Martin Rees

Lightman:

I'd like to ask you a few questions about your childhood. Can you tell me any particularly influential experiences that you had as a child, perhaps through your parents or things that you read or friends, which might have gotten you interested in science?

Rees:

I have no scientific background in the family, really [none] at all.

Lightman:

What did your parents do?

Rees:

My parents were both school teachers and I went to ordinary kindergarten. I don't recall any particular experiences that made me keen on science at that stage.

Lightman:

Do you remember when it was that you got interested in science?

Rees:

I think I was always interested in natural phenomena and puzzled about everyday things. But I don't think at any stage in my early school days I had any definite commitment that I was going to end up being a scientist.

Lightman:

Do you remember any books about science that you read?

Rees:

I was rather addicted to encyclopedias; illustrated encyclopedias of how things work were among my favorite books.

Lightman:

How things work -- so they weren't general encyclopedias?

Rees:

These were general encyclopedias, but they had nice "cut-away" illustrations of machines and things like that. I remember always being interested in that type of thing.

Lightman:

Were you more interested in those kinds of things than other sections of the encyclopedia, like geography for example?

Rees:

No, I don't think I was. I was equally enthusiastic about maps and statistics and figures of all kinds. I remember having a good memory for figures, like the heights of mountains, the wheel-bases of cars, and things of that kind. But no particular influence or pressure to go towards science.

Lightman:

When you went to secondary school, did you get particularly interested in science?

Rees:

I was quite interested, but in the English system, by the age of 15, you have to specialize in science-oriented subjects or non-science. Since I had to make a choice, I chose mathematics, physics and related subjects. But I think honestly, even at that stage, it was not a firm commitment. It was simply realizing I was, for instance, rather bad at languages. On the other hand, I was always good at English and at history, so it was somewhat arbitrary -- that I specialized in science subjects.

Lightman:

When you were in secondary school, did you learn anything about astronomy?

Rees:

Nothing in school. I read some popular science books and one or two popular astronomy books, but we weren't taught any astronomy at all in school.

Lightman:

Do you remember any of the books that you read, outside of school?

Rees:

Yes, I remember one of the first I read was by Ray Lyttleton.^[1] I also remember encyclopedia article on astronomy.

Lightman:

Did you know anything about cosmology at that age?

Rees:

In some of the popular science I'd read, I'd learnt about the steady state and the big bang. I'd read things by Hoyle,^[2] who was somewhat notorious for some radio talks he'd given in the early 1950s. So I knew that there was a debate about whether the universe was in a steady state or not.

Lightman:

Did you form an opinion yourself?

Rees:

I don't think so. I think I was just interested in the subject.

Lightman:

I'd like to ask you a little bit about your undergraduate career at Cambridge. Was it in your undergraduate work that you headed in the direction of astronomy, or was it later?

Rees:

It was really later. What happened was that in my last two years at school I had done mainly physics and mathematics because one had to specialize. When I got to Cambridge, I specialized in mathematics. Mathematics in Cambridge includes pure mathematics and applied mathematics, which is actually mainly theoretical physics. At that stage, I liked pure mathematics. I was better at that in the exams. In my undergraduate period, I did more pure mathematics than anything else. I enjoyed the puzzle-solving aspect, but I knew already that I wasn't the kind of person who was going to be an academic mathematician. In my final undergraduate year, realizing that, I concentrated on applied mathematics and theoretical physics and also went to lectures on statistics and things. Of the areas of applied mathematics and theoretical physics, I found particle physics and field theory rather difficult and fluid mechanics very dull. The things I was oscillating between were either astrophysics or statistics and mathematical economics. I did think quite seriously about going in the latter direction, and it was a somewhat arbitrary choice to try graduate work in astrophysics.

Lightman:

Were there any people that you associated with at Cambridge who were influential or particularly interesting to you in these two interests -- astrophysics and statistics?

Rees:

Sure. I had some undergraduate friends who were very keen on astrophysics in a way that I certainly wasn't at that time and who were committed to astrophysics and astronomy throughout their undergraduate career. Also, of course, I knew lots of people who were doing economics and other subjects.

Lightman:

Were there any professors that you spent time talking to?

Rees:

As an undergraduate there were no professors whom I got to know at all well. I had my lecturers and supervisors. Some of them were really very good, but there were none who I had any particularly close relation with. In retrospect, given that I was going to do that, I rather wish that I had done more physics and less mathematics as an undergraduate. Much of mathematics which I learned has not stuck in my mind nor been useful to me, given that the work I've done subsequently has been physics-oriented and phenomenological. Actually, looking back, I feel I benefited less from the undergraduate mathematics I did than from what I'd been taught in my final two years at [secondary] school, where I was very well taught in mathematics, science and mechanics.

Lightman:

When you went into graduate school, working towards your Ph.D., you already had made the decision that it was going to be astrophysics?

