught four different languages apart from English, plus assorted subjects like geography, history, biology, and chemistry.

Lightman:

This was a public school? [corresponding to a private school in the U.S.]

Silk:

It was a state school, a grammar school. [corresponding to a public school in the U.S.]

Lightman:

Was science an interest that stood out for you?

Silk:

It was mathematics. I had a mathematics teacher who inspired me and spent many hours with me after school. In those days, the apex of mathematics was at Cambridge University, and each college at Cambridge held special examinations that the ordinary state school really had no expertise or track record in preparing its students for. As a result, the public schools in England would dominate these examinations. This math teacher who took a special interest in me was able to coach me and direct me towards trying for this examination, which normally should have been completely out of my reach. In fact, I was the first person ever in [my] school to get into Cambridge University.

Lightman:

So it was mainly mathematics for you at this age?

Silk:

It was mathematics, yes. So, I read mathematics at Cambridge. Then, in my third year in Cambridge, I did run into someone who was a very good popularizer of science, Dennis Sciama. He gave a course that I attended, on general relativity and cosmology. I think that was what really inspired me to go into astronomy, in addition to whatever thoughts I might have had earlier.

Lightman:

Out of all fields of astronomy, was cosmology of any particular interest to you at this time, when you were at Cambridge?

Silk:

I was really just sitting in as an auditor on Sciama’s lectures, and I was very busy doing whatever undergraduates do — basically wasting my time rather than studying. But hearing Sciama just talk in grand terms about Mach’s principle and Olber’s paradox and all these things really opened my eyes to new ways of thinking. I would say that was the main influence that I had. After another year or so, I realized that it was astronomy that was very, very interesting.

Lightman:

Did you decide that you would want to go into astronomy at that time?

Silk:

At about that time, yes.

Lightman:

Before you left Cambridge?

Silk:

No, it took another year. I spent the year after Cambridge at Manchester University, more or less looking around for some project to get involved in. It was during that year that I realized that astronomy was going to be the most interesting thing that I could see. I managed to get a fellowship to go to Harvard.

Lightman:

Tell me a little bit about your graduate experience at Harvard.

Silk:

I was part of a class at Harvard that had a number of well-known astronomers. In my year there were people like Ben Zuckerman, Frank Shu, Pay Palmer, and Jay Pasachoff.

Lightman:

Was Dick McCray there?

Silk:

McCray arrived as an assistant professor when I was leaving.

Lightman:

And Chris McKee was there too?

Silk:

McKee arrived later.

Lightman:

So you were there before those people.

Silk:

Yes. I would say Harvard in those days was a rather smaller, more intimate environment than it is now. I was on close terms with a number of the faculty. David Layzer was my supervisor, and I worked with him and under him on cosmology. I have very good memories of Harvard. I went through my Harvard experience managing to avoid taking a lot of courses. I think I got through my entire graduate degree program by formally taking one course, on plasma physics.

Lightman:

And you were able to satisfy the requirements in the other courses?

Silk:

I was able to. The system was flexible enough so I could satisfy all the degree requirements without having to waste one’s time having to take courses.

Lightman:

I think they still have that provision.

Silk:

Yes. I managed to get through rather quickly and get into research right away.

Lightman:

When you were first exposed to cosmology at a formal level, which I guess was at this time, do you remember having any preferences for particular cosmological models? For example, open versus closed, or homogeneous versus inhomogeneous?

Silk:

The first paper^[1] I wrote was actually on a very unconventional cosmological model called Godel’s universe, which is a rotating universe. I suppose you could say that I started off in a very unorthodox way.

Lightman:

But that doesn’t mean that you believed it just because you wrote a paper on it.

