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Authors: Alex Vilenkin
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The next act of the drama was set in the medieval university town of Cambridge. By invitation of Stephen Hawking, some thirty cosmologists from all over the world converged there in the summer of 1982. They gathered for a three-week workshop on the very early universe, funded by the Nuffield Foundation. I was thrilled to be among the participants: Hawking asked me to talk about my recent work on cosmic strings.
I instantly fell in love with Cambridge. Early in the morning I would get up to stroll through the grounds of old colleges. Gothic chapels, clock towers, austere walled courts with their perfect rectangular lawns and bright splashes of flowers—vestiges of another, more contemplative age. By nine o’clock I would be back to modernity, sitting in the conference hall and waiting for the talks to begin. Thankfully, there were only two talks a day, one in the morning and one in the afternoon, with plenty of time left for informal discussions. British food was not among the highlights of the trip, but British beer is quite a different story, and I spent many evening hours discussing physics and other matters over a pint of lager.
The program of the meeting emphasized recent developments in cosmology, and inevitably the theory of inflation took the center stage. The
graceful exit problem was now out of the way, but still there was another major concern.
graceful exit problem was now out of the way, but still there was another major concern.
It is true that inflation makes the universe flat and smooth, but perhaps it does too good a job of that. No galaxies or stars would ever form in a perfectly homogeneous universe. As we discussed in Chapter 4, galaxies have evolved from small variations in the density. The origin of these primordial inhomogeneities, or density perturbations, became the central issue of the workshop.
Shortly before the meeting, Hawking wrote a paper with a very interesting idea. According to the quantum theory, the evolution of all physical systems is not entirely deterministic, but is subject to unpredictable quantum jerks. So, as the scalar field rolls downhill, it experiences random kicks back and forth. The directions of the kicks are not the same in different regions of the universe, and as a result, the scalar field arrives at the bottom of the hill at slightly different times in different places. In regions where inflation lasted a little longer, the matter density would be slightly higher.
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Hawking’s idea was that the resulting small inhomogeneities led to the formation of galaxies and galaxy clusters. If he was right, then quantum effects, which are normally important on tiny, subatomic scales, were responsible for the existence of the largest structures in the universe!
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Hawking’s idea was that the resulting small inhomogeneities led to the formation of galaxies and galaxy clusters. If he was right, then quantum effects, which are normally important on tiny, subatomic scales, were responsible for the existence of the largest structures in the universe!
Naturally, Guth was very excited by this development. Not only did it resolve the difficulty of the theory, but also it opened the tantalizing possibility of testing inflation observationally. Density perturbations can be observed through cosmic microwaves and then compared with the predictions of the theory. This was tremendously important!
The calculation of density inhomogeneities produced during inflation is a very challenging technical problem. Hawking’s paper gave very few details and was difficult to follow. So Guth joined forces with a Korean-born physicist, So-Young Pi, to work out the perturbations using a method that they could both understand. They were not quite done when Guth had to leave for the Nuffield Workshop, and he finished the calculation during his first days in Cambridge. To his great surprise, the result was very different from Hawking’s. They both found that the perturbations depended on the
form of the scalar field energy landscape. But the dependence was different, and Guth’s answer gave a much larger magnitude for the perturbations.
form of the scalar field energy landscape. But the dependence was different, and Guth’s answer gave a much larger magnitude for the perturbations.
Guth discussed the matter with Hawking, but the difference remained unresolved. Hawking insisted on his result. When Guth told me about their conversation over lunch, he looked puzzled. He was not sure his answer was correct and said he would have to recheck several points in the calculation.
To add to the confusion, there was yet another group working on the same problem. Paul Steinhardt had calculated the inhomogeneities in collaboration with two other American cosmologists, Jim Bardeen and Michael Turner. They also disagreed with Hawking, but their answer was much smaller! Finally, there was a Russian physicist, Alexei Starobinsky, who was also scheduled to talk on the subject of density perturbations. But he kept to himself, and nobody knew what result he was going to announce.