Rees:

Yes.

Lightman:

Were there any particularly influential people there?

Rees:

There were more, yes. I started my graduate work in the end of 1964, and again, for my first year, I was very diffident and undecided. I didn't know whether I was doing the right thing and I wasn't particularly happy in it. But, after a year, I became enthusiastic and realized that it was an interesting subject that I should persevere with. There were two reasons for this. First, I was in a lively group. Dennis Sciama, my advisor was a very stimulating and encouraging person. And, of course, those years were particularly interesting years because the quasars had just been discovered and the microwave background as well. One of the first seminars I went to as a graduate student was by Roger Penrose, I didn't understand much of it at the time, but it was actually one of the first occasions where he explained the singularity theorem which he'd just discovered.^[3] So, in retrospect, it was a very stimulating period in astrophysics and relativity because, on both the observational and the theoretical side, lots of new developments were just beginning.

Lightman:

Did you find cosmology to be particularly interesting as an area of astrophysics?

Rees:

I would never say that I'd been primarily a cosmologist rather than an astrophysicist. The topics I learned about when writing my thesis were the background radiation, intergalactic gas, radio sources, radiation mechanisms and things like that. It was already clear that high redshift quasars were of importance for cosmology, so I was involved right from the start in the astrophysics of these. My first student papers^[4] were, in fact, on astrophysical models for 3C273 and for radio sources and for the intergalactic medium. Also, I think I helped to talk Dennis Sciama out of the steady state theory.

Lightman:

Tell me about that.

Rees:

The steady state theory had originated in Cambridge and was perhaps never taken all that seriously outside Britain. But in Britain, because its advocates were a very loud and articulate trio -- [Fred] Hoyle, [Thomas] Gold, and [Herman] Bondi -- one heard a lot about it, and it was taken very seriously. Dennis Sciama, although not an inventor of the theory, had always been a great advocate of it. When I started research, he was defending the steady state theory against the evidence of the radio source counts. The radio source counts had had a somewhat checkered history in the 1950s, [but] by the early 1960s they were thought by Martin Ryle to be fairly reliable. The steep slope of the "log N -- log S" relation was advanced as evidence against the steady state [model]. Dennis devised somewhat contrived models to reconcile the counts with the steady state theory, postulating that many of the radio sources were local galactic objects and we were a "local hole" in their distribution. He believed that the quasar redshifts were cosmological, and therefore quasars were a population which one could not say was local. We published a little paper^[5] which was, in effect, the first use of the so-called V/Vmax test. When the first few redshifts had been discovered (including 3C9, which was the first redshift over 2), we plotted out the distribution of these objects in successive" shells" of equal volumes around us, and showed that there were more at large redshifts. This was important changing Dennis’s mind.

Lightman:

Did he struggle with you initially on this or did he accept your arguments?

Rees:

They were our joint arguments. But Dennis always had a rather different attitude to science, I think, in that he always had to believe something strongly and defend it as an advocate. So his views underwent a fairly sudden transition. My attitude towards science has never been quite like that. There were, in the mid-1960's, active debates about the microwave background, and about the interpretation of source counts and quasar redshifts. That was one reason why I became interested in cosmology.

Lightman:

Do you remember, at this time when you were still in graduate school, whether you had any preference for an open versus a closed model within the big bang context?

Rees:

I don't think so. I think at that stage the main new thing was that the microwave background offered the first evidence for a big bang.

Lightman:

Did you know about the oscillating universe model?

Rees:

I suppose so, yes, because in 1969 -- about two years or so after 1 got my Ph.D -- I wrote a paper^[6] on what would happen if the universe collapsed, showing that the collapse would be more irregular than the expansion and that stars would get destroyed not by hitting each other but by the night sky getting bright and hot. 1 was certainly aware of the different consequences of these two eschatologies, but I can't remember having either "claustrophobic" or "agoraphobic" prejudice at that stage. [I think I inclined towards a closed model because 1 liked Wheeler's idea of an ensemble of universes, which made more sense if each member of the ensemble was finite. There certainly wasn't then the present strong prejudice in favor of omega being exactly 1.] The Cambridge radio astronomy group under [Martin] Ryle was then leading the world in observational cosmology and the source counts. Historically, those counts were very important because they were the first evidence against the steady state, before the microwave background came along. I accepted that as important evidence. I was never part of Ryle's group, which had a very closed "fortress-type" mentality against outsiders. But I attended their seminars and followed the subject quite closely. I was very puzzled by the tremendous hostility shown by the steady-state advocates to Martin's results, which seemed to me very convincing. I acquired a better understanding of the psychology of this only when, at a much later stage, I read more about the history in the 1950s. Ryle had then been equally dogmatic about controversial and, in retrospect, wrong data. Back in 1954, he had been unwilling to accept that the radio sources were extragalactic at all, although Tommy Gold was urging this, [and the counts based on the earliest radio surveys were indeed erroneous]. So when I read more about this history, which wasn’t until much later, I became better able to understand the reluctance of people like Hoyle, Gold and Sciama to accept evidence which seemed to me to be quite compelling. Martin Ryle made many incorrect [conjectures in the early days], but after 1958 he didn't say anything that was basically wrong.

