Oral History Transcript — Dr. Alan Guth
This transcript may not be quoted, reproduced or redistributed in whole or in part by any means except with the written permission of the American Institute of Physics.
This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.
See the catalog record for this interview and search for other interviews in our collection
Interview with Dr. Alan Guth
Alan Guth; March 21, 1988
ABSTRACT: Parental background; education at M.I.T.; summer work at Bell Labs; history leading up to the discovery of the inflationary universe model; introduction to the flatness problems and horizon problems; initial conditions in physics; question of legitimacy of flatness and horizon problems; attitude toward de Lapparent, Geller, and Huchraís work on large-scale inhomogeneities; the question of whether the universe is homogeneous on the large scale; outstanding problems in cosmology: galaxy formation, measurements of anisotropy in cosmic background radiation, dark matter; ideal design of the universe.
TranscriptSession I | Session II
Lightman:I wanted to ask you questions that we didnít get to in the last interview, which was sort of an exploratory interview. Let me start with a couple of questions in your childhood. We did talk about that a little bit last time. Could you tell me about your parents?
Guth:Sure. Neither of my parents graduated college. My father had gone to two years of junior college. All the time I was growing up, my father was working in small businesses. In my younger years, he owned a small grocery store in New Brunswick, New Jersey. During most of my childhood — I donít know when he changed businesses, probably when I was ten years old or something — from then on he owned a dry-cleaners business, also in New Brunswick, New Jersey. My mother had never gone to college, and she had very few interests outside of being a full-time mother.
Lightman:Do you remember any experiences that you had with your parents that might have been influential in your becoming a scientist?
Guth:Thatís a good question. Itís kind of hard to tell from my family background what it might be that drove me towards science. Neither of my parents knew much about science, nor was they particularly interested in science. Iím not sure what it was that pointed me towards science.
Lightman:Tell me a little bit about your undergraduate education at MIT. We didnít talk about that at all in the last interview.
Guth:Okay, sure. What would you like to know about it?
Lightman:Iíd like to know about any particularly influential experiences that you had. Also, Iíd like to know whether you had decided from the beginning that you wanted to go into physics.
Guth:I had decided from the beginning that I wanted to go into physics, and somewhere I think pretty early on, I even went so far as to decide that I wanted to go into particle theory. I think what appealed to me about particle theory was the idea that youíre grappling with trying to discover what the fundamental laws of nature are themselves, and that idea fascinated me — just the idea that there were fundamental laws of nature that could be discovered. As far as the things that happened while I was an undergraduate, I think one of the more influential things for me was a project I got involved in my junior year and then continued in my senior year and the summer following my senior year. This was not a theoretical physics project. It was a project I did with Aaron Bernstein, who is an experimental nuclear physicist. I had met Aaron during my first year at MIT. He was a recitation instructor for one of the terms of freshman physics. Sometime around my junior year I started working with him. I think it was initially a project that we had to do for junior lab and then later it became my senior thesis and then later became my masterís thesis. It didnít have much to do in any direct way with the work I did later, but it was the first time I really felt like a physicist, and it was a very good feeling. Until then I really just felt like a student.
Lightman:Because this was original research?
Guth:It was original research of sorts. Even though he is an experimentalist and it was an experimentally-oriented thesis, I actually never really got my hands dirty. It was a theoretical analysis of an experiment to be done. [laughter] But it was original in the sense that I felt like what I was doing was important, and had never been done before, and he did a good job of making me feel that what I was doing was important.
Lightman:Is that the closest you ever got to experimental physics?
Guth:I suppose, outside of the laboratory courses themselves. Yes, outside of courses, thatís the closest I ever came. In courses, you have your hands on things, but theyíre experiments which are classic rather than current.
Lightman:Theyíre set up for you.
Guth:Right. Another experience I had as an undergraduate was that I spent one summer — it was the summer after my junior year — working at Bell Labs in Murray Hill. That was also a lot of fun and at least in terms of my feeling towards physics I got a lot out of it. I was working with lasers, and one of the things I was exploring was sort of a half-baked idea that somebody there had come up with — half-baked by his own admission. It was something he never followed up on. It was intended to be a cavity that had a continuous range of resonances, which may sound to you like itís impossible. It sounded to me like it was impossible, and I spent most of the summer trying to convince the person I was working for that it was impossible. By the end of the summer, I succeeded, and that was a very good experience for me.
