The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos

by John Brockman

  Because Stephen had made some objections to what I said, I polished the end of the paper to respond to some of his objections. I didn’t give it to my friends to smuggle from Russia. So I sent it for publication in October. It arrived eventually at Physics Letters, but because it was delayed, it was published in ’82 instead of ’81. And I also sent lots of preprints to the United States, and one of them reached Paul Steinhardt and Andy Albrecht, who both worked on similar ideas. Three months after I sent my paper for publication, they sent for publication their own paper, with the same idea described in it, and with a reference to my work.

  It was a miracle that the government allowed me to go to Cambridge. I had visited Italy previously, but then for a while, for some reasons which were not explained to me, they were unwilling to let me go anywhere outside the Soviet Union. But that time it worked, and it was the most wonderful conference in my life. It was the first conference on inflationary cosmology attended by the best people in this area. Things really happened at this conference. It was magnificent. Three weeks of intense discussion and work together.

  The conference was in summer 1982. The whole conference was about new inflation. I gave this name to the scenario I developed. But this theory, just like the old scenario proposed by Guth, did not live long. Because of this symposium, new inflation in its original form essentially died in ’82. The theory predicted too large perturbations of density. The model required modifications, and these modifications were such that there could be no thermal equilibrium in the universe, no cosmological phase transitions, so no way to realize a scenario like Alan Guth and I envisioned. Interestingly, most of the books on astronomy still describe inflation as exponential expansion during the cosmological phase transitions; this theory was so popular that nobody even noticed that it died back in ’82. But a year later, in ’83, I invented a different scenario, which was actually much simpler. It was chaotic inflation, and it did not require the universe to be hot to start with.

  In the chaotic-inflation scenario, one could have an inflationary regime without assuming that the universe initially was hot. I abandoned the idea of the cosmological phase transitions, metastability, false vacua—most of the things that formed the basis for the old inflation model proposed by Guth and for my own new-inflationary scenario. After all of these modifications, the inflationary regime became much simpler, more general, and it could exist in a much broader class of theories. In ’86 I found that if we have inflation in the simplest chaotic inflation models, then, because of quantum fluctuations, inflation would go on forever in some parts of the universe. Alex Vilenkin found a similar effect for the new-inflation scenario. The effect that I found was very generic. I called it eternal inflation.

  What Vilenkin studied was the theory of new inflation, and in new inflation, you can start at the top of the potential energy and the field doesn’t know whether to roll down to the right or roll down to the left, so while you stay at the top you’re thinking you’ll fall down, but you can think for a long time, and during this time the expansion of the universe produces lots of volume. In chaotic inflation, where the potential energy has the simplest parabolic form—no specifically flat pieces of potential are required—you just take a model like that, and if the field is sufficiently high, there are quantum fluctuations, and the scalar field wants to go down, but quantum fluctuations sometimes throw it higher. The probability of jumping high is very small, but if you jump, you are exponentially rewarded by the creation of huge amounts of new volume of the universe. You start with a tiny part of the universe, and then it just spreads and spreads. It’s like a chain reaction. It’s called the branching diffusion process.

  This is the basic idea of eternal inflation. In the paper of ’86, where I discovered eternal chaotic inflation, I also noted that if you have eternal inflation in string theory, then the universe will be divided into an enormous number of different, exponentially large parts with different properties corresponding to a large number of different stringy vacua, and that’s an advantage. That was what later became the string theory landscape.

  I should say that one of the most important predictions of inflation was the theory of quantum fluctuations, which give rise to galaxies eventually. Just think about it. If inflation were not to produce inhomogeneities, then when it blows up and the universe becomes almost exactly homogeneous, that would be the end of the game: no galaxies, no life. We would be unable to live in an exactly uniform universe, because it would be empty.

  Fortunately, there was a way around it. Before I even introduced new inflation, I knew about the interesting work of two men from Lebedev Physical Institute, Gennady Chibisov and Slava Mukhanov. Mukhanov was younger, but he was the ideological leader in this group. By studying the Starobinsky model, which, as we now know, is a version of inflationary theory, they found that during inflation in the Starobinsky model, quantum fluctuations grow and may eventually give rise to galaxies. And we looked at them and said, “Oh, come on, guys. You cannot be right. It’s impossible, because galaxies are big classical objects and you are starting with nothing. You start with quantum fluctuations.”

  What they managed to explain to us—and this was an important ingredient that influenced everything we did later—was that these quantum fluctuations become essentially classical when the universe becomes large. They give rise to galaxy formation. Their paper of 1981 was the first paper on that subject. After that, in ’82, similar ideas were rediscovered by a group of people who were all at the Cambridge symposium. This group included Hawking, Starobinsky, Guth, Bardeen, Steinhardt, and Turner. Their ideas were developed in application to new inflation, but it all started with Chibisov and Mukhanov in ’81. And then Mukhanov continued studying it more and more, and he developed a general theory of these quantum fluctuations. From my perspective, this is one of the most important parts of inflationary theory. This is most important not only for the theory of galaxy formation. Chaotic eternal inflation would be impossible if not for these quantum fluctuations.