Lightman:

So he didn't have such a good track record initially.

Rees:

[His technical inventiveness was always superb, but] his track record before 1958 in interpreting his data was not so good. Ryle's Bakerian lecture in 1958,^[7] which was the first place where he announced the steep source counts in the [more reliable] 3C survey; after that time, he had a splendid record for interpreting his data, and I was impressed by this.

Lightman:

So you came at the right time.

Rees:

Yes.

Lightman:

Let me ask you a-little bit about how you've reacted to some of the recent results in the last ten years, both theoretical and observational. Do you remember when you first heard about the horizon problem?

Rees:

Well, when was Rindler's paper?^[8] That was 1959, wasn't it?

Lightman:

That was in 1958 or 1956 or something.

Rees:

Yes.

Lightman:

I've looked at that paper and, although he gave a very finalistic and rigorous definition of the various kinds of horizons -- particle and event horizons -- I didn't see anywhere where he stated the horizon problem. Did I not read it correctly?

Rees:

No, perhaps you did. I don't know. But I certainly remember in lecture courses I gave as early as 1970, emphasizing that it's a big puzzle in the big bang why everything started off in a synchronized way, because causal contact was worse in the past. That was certainly something which we were familiar with.

Lightman:

In 1970.

Rees:

In 1970, yes. Simply because of the argument that if the universe is decelerating, causal contact was worse in the past. Then that would be the puzzle. Indeed, in 1972, I wrote a paper in Phys Rev Letters^[9] suggesting that different parts of the universe were initially unsynchronized and dissipation of the random motions released energy that was thermalized to give the microwave background. [I was impressed by] the coincidence that the radiation energy density and the matter energy density were equal at the last stage, well the radiation could be thermalized -- about a z (redshift) of 10⁴. In fact, the development which had certainly made everyone in Cambridge well aware of this problem was the so called Misner program.^[10]

Lightman:

Oh yes, the mixmaster [model].

Rees:

In fact, [Charles] Misner spent a whole academic year -- 1967-68 I think it was -- in Cambridge and lectured about this. His motivation for discussing anisotropic models was that they might [have a different horizon structure which allowed] causal contact. So the horizon problem...

Lightman:

Was at least as old as that.

Rees:

Oh, yes. Although it may have been news to the people who came into cosmology from the particle physics side, it was very familiar to relativists and [others], being the main motive of people like Misner.

Lightman:

Did you consider the horizon problem to be a serious problem with cosmology?

Rees:

Oh yes, yes.

Lightman:

You mentioned an attempt to resolve the horizon problem by having homogenization occur at z = 10,000. Did you have any other ideas about how the horizon problem might be solved?

Rees:

Not really. I think most of us in 1970 felt it was the kind of thing which quantum cosmology would have to solve. You can extrapolate back to the Planck time, but no earlier. That's why the idea of the infinite number of oscillations in the mixmaster universe, [discussed by Misner and the Moscow group], was never relevant because you could only legitimately go back 60 decades of logarithmic time. Misner was writing papers on quantizing the mixmaster universe around that time.

Lightman:

What did you think about the possibility of having the horizon problem simply explained by proper initial-conditions? Did you consider the initial conditions to be very special? Some people have said that [the horizon problem] is solved just by the appropriate initial conditions and leave it at that.

Rees:

Well, there is the "anthropic" type of argument, which has a certain appeal as a last desperate attempt; the idea that we're here worrying about the problem only because of those special conditions.

Lightman:

Yes.

Rees:

I think that people felt that there may be some answer, but it was premature to seek it at that time. It was obvious that back at the Planck time, quantum gravity would come in and that would transcend the concept of a classical horizon anyway. So perhaps we'd have to wait for quantum gravity before getting any more convincing answer to why the initial conditions for the classical cosmology were that way.

Lightman:

I can ask you the same question about the flatness problem. Do you remember when you first heard about that or first thought of it yourself?

Rees:

I've never quite understood the distinction between the horizon problem and the flatness problem.

Lightman:

So when you were thinking about the horizon problem, which would have included [the flatness problem]?

Rees:

Well, I suppose I wouldn't have thought so specifically, but the paradox I'm sure I was familiar with in the late 1960s was that the big bang must have been synchronized in a rather mysterious way. Different parts of the universe which at t = 1 second were not in causal contact with each other had to have "known" to start off expanding with the same local curvature, the same entropy per baryon, etc.

Lightman:

I see. When you phrase it that way, then it's the same problem.