Lightman:Some kind of tunable cavity?
Guth:Well it wasnít. Tunable is okay — you can in principle make a tunable cavity. This cavity was intended to be resonant at all frequencies without any tuning. I can tell you what the idea was. It was a cavity which instead of having an ordinary reflective mirror at the end, at one end it had a diffraction grating, so that the reflection was at an angle.
Guth:The intended idea was that because the reflection was at an angle you had a situation where if you made a small change in the orientation of the light beam, it looked like you then have a first-order change in the path length, and that would be enough to give you an essentially continuous range of resonances, given some finite Q. The reason it goes wrong is that a diffraction grating is not really a smooth surface that reflects at some angle; it really is a lot of little slats. It works precisely because you have a coherence of phases of waves coming back in a certain direction. What really counts is not the geometric path length between idealized smooth surfaces, but you have to take into account the grating itself, obviously. You need to think about a precise graph of the path length between the mirror and the grating, plotted against the angle of the beam. If you look at his graph crudely, it looks like a line with some nonzero slope. It was this crude picture that led to the belief that maybe continuous resonance was possible. But, if you look at the graph precisely, it has a series of discontinuities caused by the etching in the surface of the diffraction grating. At the angle of reflection these discontinuities are an integer number of wavelengths. Since the conditions for resonance are not affected by a change in the path length by a multiple of the wavelength, these discontinuities can be subtracted out. The resulting line then has zero slopes and looks essentially like what you would have for a resonant cavity with mirrors on both ends.
Lightman:Let me ask you about some of the motivating factors or influences in your development of the inflationary universe theory. We talked about this once several years ago. Could you say that again for the record?
Guth:Sure. Would you like me to tell the story that led up to the discovery of inflation?
Lightman:Yes, and with emphasis on what influenced you in particular, not just the historical things, but the story as it relates to you.
Guth:The anecdotal accidents. Sure, thatís what I understood you to mean. The story as it relates to me probably starts in the fall of 1979. It was just sort of an odd coincidence. I went to a lecture given by Bob Dicke from Princeton. It was a lecture which I might have gone to and might not have, but I decided to go, and it turned out of course to be very important in terms of what I learned during that lecture. Because one of the things he talked about in the lecture was the flatness problem. I donít remember for sure if he used that word, but itís definitely what he was speaking about, and, as he phrased it, the problem was that if you looked at the universe one second after the big bang, at that time the expansion rate had to be exactly what it was to an accuracy of about one part in 1014 or else the universe would have either flown apart without ever forming galaxies or quickly recollapsed. Now at the time I was not working on cosmology. It was just an odd fact for me, but one which struck me, and I tucked it away in the back of my mind. At the time, I didnít even understand how to derive that fact, but I believed it and was startled by it. Also during the fall of 1979 — probably later, Iím not sure if this is the exact chronology — Henry Tye came to me, and this was very important in my getting involved and working in cosmology. He had gotten himself interested in grand unified theories, which were all new to me, and he came to me with a question. The reason he asked me was because of some work Iíd done earlier on magnetic monopoles. He asked me whether or not grand unified theories would give rise to magnetic monopoles. And at the time I didnít really have any idea what grand unified theories were, so he had to teach that to me, which he did.
Lightman:You were a postdoc at Cornell at this time, is that right?
Guth:Thatís right. I was a postdoc at Cornell, and so was Henry.
Lightman:And the Dicke talk was the Einstein Centennial...
Guth:That turned out to be a mistake. I probably told you that. There was a time when I believed that, but that turned out to be a mistake. The fact that it was a mistake was discovered by Marcia Bartusiak, a very enthusiastic and diligent reporter who is writing a chapter about me. After I told her the story, she went to Cornell and tried to track down the date of this talk, and it turns out it wasnít the Einstein Centennial talk. It was another series of talks that were given in the fall of 1979.
