If we started in a red part of the universe, it does not stay red forever. Inflation is capable of producing and amplifying quantum fluctuations, and these quantum fluctuations let us jump from one vacuum state to another, and then to another. The universe becomes multicolored. The basic mechanism was understood as soon as the inflationary theory was invented. But its most interesting consequences appear in string theory.
A long time ago, string theorists realized that this theory allows many different vacua, so just by saying that we deal with a given version of string theory, one cannot actually predict the properties of our world, which depend on the choice of the vacuum state. Many people thought that this multiplicity of possible outcomes is a real problem with string theory. But in the context of the theory of inflationary multiverse, this is no longer a problem. In one of my papers, written in 1986, I said that it is actually a virtue of string theory. It allows creation of universes of many different types, with different laws of low-energy physics operating in each of them.
This simplifies the difficult task of explaining why the laws of physics in our part of the universe so nicely match the conditions required for our existence. Instead of the cosmological principle asserting that all parts of the world look alike, we found justification of the cosmological anthropic principle. It says that different exponentially large parts of the universe may be very different from each other, and we live only in those parts where life as we know it is possible. Thirty years ago, ideas of this type were extremely unpopular, but two decades later, when we learned more about properties of string theory vacua, the situation changed dramatically. The picture outlined above became a part of what Lenny Susskind called the string-theory landscape.
The way from the invention of inflationary theory to the string theory landscape has taken a very long time. In the beginning, it was pretty hard to work in this direction. Nobody wanted it. Nobody expected that inflationary theory, which was invented to explain the observed uniformity of the universe, would end up predicting that on a much greater scale than we can see now, the universe is 100-percent nonuniform. It was just too much. Even the simplest versions of inflationary theory seemed a little bit too revolutionary, providing a possibility of creating everything from practically nothing. It seemed like science fiction; it was too bold, too exotic.
For example, I found that in the first model of new inflation, which I invented back in 1981, the universe could expand 10800 times during the inflationary stage. It was surreal; we had never seen numbers like that in physics. When I was giving my first talks on new inflation at Lebedev Physical Institute, where I invented this theory, I had to apologize all the time, saying that 10800 was way too much. “Probably later,” I said, “we’ll come to something more realistic, the numbers will decrease and everything will become smaller.” But then I invented a better inflationary theory, the theory of chaotic inflation, and the number became 101000000000000. And then I found that inflation in this theory may continue eternally.
Inflationary theory explained many properties of our world, which we could not understand without it, and it also predicted many things that were confirmed experimentally. For example, quantum fluctuations produced during inflation, if the theory is right, explain formation of galaxies of the same type as our galaxy.
Secondly, there is the cosmic microwave background (CMB), which reaches our Earth from all parts of the sky. It has very interesting properties. When this radiation was discovered, almost fifty years ago, its temperature was measured, 2.7˚ Kelvin: very, very small. It comes to us from all parts of the universe in a uniform way. Then people looked at it more carefully, and they found that its temperature is 2.7˚ Kelvin plus 10-3˚ Kelvin to the right of you, and 2.7˚ minus 10-3˚ Kelvin to the left of you—a strange thing. But then an interpretation came. We are moving with respect to the universe, and there’s a redshift. One part of the sky, in the direction of our motion, seems a little bit warmer, and another part of the sky seems a little bit cooler, because of the redshift, and that’s OK.
Then people measured the temperature of the cosmic microwave radiation with an accuracy of about 10-5˚ Kelvin. That’s what the COBE satellite did, and that’s what the WMAP satellite is doing right now, and many other experiments. And they have found many tiny spots in the sky—some a little bit warmer, some a little bit colder. They classified these spots, studied their distribution, and found that the distribution of spots was in a total agreement with the predictions of the simplest versions of inflationary theory. That was quite an unexpected confirmation. Theoretically, we knew that it should be there. What was incredible is that people who make observations could achieve this absolutely astonishing accuracy. It’s just amazing.
Nobel Prizes are awarded for definitely established facts. Observers found fluctuations in the CMB, but one may wonder whether they were produced by inflation or by some other mechanism. So far, from my point of view, there are no other fully developed theories that would explain these observations. Who knows, maybe ten years from now, somebody will come up with something as smart as inflation. However, many people already tried to do it during the last thirty years, but no other theory comparable to inflation has been invented yet.
Let me tell you a little bit about how inflation was invented, from my perspective. Because things look very, very different when you look at it from a Russian perspective than from an American perspective, simply because at the time inflation theory was developed, Russia and America were not closely connected; every letter from Russia would come to the United States with an interval of about one-and-a-half or two months. You know, in the ’30s there was the record flight from Russia landing in the United States, and it took almost a day, and then fifty years later a letter from Russia to the United States would take maybe forty to sixty days, so that was the progress achieved. Whatever.