Silk:

No, but Layzer was a great believer — and still is a great believer — in a very unconventional version^[2] of the big bang [model], in which the universe is cold in the beginning. I also worked on his approach to cosmology, on his theory. It took a certain amount of effort on my part, but I eventually broke away from that. I will say this for David, that he supported what I was doing even though I began working on the hot big bang, which he was philosophically and mathematically and experimentally opposed to. The breakthrough actually came when I spent one summer at Woods Hole, where the Woods Hole Oceanographic Institute organizes around a summer school on fluid dynamics. In alternate years they tend to choose an astrophysical topic. This was in the summer of 1967, I think. They decided to look into the question of galaxy formation as a fluid dynamical problem in an expanding medium — idealized that way. I remember that there were a number of people there, but George Field, in particular, was one of the lecturers there.

Lightman:

This is when he was at Berkeley?

Silk:

Yes, that’s right. And he pointed the way to the standard view of the big bang, which I must say at Harvard one had really not been exposed to at that point. I never really looked back [from that point on.] In fact, the summer project I did at Woods Hole eventually turned into my thesis.

Lightman:

Was that the summer where you got inspired to work^[3] on the opacity of radiation and matter in the early universe?

Silk:

That’s right. The people at the Woods Hole Institute included Ed Spiegel and a number of others who worked on radiative transfer problems and associated fluid dynamical problems. And so things really gelled together in pointing the way to an application of radiative transfer and fluid dynamics to cosmology. It was just a new area that no one had looked at. Another person there at that time who was a source of great inspiration died very shortly afterwards and that was Richard Michie. He was a dynamicist who was developing an interest in cosmology. He was a source of great guidance and inspiration.

Lightman:

What was Michie’s institution?

Silk:

He was at Kitt Peak.

Lightman:

When you began breaking away from Layzer’s picture and started working in the standard hot big bang model, do you remember any early preferences for particular kinds of models — open versus closed, for example?

Silk:

I was really dealing with the very early universe, and it doesn’t really matter if it’s opened or closed, so the answer is no.

Lightman:

But you didn’t have any additional personal prejudices besides?

Silk:

I had no philosophical reason, nor any scientific reason to believe that the universe was opened or closed. When one is dealing with what happened in the first million years of the big bang, it makes no difference.

Lightman:

Yes. When you began working in 1967 on the fluctuations and the primordial fireball radiation, do you remember what was the motivation for that work?

Silk:

I had been hearing lectures on the coupling of radiative transfer to an expanding medium. The lecturers were people like Field and Spiegel and one or two other people who worked in fluid dynamics, like William Malkus and George Veronis. All of these were very inspiring lecturers, and I just started looking for a problem that would combine something that I was hearing about in the lectures with something a little bit different. It was clear that the early universe was an area that no respectable fluid dynamicist at that time would have worked on. It was really virgin territory. That was how I began trying to take elements from different areas that I’d been hearing about and to put them together in a cosmological context.

Lightman:

Did you have any ideas about what results your calculation would produce?

Silk:

Well, it was clear, I think at the beginning, that there was going to be some effect. If I started off with fluctuations in the matter and the radiation, there would be a smoothing effect — that is, the radiation would be able to stream through freely and erase some of the fluctuations. That really was the main point of the early calculations, to try to understand this a little better. What emerged, finally, was the idea of a minimum mass for survival of the fluctuations. Then, as a consequence of that, it soon became apparent that one could find a beautiful observational prediction of this theory by looking for the fluctuations that might be in the background radiation which was left over from this initial smoothing process.

Lightman:

In your Nature paper^[4] of 1967, when you start off by saying that the fluctuations are usually taken as a given and you would like to investigate what happens to them subsequently, did you at that time worry about what caused the fluctuations to begin with?

Silk:

No. I think at that time one just took the fluctuations as a given. At that time, one had no idea really of what happened in the first second of the universe. The first second of the universe was a given. At that time, the particle physicists hadn’t discovered the big bang cosmology, and it was a classical cosmology. It really only began with the application to nucleosynthesis and the predictions of the light element abundancies in the first three minutes. What happened before then was very obscure, I think. It was clear that the fluctuations must have been produced at the very beginning of the universe, before one second. And since one didn’t have a theory, one just took it as a given at that time.

Lightman:

Why was it clear that the fluctuations had to have been produced earlier?