Lightman:Okay, I mentioned it because Dicke wrote a chapter about cosmology in the Einstein Centenary volume, in which the flatness problem was mentioned.
Guth:Thatís right. As far as what Dickeís done and talked about in print, thatís right. It was a chapter by Dicke and Peebles in the. Einstein Centenary volume thatís edited by Hawking and Israel.
Lightman:Okay. Iím sorry to interrupt — letís go on with the story.
Guth:Thatís okay. So Henry explained to me what a grand unified theory was, and I donít remember how long it took me to figure it out, but I understood at that time enough about magnetic monopoles, so it was pretty obvious once I learned about grand unified theories, that they would give rise to magnetic monopoles. So I came back to Henry and told him that, yes, there would be magnetic monopoles. But I told him that they wouldnít be things that you could expect to find, because theyíd weigh something like 1016 GeV. So itís just another unobservable consequence of these theories. So he immediately came back and said why we donít try to figure out how many of those would have been produced in the big bang. At that time, it sounded to me to be an almost totally crazy thing to think about. I had never worked on cosmology. All the problems in fact that I had worked on up until that time had been very clean, well-defined mathematically-posed problems, and this was the opposite. When you talk about the early universe, itís always hard to know what the relevant physics is and whether or not youíve got it right or whether youíve missed some other fact thatís very important. Itís a much more open-ended, less well-defined problem. So I resisted working on it, — and it really wasnít until sometime in the spring that Henry finally convinced me to begin to work seriously on the problem. In between, there was one other significant influence on me which was important. And that is sometime in the spring of 1980... No, Iím off by a year all around. The Dicke talk was in 1978 and now Iím talking spring of 1979.
Lightman:The Dicke talk was in 1978?
Guth:Yes. That has to be right because all of this ends in the following December. It was December 1979 that I invented inflation.
Lightman:Yes, thatís right.
Guth:So Iíve been a year off in talking about it today. It was fall 1978 that Henry started talking to me about grand unified theories and it was the spring of 1979 that I started seriously working on it. I was influenced in part by a visit during the spring of 1979 to Cornell by Steve Weinberg, who at that time was working on baryogenesis in the context of grand unified theories. He gave several lectures about that at Cornell, and Iíd always been very impressed by Steve, and it was really his work that convinced me that the early universe was a well-enough defined question that it made sense to work on it, and that grand unified theories were interesting enough so it made sense to apply them to the early universe. So at that point I started working with Henry, and we fairly quickly came to the tentative conclusion that it looked like far too many of these magnetic monopoles would be produced. We were also in touch — I forget whether it was in the spring or early summer — with John Preskill, who at that time was a graduate student of Steve Weinbergís and who was also working on exactly this question of magnetic monopole production and the big bang. Preskill was the first to publish on this subject, and I think he did have a significantly cleaner argument than Henry and I had, although we were reaching similar conclusions. So he published whatís now the really classic paper that concludes that if you have standard cosmology and grand unified theories, you tremendously over-produce magnetic monopoles. You over-produce them by so much that itís not even a question of magnetic monopole searches. If there were that many magnetic monopoles, the entire universe would have collapsed in 30,000 years or so. So, John Preskillís paper came out — I forget when exactly — late summer, fall, something like that. In the fall of 1979, I moved to SLAC [Stanford Linear Accelerator Center]. I was on leave from Cornell, intending to spend one year at SLAC, which is also how it turned out. But Henry and I kept working. We communicated by telephone. At this point our worked shifted a little bit because the Preskill paper had been published, so it wasnít enough just to say that too many monopoles would be produced, you had to say something in addition.
Lightman:You had to solve the problem.