Rees:

Yes. There was no natural way to homogenize or synchronize things. The motive of the Misner program was to homogenize [an initially anisotropic universe] by having neutrinos or something able to communicate [over large scales].

Lightman:

Yes, but would that communication also have made the potential energy come very close to the energy of expansion, or omega equal to one?

Rees:

No. I remember worrying about [this limitation] at that time. There was a tremendous amount of activity in the late 1960s and early 1970s on anisotropic cosmologies of those Bianchi types, by George Ellis^[11] and many collaborators. [This work was pursued] in the [United] States as well, and by the Russian group of course -- Khalatnikov and Lifshitz.^[12] One of the motives was to see if anisotropies would damp out naturally. If so, this would have meant that the isotropy of the universe could easily be understood if we assumed it was homogeneous. But I remember, at least informally, arguing with people back then that that did not answer the homogeneity problem, unless one could show that [damping out the anisotropy everywhere], when only a small part of the universe lay within each horizon volume would make the universe homogeneous.

Lightman:

But you wouldn't make it flat.

Rees:

No, and I suspect that even, the specialists were a bit confused about this. They seemed to believe that if they could show that an anisotropic homogeneous universe would isotropize, that was equivalent to saying it would flatten. In the mid-1970s, a paper by Edison Liang,^[13] emphasized that density perturbations did keep on growing. On a scale very smaIl compared to the perturbation, you would see a uniform shear, but the kind of sheer that was associated with growing-mode density perturbations could not be damped out by viscosity. I remember talking to lots of people about this problem. It became clear in the 1970s that the damping out of anisotropy was not going to explain homogeneity.

Lightman:

I keep asking questions about the flatness problem because it seems to have been a principle motivation for Alan Guth who knew about the flatness problem but didn't know about the horizon problem until after he discovered the inflationary universe model.^[14]

Rees:

As I say, I've never really understood either term, and therefore I've confused them.

Lightman:

Well, as I understand the flatness problem, [it says] that in order to have omega as close to unity as it is today, then, when the universe was, say, a Planck time [old], omega had to be equal to unity to within a part in 10⁶⁰, and that requires very fine tuning.

Rees:

Yes, yes.

Lightman:

Did your feeling about, or understanding of, either of these problems change after Guth's inflationary universe model? Did you feel that the problems assumed a different significance? Or did you feel like they had been satisfactorily solved?

Rees:

The idea of inflation clearly offered an important new insight, suggesting a possible explanation. It meant that one would not necessarily have to go right back to quantum gravity to get the solution. Prior to that time, I had supposed -- in an entirely ill-informed way -- that it was the kind of question which only quantum gravity could answer.

Lightman:

Yes. Did you find a mechanism that solved the problem after the quantum gravity era -- like Guth’s mechanism -- more acceptable or preferred? Or would you be just as happy finding a solution to the problem in the quantum gravity era, at some future date?

Rees:

Guth's version of inflation offered an answer to this question without having a full theory of quantum gravity. But if we did have a theory of quantum gravity that gave us an answer, then that would be equally satisfactory. As to the way things are going, I get the impression that the distinction between those two concepts is getting rather blurred, in that the detailed features of the early version of inflation -- based on a particular phase transition -- have not survived. In a sense, things are reverting to the pre-Guth version of inflation, Starobinsky 1979,^[15] which was based on a sort of sudden transition, some sort of quantum transition, from a de Sitter universe to a Friedmann universe.

Lightman:

I guess some of [Stephen] Hawking's mini-superspace models^[16] also do the same thing.

Rees:

Yes.

Lightman:

Let me ask you another question about the inflationary universe model. Had you been aware [before Guth's work] of inflationary universe ideas or [ideas based on] de Sitter expansion as a way to solve problems?

Rees:

I had heard seminars by Starobinsky. I'd heard (but not understood) a lecture by [F.] Englert, who had done rather similar things.^[17] After the concept of the inflationary universe was introduced, I at least understood better what they were about.

Lightman:

Do you remember how you reacted to it [the inflationary universe model] at that time? Did you think it was a very good idea or a clever idea or just another idea?

Rees:

I thought it was a very good idea, yes.

Lightman:

Did you think that it was likely to be correct, or [more like other] things in astronomy, where typically only one idea out of every hundred ideas is correct?

Rees:

It didn't make a tremendous impact on me when I first heard about it. At a Baltimore Texas conference, [I remember] people telling me about this and that it was very exciting, but it didn't really sink in. Over the years since then, I've come to think that there's almost certainly something in the general idea of a de Sitter phase, but not necessarily in anything closer to Guth's first formalism than to Starobinsky's first formalism.

Lightman:

Do you have any opinion as to why the inflationary universe model has caught on so widely?