I was educated at Moscow State University. I graduated, and then became a postdoc at Lebedev Physical Institute, where I worked with David Kirzhnits, an expert in condensed matter physics, in astrophysics, in quantum field theory, and nonlocal theories, and everything else. He was a really amazing person. He discovered the theory of cosmological phase transitions, and I further developed it. So one of the things that became a basis for many inflationary models was that in the early universe, physics could be completely different, and there may have been no difference between the weak, strong, and electromagnetic interactions.
Then for a while we thought that maybe this was just an exotic idea. I mean, not exotic—we knew it was true, but we were afraid it was untestable, like a Platonic idea, of which we would never know any consequences. But we were overly pessimistic. Some consequences of what we studied included the creation of strange objects in the early universe called primordial monopoles. If the theory we developed was right, and if unified theories of weak, strong, and electromagnetic interactions proposed in the ’70s were right—and at that time everybody thought they were right—then in the process of the cosmological phase transitions in the very early universe, when the whole world cooled down from its original hot state—which is what everybody believed it was—the theory predicted creation of some strange objects that look like separate south and north magnetic poles.
If you cut a magnet into two pieces, each of its parts will always have both a south pole and a north pole; you cannot have a south pole or a north pole separately. But according to these unified theories, there would also be objects in the universe called monopoles, each of them having either a south pole or a north pole. And each of them would be a million billion times heavier than the proton. People then calculated how many there should be, and the answer was that the total number of monopoles right now should be approximately the same as the number of protons, which would make our universe a million billion times heavier than it is now. Such a universe would be closed, and it would have collapsed already, in the first seconds of its evolution. Since we’re still around, there’s something wrong with this theory. So this was the
primordial-monopole problem. It was a consequence of the theory of cosmological phase transitions we developed.
Many people were trying to resolve this problem, and they could not. Somehow, consistent solutions would not appear. Then, in parallel with these studies, we learned about a new cosmological theory proposed by Alexei Starobinsky in Russia, in ’79 and ’80. It was a rather exotic model where quantum corrections in quantum gravity produced the state of exponential expansion of the universe. This was a very interesting scenario, very similar to the inflation. But nobody called it inflationary theory, because it was proposed a year before Alan Guth proposed his “old inflation.” It was called the Starobinsky model. It was the number-one subject of all discussions and debates at all cosmology conferences in Russia. But it didn’t propagate to the United States, and one of the reasons it didn’t was that the goal of Starobinsky was to solve the singularity problem and the model did not quite solve it, and he did not attempt to solve other problems, which were addressed by inflationary cosmology. In fact, he assumed that the universe was homogeneous from the very beginning; he did not try to explain why it was homogeneous. Nevertheless, when we look back at the history of inflationary models, formally, this model had all the features of successful inflationary models, except for the problem of the beginning of an inflationary stage. And the motivation for this [Starobinsky] model was rather obscure. But it worked, or almost worked.
Then Alan Guth formulated his model. But I didn’t know about it. I attended a seminar at the Institute of Nuclear Research in Moscow, sometime in ’80. At that seminar, Valery Rubakov, one of the famous Russian scientists who still lives and works there, discussed the possibility of solving the flatness and homogeneity problems by exponential expansion of the universe due to cosmological phase transitions in the so-called Coleman-Weinberg model. They were discussing all of these things without knowing anything about the paper by Alan Guth. They explained the problems, and they also explained why their model didn’t solve these problems. Then they sent the paper for publication, but it was not accepted, because by that time the paper by Alan Guth had already come out.
Thus I learned about the idea of how one could solve many different cosmological problems during exponential expansion in the false vacuum, but I learned it not from Alan’s paper. It happened quite a while before Alan’s paper appeared in the Soviet Union, and the delay was just this mail delay. Moreover, I had known that one can have exponential expansion in the false vacuum since 1978, when we worked on it with Gennady Chibisov, but we found that this model didn’t work, and so we didn’t continue studying it; we didn’t know that it might be useful for solving many cosmological problems until Rubakov and his collaborators told us about it.
When Alan’s preprint arrived in Russia, Lev Okun, one of the famous Russian scientists, called me and asked: “You know, I heard something about the paper by Alan Guth, who is trying to solve the flatness problem by exponential expansion of the universe. Have you heard about it, anything?” And I told him, “No, I have not heard anything about it, but let me tell you how it works, and let me tell you why it doesn’t work, in fact.” So, for half an hour I explained to him Alan Guth’s paper, although I did not see it at that time, and explained to him why it didn’t work. And then, after a while, I received Alan Guth’s paper. We had been in correspondence with Alan some time before that, to discuss the cosmological phase transitions, the expansion of the universe, and bubbles. But we did not discuss the possibility of using it for solving cosmological problems.