Silk:

Because it was very hard to imagine any causal process which could have gathered matter together on these very large scales. To make a galaxy, you need to collect together 10⁶⁸ atoms, equivalent atoms. Now, that involves gathering matter from quite a large volume. If you go back to the first instants of the universe — the first few minutes or even the first year of the universe — you find that the volume that you have to gather that matter up out of is so large that light cannot travel across it, which means that you need some noncausal process, something that is beyond the domain of any physics that we understood at that time.

Lightman:

Well doesn’t that problem just get even worse as you go back to an earlier, earlier time?

Silk:

That’s absolutely correct. So, the only resolution is that it’s a property of the initial conditions of the big bang. That is, the big bang need not have begun without the fluctuations. You say, “well, I don’t understand this; let me just suppose that it was there from the beginning, that the fluctuations were laid down and galaxies began from them.” If we thought of the origin of fluctuations, that would be the origin that one would ascribe in those days. It’s only been very recently that we have developed a theory about the origin of those fluctuations, and that theory is by no means universally accepted.

Lightman:

And it produces the fluctuations much earlier than one second.

Silk:

Yes. It may even be that for that theory of the fluctuations, you still have to have some acausal process, some very special initial conditions — otherwise you would get the wrong sort of fluctuation. So, one may not have avoided the problem that we had in 1967.

Lightman:

One of the common threads through most of your work, coming up to the present time, has been the influence of various physical processes on the cosmic microwave background radiation. How confident do you think we are in our interpretation of the cosmic microwave radiation background, in the way it is interpreted in terms of the big bang theory?

Silk:

When it was discovered in 1965, one really had no idea it was black body radiation, with measurements at one or two different wavelengths, and very limited knowledge of its isotropy distribution around the sky. It was on shaky ground. But in the more than two decades that have elapsed since then, we have learned that it is to very high precision black body radiation, to very high precision isotropic. Those two properties are very strong arguments for an origin in the very early universe. The isotropy means that it has nothing to do with the galaxy, nothing to do with the supercluster of galaxies, nothing to do with the sun, or the local interstellar medium. And the black body nature is exactly what you expect in the very dense fireball in which the universe began. So, it fits in beautifully. It’s not direct proof, of course, but it’s by far the simplest hypothesis of the big bang theory for the origin of the cosmic microwave background radiation. That’s the philosophy that we take in physics. You start off with the simplest theory first, and if you can shoot that theory down — if someone can shoot the big bang down — then they’ll have to think of a different origin for the microwave background.

Lightman:

I gather from reading your papers that you take this so seriously — the interpretation of the microwave radiation background — that you feel that you can rule out theories of galaxy formation based on it and feel confident of the results.

Silk:

Yes. The reason is that the isotropy of the background — the smoothness — is now measured to better than .01% percent on all angular scales, and that statement alone makes many theories of the origin of galaxies untenable because they would predict larger fluctuations. It’s fair to say that until the fluctuations of the background are actually measured, we will not have any positive evidence for any theory. Hopefully, that measurement will come about some day. At the moment, it’s sort of a negative argument. We’re ruling out theories rather than finding the right theory.

Lightman:

I wanted to ask you a little bit about your reactions to some of the recent developments in cosmology, both theoretical and observational. To begin, let me get a little background. Do you remember when you first heard about the horizon problem in cosmology?

Silk:

I think it goes back a long way, to this question of the beginning of the origin of fluctuations. Cosmologists have been aware^[5] of the horizon problem for 20 years.

Lightman:

Do you remember when you, personally, first heard about it?

Silk:

One aspect of the horizon problem is that the fluctuations from which everything began would have to be acausal. You have to think of some process. Now that’s a limited version. I’m not sure about what you mean by the horizon problem.

Lightman:

I can take that [as a definition of the horizon problem].

Silk:

That goes back to reading papers^[6] by Ted Harrison and Charles Misner in the late 1960s and early 1970s.