Rees:

Oh, quite cynically, I think it helped that it was boosted in America, so there was more ballyhoo about it. And, of course, Guth wrote a much clearer paper than anyone else about it. But I think it was really an evolution rather than a revolution, when one looks at the earlier work of Starobinsky, Sato and others. But I think there's another reason actually. Guth's version of the delayed phase transition was tied to GUT theories, [Grand Unified Theories], and these same theories introduced the idea of baryon non-conservation. The weakness in the early models was that if baryon number is fixed, then it's no good exponentially inflating a region and diluting all the baryons in it. You've still got to explain how we ended up with 10⁸⁰ baryons in the [observable] universe. So the most important change around 1980, perhaps, was that the GUT models gave people reason to doubt baryon conservation. Had that other consequence of GUT not come along suppose that Guth had just talked about delayed phase transition as a way to get exponential growth then his model wouldn't have carried any more weight than Starobinsky's.

Lightman:

Of course, he didn't talk about baryon non-conservation at all in his paper.

Rees:

No.

Lightman:

It was just the association between GUT's and recent work, you're saying.

Rees:

Yes. But what certainly made me think about [inflation] much more seriously was the concurrent idea of baryon non-conservation, which came from the GUT theories. I think without that there would still have been a tremendous puzzle.

Lightman:

You couldn't have started off with enough baryons to end up with 10⁸⁰ in an inflated universe.

Rees:

Yes, it's no good just inflating unless you could create the baryon content after inflation. So it was the realization of baryon non-conservation which made palatable the idea of starting with something very small.

Lightman:

On an observational discovery now, do you remember how you reacted to the results of de Lapparant, Geller and Huchra^[18] and of Haynes and Giovanelli^[19] on large scale structure?

Rees:

There again, I regard it as an evolution rather than a revolution. I think the first detailed discussions in print about large-scale structure, and particularly about voids were at the IAU symposium in 1977, the one in Estonia, with the proceedings edited by Einasto and Longair.^[20]

Lightman:

With this early work, were you beginning to accept the idea of a very inhomogeneous universe? What was your feeling about these structures in terms of a homogeneous versus an inhomogeneous universe?

Rees:

Everyone knew that galaxies were clustered and things were generally smoother on still larger scales. A popular scenario at that time was, of course, the adiabatic pancake model^[21] which naturally gave these structures. So in the late 1970s, "sheets" and "voids" were taken as support for the pancake model, and therefore the adiabatic perturbations. It was clear that the actual statistics of clustering were going to be important as a discriminant. Clearly, one wanted to know whether there was a characteristic scale associated with, for instance, the Silk mass or its neutrino versions. When people talked about voids and sheets, which certainly came in 1977-78, then theorists related them to those ideas.

Lightman:

What about the question of the sharpness of the features? Did you agree or disagree with the notion that [the sharpness of the features] might indicate that gravity was not the dominant force in forming these structures?

Rees:

I think it's suggestive. I first saw [de Lapparent et al.’s maps] in Time magazine, but I'd seen things like this before. In this plot, the radial coordinate is velocity (or redshift), not distance. When one subtracts off the obvious "fingers" that the Coma cluster gives in such a plot, and also when one realizes that systems which are in the process of turning around -- expanding stellar [systems] -- will naturally give you apparent sheet-like features perpendicular to the plane, the evidence becomes a bit ambiguous. When I first looked at the picture, I realized that the key thing was whether the sheets were or were not preferentially perpendicular. If they were preferentially perpendicular to the line of sight, then the natural thing would be to say that they are the effects of the region turning around, with a small velocity gradient. Clearly, when you're looking in velocity space, it's not the same as [physical] space. As you know, since then there have been all kinds of different simulations of clustering. I'm still not convinced that we have firm evidence for non-gravitational effects. The pancake picture, which goes back to the 1970s, predicted collapsed sheets, where there would be dissipation, galaxies being formed only in those sheets.

Lightman:

So you form galaxies on two-dimensional surfaces in that case.

Rees:

Yes.

Lightman:

I talked to Margaret Geller recently about this. She said that, in her view, in the pancake model you formed galaxies only where two of the pancakes intercepted, because then you would have a much higher density along the points of [intersection]. That [process] would [form] a line, and of course a line is not consistent with the structures seen. Any two dimensional structure is consistent, but a line would not be.

Rees:

I think that may be true, but certainly in the early history in the pancake model, the idea was...

Lightman:

...that galaxies would form in a whole sheet.

Rees:

The idea was that you get sheets or, in general, caustic surfaces. In fact, there were papers^[22] by the Russians on caustic surfaces. That that may not be the way things actually happen even in the context of that picture, because galaxies get a still better opportunity to form where two of those sheets cross. The idea of galaxies in sheets was first discussed in the context of the pancake model. And the idea of their being thin is something which would have been expected by people who'd thought about...

Lightman:

Dissipative mechanisms.