Guth:I donít know about solve the problem, but we did gear our work towards trying to figure out whether there was any way around it, whether there was anything that you could change in your assumptions to make grand unified theories compatible with cosmology and the fact that the universe is not swimming with magnetic monopoles. We were led to the idea of super cooling. The magnetic monopoles are produced in this phase transition of the grand unified theories, and the argument which both Preskill and we were using to convince ourselves that there would be far too many magnetic monopoles was really just based on timing. The argument said that if the grand unified theory phase transition happened when we expected it to happen that there just wasnít enough time for the Riggs field to become organized well enough throughout space, so the Higgs field would necessarily be left with a lot of knots and twists. And a magnetic monopole really is just a knot in the Higgs field. So if the Higgs field didnít have time to organize itself, it would necessarily be left with a lot of knots and a lot of magnetic monopoles. Super cooling clearly gets you around that argument — it gives you extra time, and the more super cooling you have, the more time you have to buy. So Henry and I wrote a paper about this which we finished in December of 1979. We wrote it as a Physical Review letter, and in that paper we totally ignored any effect that the phase transition might have on the evolution of the universe. We assumed that the scale factor R(t) just evolved exactly as it would if the phase transition were not taking place. Somewhere while we were doing that, Iím quite sure that — I never really checked back with him — Iím quite sure that it was Henry who suggested that we should look at that assumption. It was the beginning of December 1979 that I sat down and looked at it. And when I did in one very exciting night, I realized that it would lead to exponential expansion, and I also by that time understood the Dicke problem, the flatness problem, well enough to realize that that would solve the flatness problem. That was the first time I sat down and really went through the arithmetic of the Dicke problem, but I understood enough about how it worked to realize that exponential expansion would avoid the problem. And that was the beginning of inflation. Meanwhile, Henry and I were still mainly concerned about finishing off this paper. He was about to go on a rather long trip to China, so we wanted to be sure to get the paper tied up before he left. So in our conversations we were still concentrating mainly on just getting the draft of the paper in order, which we finished and submitted to Physical Review Letters just before Henry left on his trip. Then I continued working on this exponential expansion idea.
Lightman:You talked in your last interview about how the idea was received and publicized.
Guth:Right. There was one other freak coincidence, which I donít remember if I talked about last time or not. Iím pretty sure it was sometime in January of 1980 that I learned about the horizon problem for the first time, and that was also just an odd piece of information that just fell in my lap one day.
Lightman:I was going to ask you about that. So you learned about that problem after you had invented the inflationary universe theory. Most people knew about [the horizon problem] before the flatness problem.
Guth:Thatís right. Most people with a more normal education, right.
Lightman:I was going to ask you some questions about the horizon problem, but since you learned about that after inflation, let me refer those questions to the flatness problem, since the two are so closely related. You mentioned or stated that the flatness problem was very influential — planting a seed of a cosmological problem. When you first heard about the flatness problem from Dicke — I know Dicke has his own point of view on the significance of the problem — did you think that it was a problem that demanded a physical explanation as opposed to a special set of initial conditions, or how did you imagine in your head that the problem might be resolved?
Guth:Good question. I donít know if Iím completely sure. I think that the way I probably looked at it was initially in a vague way, but I think pretty close to the way it turned out. I think I initially interpreted it as an indication that there was something missing in the picture of the big bang and that some little ingredient would be invented to explain this fine tuning.
Lightman:I assume from that you mean some ingredient other than a special set of initial conditions, or did you distinguish between initial conditions and physical mechanisms?
Guth:Iím not sure that I would have distinguished between initial conditions and physical mechanisms, but I would have probably said that you would need some new element of physics. I think Iíve always viewed the initial conditions as something that probably is determined by physics — physics that we may not understand yet. Itís hard for me to tell when these ideas emerged in my head, but at least from my present point of view, the ideas of quantum cosmology and that the universe has a well-defined wave function determined by quantum cosmology sound very attractive to me, even though I think weíre still only at the primitive stages of trying to understand exactly how that works.
Lightman:Did you know about quantum cosmology at the time that you heard the Dicke talk?
Guth:Probably not. So Iím not sure of what I would have said then, but Iím not sure I would have drawn a sharp distinction between mechanisms and initial conditions — I think I might have said just Ďsome physicsí that happens in the early stages that would have to control the mass density of the universe and create this fine-tuned situation.
Lightman:So you were not willing to accept it then as an accident?
Guth:No, I would certainly not be willing to accept it as an accident, right.
Lightman:Which is the view that some people had?