Lightman:

He would have had to have defined a certain scale on which these structures occurred. I assume that he was talking about structures that were much larger than the horizon scale at any reasonable time.

Silk:

At some early time, yes that’s right. As far as the question of the horizon of the present universe — why is the present universe so smooth — all of this was focused, of course, very well by Alan Guth around 1981 or so, when he developed the inflation theory. ^[7] But he was just gathering together a problem that was well-known.

Lightman:

Yes. When you first encountered the horizon problem in reading Harrison, which you said was in the early 1970s, were you persuaded that it was a serious problem?

Silk:

Yes. I guess at that time the solution was simply one of initial conditions. You simply said the universe began smooth, or began with certain fluctuations which were almost smooth. Those fluctuations gave rise to the structure and the smoothness that we have today.

Lightman:

Was that a solution to the problem that you accepted?

Silk:

It was not a completely satisfactory solution. But remember that we only have one universe, so you don’t exactly have a statistical ensemble on which to start making comparisons and calculating the most likely possibility. So, yes, with reservations, I accepted it.

Lightman:

You mentioned a moment ago that Alan Guth really focused these various arguments relating to this problem in his proposed solution. Did your view of the horizon problem change after the inflationary universe model?

Silk:

It did change in the sense that I found Guth’s arguments and the subsequent developments in inflation theories very persuasive, and I still do. I think if one really has some sort of “no-hair theorem” for cosmology, then you can start off with more or less anything and end up with solving the horizon problem with a very smooth universe. That is really a marvelous development. I don’t think we’ve achieved that today, but it is still a possibility.

Lightman:

When you say that you don’t think we’ve achieved that today, do you mean that the inflationary universe model is not worked out to a satisfactory degree or it still has problems?

Silk:

I think that you need to specify particular conditions in order for inflation to work. If the universe is too inhomogeneous initially, then inflation may not work. That’s one of the worries, and there are other related problems too. Now, it may be that we don’t yet have a satisfactory theory of inflation. I think one is still waiting. If you really need to push inflation back to the Planck epoch, as some people believe, then you need to know the theory of quantum gravity at the same time.

Lightman:

Do you think that some version of the inflationary universe model is likely to be true?

Silk:

I think it’s sufficiently powerful as, if you like, a scientific philosophy… I find it very compelling, and it would seem to me that the ultimate theory of cosmology has got to contain something like inflation. Some vestige of inflation will be recognizable.

Lightman:

Why do you think it’s been so popular?

Silk:

It’s been popular because its advocates, at least, have made it seem compelling, natural, and able to explain many things — not just the smoothness of the universe, but the fact that the universe must have had fluctuations. If the fluctuations in the beginning were the wrong amplitude, either too small or too big, we wouldn’t end up with the universe that we have today. The inflationary advocates make persuasive arguments. They’re still having some trouble in getting the right amplitude of the fluctuations, but they get the spectrum, the distribution, the sizes to come out remarkably well, close to what we think we see.

Lightman:

The Zel’dovich spectrum.

Silk:

Right. The Zel’dovich-Harrison-Peebles spectrum, to give full credit where it is due.

Lightman:

Let me ask you a related question. I asked you when you had first heard of the horizon problem. Do you remember when you first heard of the flatness problem?

Silk:

I think the flatness problem, stated that way, was really concocted by Alan Guth. I first heard of it when I read his paper. Certainly, long before his theory of inflation came out, there were advocates of a flat universe, who regarded that as being the only natural model.

Lightman:

Well, that’s different from the flatness problem.

Silk:

That is right.

Lightman:

So, as you recall, the first time you heard about it stated as a problem was from reading Guth’s paper?

Silk:

Yes. But it’s not so different from the question of why is the universe so old or why is the universe so isotropic. They’re somewhat related aspects of the flatness issue.

Lightman:

So when you heard about the flatness problem, you heard the problem and received its solution at the same time?

Silk:

Yes, that’s right.