Rees:

... about dissipative mechanisms. Also, biased galaxy formation, being discussed in other contexts, was inevitable in [the pancake] model because the only gas that would ever fragment into galaxies was the gas that was squeezed and then cooled. So sheets were the kind of thing which, rightly or wrongly, anyone who had thought about the pancake model would not have been surprised by. I'm sure that was the case for the Soviet group who had spearheaded this work.

Lightman:

If we average over a time scale of say 10 or 15 years and don't point our fingers at these particular discoveries and observations, in what way do you think cosmological thinking has changed? Can you characterize that in a general way?

Rees:

Perhaps in two ways. First, the realization, triggered by developments in physics, that features of the universe which previously we had to regard as initial conditions or premature to study, can now be seriously discussed. The baryon-to-photon ratio clearly couldn't be discussed at all until one had the idea of baryon non-conservation. The same goes for specific versions of inflation, and possibly the origin of the [density] fluctuations. These are fundamental questions which everyone accepts can now be discussed seriously. So there have clearly been very important developments over the last 10 or 15 years in the scope of theoretical cosmology. On the observational side, there have been developments, as you say, in the quantitative analysis of clustering, starting with the correlation function and going on to realizing that that's not just all there is to it. There's also evidence for intergalactic gas, Lyman alpha lines, and quantitative evidence of quasar evolution. Also, there is the realization that the microwave-background-isotropy limits are a serious constraint on scenarios for galaxy formation: a baryon-dominated adiabatic pancake model can already probably be ruled out on the basis of the present microwave limits. So one can now confront different theories with observational tests -- studies of clusters, high z galaxies and quasars, and the better microwave background limits. And, as I mentioned, one can now talk seriously about what caused the baryon-to-photon ratio and the isotropy and the fluctuations. At the same time, there have been new ideas like strings which may be relevant to the fluctuations.

Lightman:

Most of what you've mentioned seem to be [questions] that we knew were interesting [but] just didn't have the tools to answer. Do you think that any completely new cosmological questions have emerged in the last 10 years -- [questions] that we weren't even asking before the last decade.

Rees:

I suppose many of the questions we didn't ask seriously -- or not in our working hours, as it were -- because we thought they were premature. But I can't really think of any of the questions that we think about now which one didn't hope one would be thinking about.

Lightman:

Do you think our overall view of the universe has changed in the last 10 years, on the largest scale?

Rees:

I would say the most significant change is the evidence for dark matter. There again, as you know, this goes back 50 years. But the firm quantitative evidence goes back to the mid-1970s -- the rotation curves and the more detailed studies of clusters. The general realization that the universe is dominated by dark matter, which may or may not be enough to give critical density, is an important change over the last 10 years. Again, there's been an evolution in the seriousness with which the idea has been taken. I can't think of anything that's been a really sudden transition -- such as we'd have if strings turned out to be dominating the universe or we discovered some entirely new piece of physics that created fluctuations from nothing. But I don't think that the observations have pointed to anything that's really qualitatively new. There are some things which we haven't found. We haven't found any objects with a z of 20, for instance, though one wouldn't have been amazed if we had. It's surprising actually, how well the standard model has stood up. The standard 14 big bang [model] -- as [David] Schramm was quite rightly emphasizing in his talk^[23] yesterday -- is a success. There are things that could have happened over the last 15 years which would have discredited the big bang [model]. We might have found zero helium abundance somewhere. We might have found some objects of an age vastly greater than 15 billion years. We might have found the Hubble constant to be at least 100, in which case there'd be a big age problem. The fact that none of those things has happened is gratifying for the extrapolation back to t equals one second. With the advances in physics, one has got a new set of questions to address about what happened before the first second. The status of those discussions is rather like the status of the hot big bang at the time, before my time, when [Georges] Lemaitre and [George] Gamow were talking about it. They didn't have quite the right physics for the neutron-proton ratio and things like that. But over the subsequent decade or two, the physics of the first few minutes got firmed up. Likewise, we can now hope that ideas on the earlier stage will firm up. In one respect, we should be less hopeful, since the relevant physics is going to be more exotic and harder to confront with laboratory tests. But, on the other hand, in the 1950s only a few "eccentric" physicists took much interest in cosmology, whereas the subject now clearly engages the interests of a substantial fraction of the best main-stream physicists, and that ought to accelerate progress.

Lightman:

You've touched on this already, but how well do you think theory and observations have worked together in the last 10 or 15 years? Do you think that the theorists and observers are paying enough attention to each other? Do you think the theorists are considering too many ideas?