Guth:Yes, thatís right. I guess Dicke himself at the time — and I donít think he said this in his lecture, but it certainly is in the written version of the Einstein Centennial Volume — at that point he was pushing the point of view that [the flatness problem] indicated an oscillating universe, that the entropy builds up from cycle to cycle. I donít know if that idea would have appealed to me or not. I guess I would have considered it a possibility. From my present point of view, the short-coming of that idea is the fact that thereís just no physics behind the bounces of the oscillating model. Thereís no physical theory.
Lightman:No physics that we know of.
Guth:No physics that we know of. Thatís right, itís not ruled out by anything we know, but thereís no physics that we know of that indicates that nature works that way.
Lightman:Itís not immediately obvious to me how an oscillating universe would solve the flatness problem...
Guth:Well, thatís right. It may solve a certain aspect of the flatness problem. That is, it may explain why the total entropy of the universe is very large, which is one way of saying the flatness problem.
Lightman:Oh yes, it could certainly explain that.
Guth:But thatís not really the whole problem in the sense that itís linked closely to the homogeneity problem and you would certainly not expect that a universe that had just been oscillating for ages and ages [would] have any good reason to be homogeneous. That is another flaw in that picture, I think.
Lightman:When you heard about the horizon problem, which was after inflation, did you consider it to be a legitimate problem in the same way that you considered the flatness problem to be legitimate — that is to indicate something missing in the standard big bang model? By that time, you already had a solution to the horizon problem so you were in a special position.
Guth:Actually, it really did take a while before I regarded the horizon problem as being as significant as the flatness problem. Now I think I regard the two as being equally significant.
Lightman:Why was that? Why did you regard it initially to be less significant than the flatness problem?
Guth:I think the reason I certainly would have given at the time was that itís a problem whose statement is less quantitative. With the flatness problem, you have this number, the expansion rate at, say, one second, which has to be tuned to 14 decimal places. If you want to phrase it at grand unified theory times itís even more — 49 decimal places. The horizon problem is always just qualitative — you donít understand why the universe looks the same here as it does there — and for that reason it impressed me somewhat less.
Lightman:Because what you mean by uniformity of temperature is hard to quantify. You can certainly state quantitatively what is the largest angular size over which things were in the horizon at a z of 1000, when the microwave radiation was produced. You can certainly state that quantitatively.
Guth:Yes, thatís right. But I guess I was less impressed because thereís no colossal number that you have to explain. The horizon is not that small. The causal horizon is maybe a factor of 100 smaller than what youíd need to cover the entire observable universe, but not a factor of 1015 or 1029. I suspected that there was enough ambiguity so you could find some minor way of changing the physics of the early universe, without something dramatic that might very well get around the horizon problem.
Lightman:I see. But you say that since youíve been thinking about it more, that you have come to elevate its importance to that of the flatness problem?
Guth:Yes, I think so. Why? [pause] Well, certainly the strongest influence is probably just the psychological importance of time. In the end, of course, itís not really a scientific question to ask which of these problems is more important. It doesnít affect anything that youíve concluded about whatís true or false. Emotionally, I regard the two as on equal footing. In terms of the role they play, I think the role of the horizon problem has enlarged somewhat. It enlarges when you consider a question that came up after inflation was proposed. The people who were strongly convinced that omega was really 0.2 rather than 1, some of those people advocated the idea of limited inflation — inflation that would cut off at just the right point so that omega doesnít get driven to one. I think the strongest argument against that is basically the horizon problem of sorts, that is, the uniformity of the cosmic microwave background. If you have inflation which cuts off before it drives omega to one, it means that you're still leaving significant influences from the initial conditions, before inflation, in the observable universe.
Lightman:And you donít solve the horizon problem.
Guth:You can marginally solve it I think. I havenít really looked into this. I think you may or may not be able to marginally solve it in the sense of making the horizon just big enough to cover the universe, but I think itís very hard to explain the fact that the universe is really uniform. It is not enough just to have the horizon cover it. You somehow have to explain why the cosmic background radiation is the same temperature at one end as at the other to accuracies of a few parts in 105. And that I think you canít really explain, although it becomes not a terribly well-defined question because if you only have enough inflation to drive omega to 0.2, what youíre left with depends strongly on what you started with before inflation. Thatís sort of the name of the game if you havenít driven omega to one, because the end point of inflation — if you let it go for a long time — is certainly omega equals one. Itís only the asymptotic end point thatís independent of the initial conditions.