Lightman:

Let me ask you this, which requires a certain amount of speculation. Suppose that you had not been presented with the solution to the flatness problem at the same time that you heard it. Suppose that you had heard about the flatness problem before Guth’s paper. You heard someone state that “isn’t it amazing that the universe is so close to an omega equal one universe, because if it had been started with omega slightly bigger than one or less one at the Planck time, omega would have been far from one today, at 10⁶⁰ Planck times.” Would you have found that a compelling problem?

Silk:

Not at all because, in my view, it’s all initial conditions — given the fact that we have only one universe. I see nothing whatsoever wrong with starting things off within an amount epsilon of being flat at the beginning so as to arrive today flat or almost flat. You may well have to make assumptions like that to get inflation itself to work — that is fine tuning. The flatness problem is just another version of fine tuning.

Lightman:

So you wouldn’t have been particularly bothered by it?

Silk:

No. I don’t lose sleep at night worrying about the flatness problem. In fact, I personally think it’s unlikely the universe is flat. One solution to the flatness problem, of course, is that the universe was born flat and always is flat. It may or may not be inflation that does that for us. That argument means nothing to me. In fact, I suspect that it could well be that the universe is open today. All the observations point to that of course.

Lightman:

Let me ask you this. If you are willing to make the argument that we have only one universe, so there is no reason a priori to expect one set of initial conditions to be more probable than another, and [if] therefore you are willing to take whatever initial conditions are needed in order to produce a homogeneous universe and a universe that is very [nearly flat], then what does inflation really buy you?

Silk:

Very little. In fact, initial conditions are an alternative to inflation. Inflation is really a way of trying to erase arbitrary initial conditions. And it hasn’t succeeded. That’s the worry about inflation. It erases a large subset of possible initial conditions, but it doesn’t erase them all. So, I think one really has a choice. One can go with inflation and hope that someday someone will come along with a better inflationary theory, or else just take the simple view and say that the universe began in a certain way and here we are today.

Lightman:

Do you personally have any preference for one of those two approaches?

Silk:

I would be delighted if inflation turned out to be right. It’s such a beautiful mixture of particle physics and cosmology. So, philosophically I sort of lean toward inflation. On the other hand, as an astrophysicist, I am perfectly willing to contemplate another universe, just because that’s what astronomers tell us is out there.

Lightman:

If you say philosophically you would like for inflation to be correct, does it worry you any that we then have to reconcile the observations of omega equal 0.1 with the inflationary requirement that omega is equal to one?

Silk:

Presumably, if there is omega equal one worth of matter out there somewhere, we’re going to detect it someday. It may be that we haven’t been clever enough yet. But there are possible ways to look for this extra dark matter. Eventually there’s going to be an experimental test of this question. I would simply defer to that point. It’s not really a worry because our experiments just are not good enough yet. We haven’t gone deep enough.

Lightman:

So you wouldn’t really say that there is a conflict at the moment because you don’t think that the observations are really complete enough?

Silk:

The observations are only reasonably complete out to 10, 20 or 30 megaparsecs. Within that range, it’s clear there is not enough matter to close the universe if I use my local ratio of dark matter to light matter as a guide. However, it's completely an open question, excuse the pun, as to what might be on larger scales. We need better observations. It will take a long time. For example, I mean doing very deep counts of galaxies, measuring many redshifts. You probably need thousands or even tens of thousands of redshifts. You have to go out to distances of 200 or 300 megaparsecs. A project like that will, I think, eventually directly measure the curvature of space. That’s probably a few years off. I see the answer coming.

Lightman:

So something like a Sandage program?

Silk:

I was thinking of the Loh and Spillar experiment, ^[8] ultimately, because they’re measuring the volume element of the universe directly, and that’s a beautiful approach. If I had to put money on an [investigation], I think that would be the one I would want to back. They haven’t got accurate enough redshifts. A repeat of their experiment getting real redshifts for many more galaxies would be the way to go.

Lightman:

Let me go back a little bit. When you first heard about the results^[9] of de Lapparent and Geller and Huchra on the large scale structure, and the related results of Haynes and Giovanelli,^[10] did those results surprise you?