Rees:

I think to some extent they've evolved independently. Obviously, there always will be theoretical investigations of things that are not yet ripe for comparison with observations, but I think there are now areas where there is a fruitful confrontation. One rather good example is that the detailed calculations of microwave background isotropy predicted in different models can be compared with the fluctuation amplitudes needed to get galaxies in clusters now: one needs quite sophisticated calculations in order to get a precision better than a factor of two, and that level of precision is merited by the observations now. [And observers can now measure helium abundances to almost one percent accuracy, which allows important constraints to be set on the conditions at the epoch of primordial nucleosynthesis. N-body simulations of galaxies and galaxy clustering have been important in guiding the observers. And the studies of galactic evolution, which started with Beatrice Tinsley,^[24] have clearly been important. Observers know what they can really expect to get out of their data and what is premature to try and get. The fact that in 1970 astronomers were optimistic about getting q₀ [the deceleration parameter] showed that they didn't then have an adequate appreciation of the complexities of galactic evolution. We now realize that the astrophysical theory and the studies of galaxies and the cosmology have to advance on a broad front, in the hope that all parts will come into focus. One won't, for instance, make any progress on geometrical cosmology until we have a better understanding of galactic evolution. Many of the observations are made regardless of theory, but I think we're getting to the stage where it's often prudent for observers to seek theoretical input in formulating systematic programs. Theorists can advise on how important it is to get an extra factor of two in the microwave background, how surprising it is to see gravitational lenses, just how puzzled we should be by the distribution of galaxies, and how to get a better handle on the large-scale velocities of galaxies.

Lightman:

Do you think that the theorists are justified in extrapolating backwards to t equals one second?

Rees:

To t equals one second? Well, I would certainly bet more than 50% odds on that extrapolation and on the standard big bang nucleosynthesis story being OK. I think one should still be slightly open-minded about other possibilities, however.

Lightman:

What about earlier than one second?

Rees:

Earlier than that, then I would have less confidence of any particular picture, simply because the physics is uncertain at energies above 100 Me V, the quark -hadron transition, when t is about 10^-4 seconds. Going further back, it becomes still more uncertain. It will be a very long time before we can have the same confidence in any detailed calculations of, say, the photon-to-baryon ratio, in the way we now have confidence in the predictions of helium abundances in the standard big bang. Similarly, any really detailed theory for incorporating inflation in a precise-cosmological model or understanding the origin of fluctuations will -- have to wait quite a long time. What's encouraging is that these topics can be addressed. I suppose there are really three ways in which one could advance. Developments in particle physics may allow us to understand the early universe more definitely and may, for instance, predict some species of stable particle that survives in sufficient quantities to provide all the missing mass now. The other line of attack is to explore the cosmogenic consequences of different postulates about the very early universe: to see if galaxy formation and clustering accords best with what we observe in models dominated by cold dark matter or hot dark matter, or if fluctuations were triggered by strings or random Gaussian noise. I suppose a third line is the more speculative discussion of the very early universe, which might lead to some new insight.

Lightman:

You've already covered this in different forms, but could you give a list of what you consider to be the most outstanding problems in cosmology today. Maybe you can mention one or two theoretical [problems] and one or two observational, whatever you wish.

Rees:

Clearly, there are the fundamental issues, whose solution will have to await a better understanding of ultra-high energy physics. Those are the problems of initial conditions, fluctuations, etc., which depend on as yet uncertain physics.

Lightman:

Like quantum gravity.

Rees:

Yes, quantum gravity, phase transitions and so forth. The questions of isotropy and flatness, the baryon-to-photon ratio and fluctuations all depend on the ultra-early universe in some sense. So that's one set of problems. And there is the other set, where the basic physics is known, but where things are complicated simply because they're non-linear. Here I think we can hope for progress via a combination of theoretical modeling, numerical simulations and observations. What we'd like to do is to understand the evolution from amorphous beginnings to a universe structured in galaxies and clusters of galaxies. There's a reasonable hope of getting detailed pictures of the universe back at a z of 1, maybe higher with quasars, and understanding how structure emerges and how galaxies evolve.

Lightman:

To understand structures.

Rees:

Yes, and see what the clustering was like to a z of 0.5 or 1. Did galaxies have their disks at a z of I? Are there quasars beyond a z of 5? If not, are there sources of energy beyond a z of 5? Clearly, the different models -- the pancake model, the string model, the cold dark matter model, and so on -- make very different predictions, and I think we can hope, by better observations of high z objects and background radiation, to gradually firm up on an evolutionary scenario. The kind of cosmology that I'm mainly concerned with the evolution of galaxies and why the universe was different at high z - is not really different in conceptual status from what's done by a geologist or a paleontologist. The nature of the evidence we have may be slightly more tenuous but, in principle, it's no different. We’re trying to place our Solar System -- and our Galaxy in a grander evolutionary context and to trace the chain of cause and effect back further. But it's not different from the methodology of many other sciences, although the first set of problems I mentioned the more "fundamental" ones -- are perhaps different in that they do confront, for instance, the question of whether the universe is inherently unique or not.

Lightman:

Do you think: about questions like that, also?