Lightman:Yes, right. Let me ask you about your reaction to some recent observational work in cosmology, in particular the work on the large scale structure by Geller, Huchra, and de Lapparent and by Haynes and Giovanelli, which is similar. Do you remember when you first heard about that work? How did you react to it?
Guth:I guess I first heard about that work pretty early on, since Iím here [at the Harvard-Smithsonian Center for Astrophysics] and people were talking about it. I think my initial reaction was not too different from my feeling about it now. I think itís clearly important work, and work which has everybody working on trying to understand the large scale structure of the universe. It was certainly advertised by some people as being sort of definitive evidence that we didnít understand at all what was going on and that we were back to ground zero in terms of building theories. I donít think thatís true. I think we still have the possibility of understanding this. Itís a very complicated problem. I think after the initial observations, the simulations were pursued a little bit more using ideas like cold dark matter, and they began to look a little bit more like the data. I donít think thatís entirely artificial. People started to look at circumstances that more closely resemble the circumstances of the observations themselves. The other idea was — Iím not sure of the precise chronology — the idea of biasing. I forget when it was first proposed, but it certainly didnít really catch on until after these observations. Once you assume that biasing is at least a possibility, then it becomes much harder to compare theory with observation because you donít really understand what you mean by biasing, so it gives it a lot more uncertainty on the theoretical side. So I think what these observations are showing is that there are certainly things about the large scale structure of the universe that we donít understand yet. I donít think they mean that inflation or even inflation plus something as specific as cold dark matter is necessarily in trouble. But it certainly shows there are things that we need to understand. And in particular, I think the biggest uncertainty on the theoretical front right now is the nature of galaxy formation biasing.
Lightman:Did you have any views about the homogeneity or non-homogeneity of the universe prior to learning about the bubble work? [Did you] simply assume that the universe was homogeneous on the large scale or...
Guth:I always assumed that the universe was homogeneous on the large scale, and I still do. I think that the evidence from the microwave background is still very compelling, no matter what we see at the scales where we actually observe galaxies, which are shorter scales than what we see in the microwave background. Unless you believe that the microwave background is something totally different from what we think it is, I think youíre still forced to believe that on the largest scales the universe still appears homogeneous. By the largest, I mean scales like the microwave background. Obviously, if inflation is true, there are scales much, much larger in which the universe might be very inhomogeneous.
Lightman:Which weíll probably never know about.
Guth:Right, exactly: But on the largest scales which we can conceivably observe, it appears that the universe is homogeneous, and I think thatís what inflation should lead to. How all this complicated structure emerged on smaller scales is obviously a very intricate problem.
Lightman:So you regard that as a more detailed question, in some ways.
Guth:Yes, thatís right.
Lightman:Let me ask you, just for a minute or two, about what you consider to be the major problems in cosmology today. You mentioned one just a minute ago — the theoretical understanding of galaxy biasing, or how you determine at what scale of density and homogeneity a galaxy will form, whether it needs to be 10% above the average density or 10 times the average density...
Guth:Thatís right, or whether itís not that local a question and is influenced more by nonlocal effects.
Lightman:Yes. What are some of the other outstanding problems in cosmology today as you see it?
Guth:On the observational side, it would be tremendously important if we could get good measurements of the anisotropies in the cosmic microwave background radiation. Certainly at levels that are just a little bit beyond what weíve observed, there had better be anisotropies in the cosmic background radiation, or else all of our possible theories of galaxy formation fall apart. So from an observational point of view I would say that that is the most important thing that we would really like to learn about. The other thing which is also important is just more and better data of the same type as Huchra, Geller, de Lapparent and the Seven Samurai. It would help a lot if we had a better map of what the universe in Sour vicinity looks like. Apparently thereís a lot that can be done there — itís just a question of availability of funds and astronomers. Actually, astronomers are available. I think itís really a question of availability of funds more than anything else. But a really good redshift map, many times the amount of data thatís currently available, would be a big help. It would not answer the questions by itself, but would provide the data that would help a lot for people who are trying to build models of galaxy formation.