Silk:

I think it’s fair to say they did. The idea of large voids had been around before the CFA and the Aerocibo surveys were made. You have to go back a ways, but the Russians, Einasto^[11] in particular, have been pushing this sort of thing for a long time, for several years before the US papers appeared. However, Einasto’s work was very incomplete. He used very biased samples. Nevertheless, the suggestion was there. The beautiful images that Geller and co-workers produced not only were convincing, but when one looked at those shells and spherical seeming voids, they were convincing and surprising I think. No one had really expected structures like that.

Lightman:

Did those structures change your view about the homogeneity of the universe, or shake your faith in the big bang model, which depends upon an assumption of gross homogeneity?

Silk:

No, because the structures that we are talking about are really on scales that are no larger than the super clusters of galaxies, 10s to 20 or 30 megaparsecs at the outside. Inhomogeneity on that scale is perfectly consistent with the big bang theory. If one were seeing inhomogeneity over the Hubble scale, then one would start getting very worried. If one measured a gradient or large void that extended over a thousand megaparsecs, then I think he or she would have to seriously question the big bang theory. But, we’re a long way from anyone ever claiming that sort of structure. There are only small-scale structures. The small-scale structures of Geller and company are simply a measure of the large scale inhomogeneties in the universe on intermediate scales really. It’s just a property of the fluctuation spectrum at the beginning.

Lightman:

So do you personally still believe that on the large scales, let’s say 100 megaparsecs up to a thousand, that the universe is well approximated as a homogeneous medium?

Silk:

The only evidence we have is that on scales of several thousand megaparsecs — those scales are probed by the microwave background radiation — the universe today is extremely homogeneous and smooth. We also know that in the very early universe, on scales of hundreds of megaparsecs, it was probably very smooth too — again, because of the smaller scale fluctuations in the microwave background. What we’re not too sure about, I would say, are whether on scales of 100 or a few hundred megaparsecs today, the universe is smooth. We have no direct measurement of that. I suppose the closest direct measurement one has of the intermediate scales, hundreds of megaparsecs, involves looking at the X-ray background, which to some extent monitors the distribution of matter on these scales. This suggests that [the intermediate scales are] rather uniform too.

Lightman:

So you don’t see any conflict here?

Silk:

I don’t see any worry.

Lightman:

And you think that the standard big bang model has not really been threatened by these observations on scales of 10s of megaparsecs.

Silk:

What is at issue at the moment is not the standard big bang model, but the fluctuations in the standard big bang model. Now if one takes a Zeldovich spectrum of fluctuations, that is being challenged by some of the observations of large-scale structures. The particular observations that seem to be a challenge to it are the large-scale velocity fields which have been measured by several groups, although I have my reservations about those. That's one issue. And, there is another observation — the first report of anisotropy in the microwave background on large angular scales, of about eight degrees. If both of these observations turn out to be right, or if even one of them turns out to be right, then the standard fluctuation models, the Zeldovich spectrum, would have to be rejected. But I don’t think that will mean rejecting the standard big bang model. That’s a very minor part. It means rather than one has to rethink the inflation theory. But I think the big bang is going to survive. This is a rather minor perturbation.

Lightman:

Other than the inflationary universe model and some of these observations of large-scale structure and streaming motion, have there been any other discoveries, either theoretical or observational, that have changed your thinking in the last ten years or so?