Rees:

Oh, I wonder about those questions a lot, yes. And, when ruminating on the anthropic principle, the Universe and the place of planet Earth in it, one's attitude depends very much on whether one thinks that there is something unique in the laws of nature -- whether there is going to be a unified theory, which is going to tell us that the constants could not be otherwise or [alternatively] could there have been a universe or different parts of our universe beyond our present horizon where things were different? That question, whose answer will have to await further developments in physics, is going to make a difference to one's attitude to the anthropic principle.

Lightman:

Oh, yes.

Rees:

The question of whether the universe is an infinite or a finite system with an actual boundary is something, which some programs in quantum gravity address. That's clearly a fundamental question which is relevant' and which we like to speculate about.

Lightman:

Let me end with a couple of questions that are of a more philosophical nature. I'm going to ask you a question that was asked of Dennis Sciama in his interview with Spencer Weart, although not exactly in this form. If you could design the universe the way that you wanted to, how would you do it?

Rees:

You mean so that I would enjoy contemplating it more?

Lightman:

If that would be a criterion that you would use. You can use any criteria that you want to.

Rees:

I saw a title of a paper by someone -- "Is our universe the simplest possible interesting universe?" -- by which it meant did we have to have all the laws of physics we have, and four different forces, in order to get any kind of "interesting universe." I think the answer is probably 'yes' in the sense that we do need to have some quantum mechanics and some microphysical laws, and we also need a force like gravity which can organize things on a large scale, generate thermodynamic disequilibrium, and [allow] large structures to evolve in the initial featureless universe. But, paradoxically, the weaker gravity is, the better: the weaker it is the more powers of ten there are between the microphysical and astronomical scales, and the more powers of ten differences between the time scales for global evolution and the time scales for the microphysical processes. We have - and our very existence requires - a universe that allows complexity and structure on many scales to evolve within it, starting off from simple beginnings. That universe has to have the property of flatness a force like gravity, which can produce large scale organization without being so strong as to crush out all small scale structure. I'm not sure if that's the kind of answer Dennis gave. As to whether one wants the universe to be infinite, to go on forever, I suppose I would like a complex and varied universe where different laws of nature might prevail in regions beyond our present observational horizon. If the universe went on expanding forever, other domains with different laws of physics and different properties could conceivably, in the very distant future, come within our observable range. Some people, of course, prefer the idea of a collapsing universe. Then you could put the idea of an ensemble of universes on a more serious footing, but you could do the same, I suppose, for this one universe, if you allow different parts of it...

Lightman:

To be disconnected.

Rees:

To be disconnected, yes, and have some oases with laws of nature propitious for complexity, and others not.

Lightman:

Do you like the idea of having different parts of the universe with different laws, or would you prefer the entire universe to have the same laws?

Rees:

I rather hope that there are unending complexities in the physical laws, rather than what many physicists hope for, which is that there will be a unique theory which is attainable. On the other hand, it's probably best that many physicists should believe in the final theory, because that's the strongest motivation that keeps them working hard at it. It's good that they should be looking for such a thing, even if one hopes they may never find it.

Lightman:

Let me end with one other question. There's a place in Weinberg's The First Three Minutes where he says that the more the universe is comprehensible, the more it also seems pointless.^[25]

Rees:

That's right. He ends the book that way doesn't he, more or less?

Lightman:

It's not quite at the end, but it's somewhere near the end. What is your attitude about that idea?

Rees:

I don't understand the remark at all. First of all, I'm not quite sure what's meant by saying the universe has a "point" or not. Some people react that way because life and intelligence seem a small, unimportant part of the universe. I think that's an invalid reaction for two reasons. First, of course, there may be intelligence all over the universe -- that's something we'd like to find out. But if there's no life elsewhere -- if life is such a rare accident that it got started only on this one Earth (which is an" entirely tenable point of view), then we shouldn't think of [ourselves] as being the culmination of evolution. Even if the universe is going to recollapse, it's run less than half its course, and it may have an infinite expansion ahead of it. It's quite conceivable that, even if life now exists only here on Earth, it will eventually spread through the galaxy and beyond. So life may not forever be an unimportant trace contaminant of the universe, even though it now is. In fact, I find that a rather appealing view, and I think could be salutary if it became widely shared. Then one could properly regard the preservation of our biosphere as a matter of cosmic importance. Despite our own species' unprepossessing characteristics, it may have potentialities. If you had clobbered the first fish that crawled onto dry land, you'd have destroyed the potentialities of all land-based life. Likewise, if we snuffed ourselves out, we'd be destroying genuine cosmic potentialities. So even if one believes that life is unique to the earth now, then that doesn't mean that life is forever going to be a trivial [piece of the universe]. And, if life exists elsewhere, then there's still less reason to believe that, in any sense, the universe is pointless. So I've never really felt the pessimistic connotations of Weinberg's remark.