Lightman:What about on the theoretical side?
Guth:On the theoretical side, it seems to me that the primary question is the formation of structure. I think thereís very little that weíre in a position now to learn about [regarding) the very early universe, except possibly about things that we might get clues about from trying to understand the structure of the universe. I think on the theoretical side the key question really is the question of biasing and that could be phrased in other ways. Itís really the whole question of how galaxies actually formed. The simulations that have been done today for the most part simply trace the evolution of mass under the force of gravity. Obviously in the final stages in which galaxies actually form, thereís a lot more physics going on than just gravity, and somehow all of that physics has to be pulled together so that we can get at least some crude understanding of what it is that causes galaxies to form. Hopefully, in doing that, youíd understand how galaxies of different morphologies form and how you can relate the map of mass density you produce in these simulations to a map of the light density you expect to observe.
Lightman:Or maybe getting a handle on the dark matter.
Guth:Thatís right. The dark matter looks crucial to the whole thing.
Lightman:Figuring out what it is.
Guth:Figuring out what it is. Certainly thereís a lot of work to be done before we know what it is, by just trying different things and seeing what the consequences would be. In fact, that may be, in the end, how weíll determine what the dark matter is, by understanding more about the physics thatís involved in galaxy formation and then just seeing how it would work for different assumptions about the dark matter. But certainly the dark matterís absolutely crucial in this, and we wonít fully understand it until we know what the dark matter is.
Lightman:Let me ask you a final question. This is much more speculative than the ones that Iíve asked you so far, and I might ask you to put your ordinary scientific caution aside. If you could design the universe any way that you wanted to, how would you do it?
Guth:[laughs] Gee, I donít think thatís a fair question.
Lightman:You donít have to answer it if you donít want to, but...
Guth:Youíre giving me the opportunity [laughs].
Lightman:Iím giving you the opportunity. It was actually asked to Dennis Sciama in one of Spencer Weartís interviews of astrophysicists in the late 1970ís.
Guth:I see and what was his answer?
Lightman:We thought it was such a good question...
Guth:Youíd add it to your list.
Lightman:Weíd add it to our list.
Guth:I really donít think I have any answer.
Okay. I want to thank you again for coming back a second time.
 Editorís Note: Dickeís lecture was actually in the Fall of 1978, as Guth realizes later in the interview.
 Marcia Bartusiak, Thursdayís Universe: A Report from the Frontier on the Origin, Nature, and Destiny of the Universe (Times Books: New York, 1986)
 Editorís Note: Guth is mistaken again here, and corrects the year to 1978 later in the interview
 R.H. Dicke and P.J.E. Peebles, ďThe Big Bank Cosmology Ė- Enigmas and Nostrums,Ē in General Relativity: An Einstein Centenary Survey, ed. S.W. Hawking and W. Israel (Cambridge University Press, 1979); the flatness problem was actually stated earlier in R.H. Dicke, Gravitation and the Universe, the Jayne Lectures for 1969 (American Philosophical Society, 1969), pg. 62
 A. Guth and S.H. Tye, Physical Review Letters, vol. 44, pg. 631 (1980)
 V. de Lapparent, M.J. Geller, and J.P. Huchra, ďA Slice of the Universe,Ē Astrophysical Journal Letters, vol. 302, pg. L1 (1986)
 H.P. Haynes and R. Giovanelli, ďA 21 centimeter survey of the Perseus-Pisces Supercluster. I. The Declination Zone +27.5 to 33.5 degress,Ē Astronomical Journal, vol. 90, pg. 2445, (1985)
 The ďSeven SamuraiĒ refer to the astronomers D. Lynden-Bell, D. Burstein, R.L. Davies, A. Dressler, S.M. Faber, R.J. Terlevich, and G. Wegner, who have found evidence for large-scale motions of nearby galaxies that cannot be explained by the expansion of the universe and that may indicate large inhomogeneities in the distribution of cosmic mass. See ďSpectroscopy and Photometry of Elliptical Galaxies.Ē V. Galaxy Streaming toward the New Supergalactic Center,Ē Astrophysical Journal, vol. 326, pg. 19 (1988).