Silk:

The most interesting discovery^[12] has been, over the past year, the possible distortions in the microwave background. They, in fact, were reported five or six years ago by an experiment which has since turned out probably not to have been correct or given correct results. The new experiment, however, seems to be rather convincing. It shows evidence for distortions near the peak of the black body spectrum. The distortions amount to about 10% in energy of the cosmic black body spectrum, and it’s extremely hard to understand what could have produced that large a distortion in the standard big bang model. I would say that observation comes closest to shaking one’s faith a bit in the standard big bang model. Let me give you my speculation as to what could be going on. Either you say there ought to be some sort of astrophysical sources, stars basically, giving rise to this energy. If you make that claim, then you need to put an enormous amount of stars in the very early universe, when the universe was dense enough and opaque enough to make the radiation nearly black body. It is stretching one’s creduity quite a bit to do that, but it’s not impossible. The other possibility is that there might be some new particle field, or scalar field, or a type of particle that is not in the standard picture of the big bang, but is coupled somehow to radiation. Perhaps it’s a particle that decays or a field that decays and gives you a uniform source of energy everywhere in the universe, starting in the very early universe. But as the matter density falls off as the universe expands, this particle source would become more and more important and could result in a heating up of the matter by the present time. That heating up of the matter is the sort of thing one needs to explain the distortion, because the microwave photons pass through the hot matter and get shifted slightly in energy, and this can give you the distortion. So, that’s one way I can imagine the big bang changing. But again, it’s rather a minor variation in the big bang to add some new particle or field.

Lightman:

Do you think that the explanation of this observation is one of the major outstanding problems in cosmology right now? What would you say are some of the major outstanding problems?

Silk:

That’s one of them. Probably the greatest challenge at the moment is to find a theory of galaxy formation that simultaneously fits in with the observations of large-scale structures and the theories of large-scale structures. Large-scale structure — clusters of galaxies and larger scales — formed by gravitational processes alone. And those are rather easy to understand. You can in principle simulate points on a computer under the force of gravity. You think you understand well what’s going on. Galaxy formation is something quite different. There you have many other complex things going on, including star formation, and we just have the vaguest glimmerings of a successful theory. Our hope is that if you start off with some basic spectrum of fluctuations in the very early universe, on the large scale end you make the galaxy clusters and the super clusters, and on the small-scale end the same spectrum produces the galaxies. So you have a unified picture. Our progress in developing this has not been overwhelming so far. I think one doesn’t know that one has a unique set of initial conditions. One has the Zeldovich spectrum, which can explain the large-scale structure. However, if one takes the new observations of the large-scale velocity field and the possible anisotropies in the background radiation, the Zeldovich spectrum is in serious trouble. One has to abandon that. At the same time, we haven’t really got very far in going from the Zeldovich spectrum, or any other spectrum of initial fluctuations, to the observed galaxies. We see spiral galaxies and elliptical galaxies and many other complex things out there. Theory leaves a lot to be desired. One of the key things that one can hope to look for, actually, if you wanted to test galaxy formation, is a galaxy that is actually forming. If we can find forming galaxies, that would at least give us some yardsticks on the theory of galaxy formation, to try to pin the other end of the fluctuation spectrum down a bit. Again, only in the past year there have been some intriguing developments in this area. Galaxies have now been discovered at redshifts larger than 3, which is getting quite a way back towards the big bang, and that’s precisely the redshift range that you expect forming galaxies to be at. What we are not too clear on yet is whether these are really protogalaxies or very peculiar objects closer in spirit to quasars than forming galaxies. When that all becomes clarified, observationally, I expect we will then be able to make a little more progress theoretically. But the greatest problem, at least, is in getting some suitable initial conditions to coherently give you both the large and the small scale pictures of the universe.

Lightman:

Any other problems you would like to add to the list of important, outstanding problems? Either theoretical or observational.

Silk:

I suppose that there is, of course, getting a satisfactory theory of inflation, and, tied to that, what happened in the very beginning. The other key question, observationally anyway, is not just what is the Hubble constant, which we would obviously like to know, but whether the universe is open or closed. What does that mean? Where is the answer? That will tie down the theories a little better.

Lightman:

One of the things I’m interested in is whether or not scientists use visual images in their thinking. How important are images or metaphors. Are they important to you at all in your scientific work?

Silk:

I find metaphors rather inspiring. That is, verbal descriptions can make a huge difference. One can take a fairly mundane theory, and if you can dress it up in suitably vivid language, that can make enough of a difference to make one read the theory. One is bombarded with so many papers these days. It used to be that one had time to read the abstract, and now you barely have time to read the titles. Vivid language and the title of the abstract are very important. I am impressed by metaphors. I personally don’t use computers in my work at all, so I don’t dabble at all with visual images really.