  Some of the most interesting recent developments of inflationary cosmology are related to string theory. My understanding of this theory is based in part on my collaboration with my wife, Renata Kallosh. She is also a professor of physics at Stanford; she studies supergravity and string theory. So let me tell you about these theories just a little.

  During the last years of his life, Einstein dreamed about a final theory which would unify symmetries of space with symmetries of elementary particles. And he failed. I was told that during the last years of his life he continued writing on the blackboard, filling it with equations of the new theory, and although he could not successfully finish it, he was still happy. Then people learned there was a no-go theorem. One just couldn’t do it, period. It was impossible to realize Einstein’s dream of a unified theory of everything. Then other people found that there was a loophole in the no-go theorem. If the theory has a special symmetry, supersymmetry, relating to each other bosons (scalar fields, photons) and fermions (quarks and leptons), then these no-go theorems go away.

  Thus, it all began with supersymmetry, and then it became a more advanced theory: supergravity. One could unify the theory of gravity with the theory of elementary particles. It was fantastic! The theory flourished in the middle of the ’70s up to the ’80s. It resolved some problems of quantum gravity. Some infinite expressions, which appeared in calculations in a quantum theory of gravity, disappeared in supergravity. Everybody was ecstatic, until the moment they found that these infinities might still appear in supergravity in the third approximation, or maybe in the eighth approximation. Something was not quite working—although some very recent results suggest that maybe people were too pessimistic at that time and some versions of the theory of supergravity are quite good.

  But at that time, they looked at it and said, “OK, it doesn’t work, there are some problems with the theory, can we do something about it?” The next step was the theory of superstrings. The development of science was not like
, “Oh, come on, we can go to the right, we can go to the left, we can go anywhere, let’s go straight.” No, it was not like that. We wanted to achieve the unity of all forces, and because of the no-go theorem there was no way to do it except by using supersymmetry. Then it becomes supergravity. You just must have it, if you want to describe curved space in supersymmetric theories. Now you have supergravity, but . . . sorry, it doesn’t quite work, you need to somehow generalize it. And then string theory was developed.

  This was like a valley in the mountains. It’s not about going to the right or to the left. Your valley shows you the best way, maybe even the only way. That’s how people came to string theory, and then they became very optimistic. This was ’85. They were thinking they would do everything pretty quickly. I must say that not everybody was so optimistic at that time. In particular, John Schwarz, one of the fathers of string theory, said, “Oh, well, it may actually take more than twenty years for string theory to come to fruition as a phenomenological theory of everything.” He made a warning. Well, enthusiasm was nevertheless overwhelming, which was good and bad. It was good because so many talented young people entered the field. It was bad because the supergravity tradition was partially forgotten. In Europe, the supergravity tradition is still alive, very much so. In the United States, it’s not that much.

  String theory is based on the idea that our universe fundamentally has more dimensions, not just four. This idea was also part of some versions of supergravity. It was also a part of Kaluza-Klein theory, a long time ago. The standard attitude was that string theory required an assumption that our space is ten-dimensional and six dimensions should be compactified. After that, we have three large dimensions of space and one of time. The other six dimensions would be very small. Superstring people often use Calabi-Yau space to describe compactification of the six extra dimensions; this space can have a very complicated topology.

  The question, though, was “How do we know that this is true?” For a long time, nobody could construct a working mechanism that would allow Calabi-Yau space to be really small. Why do we need it to be small? Because we cannot move in these six dimensions, we are too big for that. We can go to the right, to the left, and upward, but we cannot go in six other directions. At least, nobody told me that they tried, had been there.

  Six dimensions. We needed to explain why they’re small. There was a property, an unfortunate property, of string theory that if treated naïvely, without any special effort, these six dimensions actually want to decompactify—want to spread out, become large. There could be many ways of compactifying space, which is the origin of many different vacua in string theory. But nevertheless the problem was how to keep these extra dimensions small.

  The attitude was, “Oh, well, we’ll do something. We will do something.” But this “we will do something” continued for almost twenty years. Nobody took this problem too seriously, because there were lots of other problems in string theory and they thought, “Let’s just go forward.” But we needed to study these string vacua. “The vacuum” means the state that looks empty from our four-dimensional point of view, but its properties depend on the properties of the compactified Calabi-Yau space, the compactified six-dimensional space. The vacuum does not contain particles. If we add the particles, then we can have our universe. This vacuum, this place without particles, galaxies, us—what properties does this vacuum have? As I said, in order to study it consistently, we need to have stable compactification of the extra six dimensions of space. There are also other fields in this theory that need to be stabilized. People didn’t know how to do it, but for a while it was not such an urgent problem. But then, at the end of the ’90s, cosmologists discovered the exponential, accelerating expansion of our universe, which happens because of what people call dark energy—or the cosmological constant. This discovery made a very strong impact on the development of string theory.