So that was it. If I had wanted to write a paper about that, I could have done it immediately after Okun called me, but it would have been completely dishonest, so I didn’t even think about it, but of course I knew the basic ideas and I knew why they wouldn’t work. So I didn’t do anything. In fact, I was in a pretty depressed state, because the idea, obviously, was very beautiful and the idea that I’d learned from Rubakov was very, very beautiful. And it was a real pain that one could not make it work. The problems were extremely difficult.
A year later, Alan Guth, together with Erick Weinberg, wrote a seventy-page-long paper proving that it was impossible to improve Alan’s model. Fortunately, I received it after I had already improved it—again, many thanks to the slow mail from the United States to Russia. While they were working on the paper, I was working on a solution. I found the solution somewhere in the summer of ’81. In order to check whether this solution was obvious or not, I called Rubakov to check with him, because he had first introduced me to this set of ideas. In fact, my solution was so obvious that when it occurs to you, it seems simple. You cannot understand why you didn’t think of it before.
I called him late in the evening. Because at that time my wife and kids were sleeping, I took the phone to our bathroom, and I was sitting on the floor there dialing him, checking with him, and he told me, “No, I haven’t heard about it.” So at that time I woke up my wife and I said, “You know, I think I know how the universe was created.” And then I wrote—this was the summer of ’81—I wrote a paper about it. But it took about three months to get permission for its publication.
At that time in Russia, if you wanted to send a paper in for publication abroad, first of all you had to do a lot of bureaucratic work in your institute. You typed it in Russian and got lots of signatures, then you sent it to the Academy of Sciences of the USSR, and they would send it to some other place that checked whether it was possible to publish it, whether it contained some important secrets which should remain that way. Then you received it back, typed it once in English, for a preprint, and typed it the second time, the same thing—no Xeroxes. And only then could you send it for publication. The whole procedure usually took two months, sometimes three months. I got permission only in October. But in October there was a conference in Moscow, on quantum cosmology. And the best people came. Stephen Hawking came, many other good people came, and I gave a talk on this. And people were kind of excited, really excited about this, and they immediately offered their help to smuggle it from Russia as a preprint, and they would send it for publication, permitted or not. You know, friends, OK? Good friends can do it for you. But then there was an unexpected complication.
The morning after I gave the talk at this conference, I found myself at the talk. . . . Oh my god, this is going to be a funny story! I found myself at the talk by Stephen Hawking at the Sternberg Institute of Astronomy in Moscow University. I came there by chance, because I had heard from somebody that Hawking was giving a talk there. And they asked me to translate. I was surprised. OK, I will do it. Usually, at that time, Stephen would give his talk well prepared, which means his student would deliver the talk and Stephen from time to time would say something, and then the student would stop and change his presentation and say something else. So Stephen Hawking would correct and guide the student. But in this case they were completely unprepared. The talk was about inflation. The talk was about the impossibility of improving Alan Guth’s inflationary theory.
So they were unprepared; they had just finished their own paper on it. As a result, Stephen would say one word, his student would say one word, and then they waited until Stephen would say another word, and I would translate this word. And all of these people in the auditorium, the best scientists in Russia, were waiting and asking: “What is going on? What is it all about?” So I decided let’s just do it, because I knew what it was all about. So Stephen would say one word, the student would say one word, and then, after that, I would talk for five minutes, explaining what they were trying to say.
For about half an hour, we were talking this way and explaining to everyone why it was impossible to improve Alan Guth’s inflationary model and what are the problems with it. And then Stephen said something and his student said, “Andrei Linde recently proposed a way to overcome this difficulty.” I didn’t expect that, and I happily translated it into Russian. And then Stephen said, “But this suggestion is wrong.” And I translated it. . . . For half an hour, I was tran
slating what Stephen said, explaining in great detail why what I’m doing is totally wrong. And it was all happening in front of the best physicists in Moscow, and my future in physics depended on them. I’ve never been in a more embarrassing situation in my life.
Then the talk was over, and I said, “I translated, but I disagree,” and I explained why. And then I asked Stephen, “Would you like me to explain it to you in greater detail?” and he said, “Yeah.” And then he rode out from this place and we found some room, and for about two or three hours all the people in Sternberg Institute were in a panic, because the famous British scientist had just disappeared, nobody knew where to.
During that time, I was near the blackboard, explaining what was going on there. From time to time, Stephen would say something, and his student would translate: “But you didn’t say that before.” Then I would continue, and Stephen would again say something, and his student would say again the same words, “But you didn’t say that before.” And after we finished, I jumped into his car and they brought me to their hotel. We continued the discussion, which ended by him showing me photographs of his family, and we became friends. He later invited me to a conference in Cambridge, England, which was specifically dedicated to inflationary theory. So that’s how it all started. It was pretty dramatic.
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