Lightman:

What about in your own head? Do you find it is important for you to be able to visualize what you are working on?

Silk:

When one tries to define a problem, I think trying to make some picture, some global picture, often helps.

Lightman:

Can you remember any specific instances where you were helped by being able to see something in your head?

Silk:

Usually I tend to think more in terms of equations.

Lightman:

I don’t want to force this issue on you.

Silk:

No, I would say when I’ve had major ideas, it’s really been “Aha! There is a time scale for something and a time scale for something else. What if those two time scales were the same, or were not the same. What would that imply?” I tend to work in terms of mathematical or physical concepts rather than visual concepts.

Lightman:

Changing gears a little bit, we have talked a lot about theory and observations. How well do you think theory and observations have worked together in cosmology, say in the last ten or twenty years?

Silk:

Well, it’s been a bit of a rollercoaster I would say. Sometimes the theories have been ahead of the observations, and sometimes the observations have been ahead of the theories. Theory was ahead of the observations for a while with the big bang. There was an awful lot of theoretical work done in the 1940s and 1950s, and one had to wait for the 1960s before the observations came along. Today, it seems clear that observations are well ahead of the theory. We’re grasping for straws in theory at this point to try to understand some of the observations.

Lightman:

For example, the large-scale structure or the microwave distortion you were talking about?

Silk:

The microwave distortion and the large-scale structure problems. The only note of caution is that the observations, which are perturbing to theoreticians, are all still rather tentative. They could solidify or they could disappear in a year or two. It’s not inconceivable that all the problems could go away. If one had real faith in one’s theory, it may be that one might just persist with the theory, as some of my colleagues seem to do, and ignore the observations. That’s one approach, to just keep on going.

Lightman:

Einstein’s approach.

Silk:

Einstein did that, yes. Some of the cold dark matter advocates do that too. They may turn out to be justified in the end.

Lightman:

Let me ask you to take a big step backwards, or I should say to take a very broad perspective and to think perhaps as an ordinary person here. If you could have designed the universe any way that you wanted to, how would you have done it?

Silk:

I wouldn’t have bothered to make it expand. I think that’s a complication really. It makes it hard to calculate things. I would have made a simple static universe — steady state or static, just like the galaxy is. You can imagine that life proceeds very nicely in the solar system or the galaxy, and why bother with the complication of the big bang.

Lightman:

When you say life proceeds, do you mean literally?

Silk:

No, no. I just meant the evolution proceeds. We have a fairly good comprehension of evolution in the solar system. We have a fairly good comprehension of how the galaxies evolved over the past few billion years. The puzzles come in trying to understand the first few minutes of the big bang, and one could well do without those rather horrifyingly high densities and high temperatures that made one’s ideas so hard to work out.

Lightman:

So you would have designed a steady, or static, universe.

Silk:

It’s really just a throwback to the cosmology that prevailed at the time of Einstein — a natural view of the universe. Expansion seemed very bizarre. But, of course, it turned out to be a very natural solution of the gravitational equations, given suitable symmetry assumptions. In retrospect, it led to the big bang.

Lightman:

Let me end with one other philosophical question. There is a place in The First Three Minutes, Steve Weinberg’s book, where he writes that the more the universe seems comprehensible, the more it also seems pointless.^[13] Have you ever thought yourself about whether the universe has a point or not?

Silk:

I have thought about that, and I think that’s an overly dramatic statement on Weinberg’s part. Evolution proceeds regardless of whether our puny brains can comprehend it or not, and there seems to be no question that structure develops in a very organized way, whether we are talking about human scales or galactic scales. So I don't find what I see around me as pointless at all. I’m sure that we’re a very long way from comprehending where it all began or for that matter where it’s all going. I think it’s very dangerous to try to compress everything into a phrase like that.