  The rumor was that at a conference in India, Ed Witten, who is the leading authority in string theory, said that he didn’t actually know how to explain the acceleration of the universe in the context of string theory. And when Ed Witten doesn’t know something, people start taking it seriously and panicking a bit. So at that moment they started really paying attention to the properties of the vacuum in string theory. They wanted to explain this exponential expansion of the universe, which apparently started about 5 billion years ago and goes on very slowly. So people started trying to explain it, and it didn’t work. Then they started thinking even more attentively about this problem—what actually defines this vacuum state, what stabilizes it.

  In 2003, I was part of a group at Stanford—Kallosh, Kachru, Linde, and Trivedi—that proposed a possible solution of this problem. There were some earlier works, I must say, which came very close to a solution, and now there are some other ways of doing it. But this was kind of like a point of crystallization, where people realized that we could actually solve the problem and stabilize the vacuum in string theory.

  When we found a way to do it, it was immediately realized that there are exponentially many ways to do it. People who estimated the total number of different ways to stabilize the vacuum in string theory came up with astonishing numbers, like 10500. Michael Douglas and his collaborators made this estimate. And this fact has profound cosmological implications. If you marry string theory with the theory of eternal inflation, then you can have one type of vacuum in one part of the universe, another vacuum in another part of the universe, and it is possible to jump from one vacuum to another, due to quantum effects. Lenny Susskind gave this scenario a very catchy name, the string theory landscape.

  What I mean is that when we’re talking about this vacuum state, “vacuum state” means the homogeneous state describing our three dimensions—three dimensions plus one [of time]. But the remaining six dimensions, they may squeeze like this or they may squeeze like that. There are lots of different topologies. In addition to different topologies, there are different fields that can exist in this six-dimensional space—so-called fluxes.

  There are other objects that can exist there and that can determine properties of our space. In our space we do not see them; they are in this tiny six-dimensional compactified space. But they determine properties of our vacuum—in particular, the vacuum energy density. The level of this vacuum energy depends on what is going on in the compactified space. Properties of elementary-particle physics depend on what happens there. If you have many different ways of compactification, you have the same string theory fundamentally, but your world—your three-dimensional space and one dimension of time—will have completely different properties. That’s what is called the string theory landscape. You have the same string theory, but you have many different realizations of it. That is exactly what I envisioned in my paper on eternal chaotic inflation in ’86: We have lots of possibilities, and this is good.

  But in ’86 we didn’t know a single example of a stable string theory vacuum; we just expected that there should be exponentially many such vacua. In 2003, we learned how to find such vacua, and then it was realized that indeed there are lots and lots of them. So that is the present view.

  Let me say a few words about what I’m studying right now. 10500 is an abnormally large number; it tells you how many choices of vacua you have. You have this huge amount of possibilities. And by the way, there’s a question which many people ask: “How do you know?” How do we know that we have this multitude—that these other parts of the universe are somewhere inside our universe?

  This is the picture: The universe is very, very big, and it’s divided into parts. Here is one realization of the string of vacua. There, in the same universe, but far away from us, it’s a different vacuum. The guys here and there don’t know about each other, because they’re exponentially far apart. That’s important to understand in order to have a vision of the universe. It’s important that you have a choice. But if you don’t see these parts, how do you know they actually exist, and why do you care?

  U
sually I answer in the following way: If we do not have this picture, then we cannot explain the many strange coincidences that occur around us. Like why the vacuum energy is so immensely small, incredibly small. Well, that’s because we have many different vacua, and in those vacua where the vacuum energy is too large, galaxies cannot form. In those vacua where the energy density is negative, the universe rapidly collapses. And in our vacuum the energy density is just right, and that’s why we live here. That’s the anthropic principle. But you cannot use the anthropic principle if you don’t have many possibilities to choose from. That’s why the multiverse is so desirable, and that’s what I consider experimental evidence in favor of the multiverse.

  I introduced the anthropic principle in the context of the inflationary multiverse back in ’82. The idea of new inflation was proposed in ’81, and then in ’82 I wrote two papers where I emphasized the anthropic principle in the context of inflationary cosmology. I said that the universe may consist of many different exponentially large parts. I did not use the word “multiverse,” I just said that the universe may consist of many, many mini-universes with different properties, and I’ve studied this possibility since that time, for many, many years.

  But what is important is that when we studied inflationary theory, we started asking questions that seemed to be metaphysical, like why parallel lines do not intersect, why the universe is so big. And if we had said, “Oh my god, these are metaphysical questions and we should not venture into it,” then we would never have discovered the solutions. Now we’re asking metaphysical questions about the anthropic principle, about stuff like that, and many, many people tell us, “Don’t do it, this is bad, this is the ‘a’ word. You should avoid it.”