Starobinsky was not a novice to cosmology. Among other things, he was known for inventing a version of inflation about a year earlier than Guth. The rub was that he invented it for the wrong reason. He thought his model could remove the initial singularity—which it could not. But he did not realize that it could solve the horizon and flatness problems. Without this crucial insight, the model did not get much notice at the time, but now it is regarded as a viable alternative to the scalar field models of Linde, Albrecht, and Steinhardt.
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Starobinsky was scheduled to speak first. His style of presentation was typical of the Russian school of physics and could be traced back to one of its originators, the Nobel Prize laureate Lev Landau. At Landau’s famous weekly seminar, the speaker was presumed to be an idiot and had a narrow window of opportunity to prove otherwise at the beginning of the talk. So the seminars were given mainly “for Landau,” to convince him that the speaker knew what he was talking about, and without undue concern that the talk might go above the heads of almost everybody else. Now, add to this a Russian accent and a strong stutter, and you will not be surprised that Starobinsky’s talk was not easy to follow. Yet, by the time he was finished, one thing was clear: he had found the inhomogeneities to be large, pretty close to Guth’s result.
The next day it was Hawking’s turn to speak. The legendary physicist suffers from Lou Gehrig’s disease and has been wheelchair-bound since the early 1970s. He now communicates through a voice synthesizer, selecting
words one by one from a menu on a computer screen. At the time of the meeting he could still speak, but barely. Most people could not understand him, and one of his students served as an interpreter during the talk. Hawking’s lecture followed the line of argument in his paper, but at the end there was a surprise. The last step of the calculation was now different, and the result was the same as found by Guth and Starobinsky! After talking to Guth and hearing Starobinsky’s lecture, Hawking must have spotted an error in his calculation. He never mentioned, though, that he was correcting an error in his paper, or that his new result was also derived by Starobinsky and Guth.
words one by one from a menu on a computer screen. At the time of the meeting he could still speak, but barely. Most people could not understand him, and one of his students served as an interpreter during the talk. Hawking’s lecture followed the line of argument in his paper, but at the end there was a surprise. The last step of the calculation was now different, and the result was the same as found by Guth and Starobinsky! After talking to Guth and hearing Starobinsky’s lecture, Hawking must have spotted an error in his calculation. He never mentioned, though, that he was correcting an error in his paper, or that his new result was also derived by Starobinsky and Guth.
The majority of the talks at the Nuffield Workshop were on the subject of inflation, and despite much excitement about the new theory, it was a bit of an overdose. The talks on other early-universe topics provided a welcome relief—the sentiment I tried to express in the opening slide of my lecture on cosmic strings (
Figure 6.6
). Strings are line-like relics of the hot, high-energy epoch in the early universe. They are thin tubes of false vacuum, which are predicted in some particle physics models. In my talk I discussed the formation of strings and their possible astrophysical effects. The talk was well received, and I could now sit back, relax, and watch the final stretch of the race to figure out the density perturbations.
Figure 6.6
). Strings are line-like relics of the hot, high-energy epoch in the early universe. They are thin tubes of false vacuum, which are predicted in some particle physics models. In my talk I discussed the formation of strings and their possible astrophysical effects. The talk was well received, and I could now sit back, relax, and watch the final stretch of the race to figure out the density perturbations.
Figure 6.6
.
Inflation overdose—the opening slide of my talk on cosmic strings.
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Inflation overdose—the opening slide of my talk on cosmic strings.
Steinhardt and his friends were still holding out. They were concerned about some subtle points in their calculation and kept working furiously to
clear them up. The answer they were getting was still much smaller than Hawking’s original result.
clear them up. The answer they were getting was still much smaller than Hawking’s original result.
Guth was scheduled to speak during the third week of the meeting. He worried that Steinhardt and company might give him a hard time and used every opportunity to retreat into his room and check various parts of his calculation. He later realized that he had even missed the conference banquet while preparing for his talk.
Despite mounting tension, the battle was not to happen. A few days before the talk, Steinhardt and his collaborators conceded defeat. They found some errors in the approximations that they had used, and now their result was in agreement with the other contestants’. Guth’s talk went very smoothly: he reiterated the original result that he had obtained earlier. Thus, by the end of the workshop, all four participating teams had reached a full consensus.
The final surprise of this remarkable race came long after the workshop was over. Much to their dismay, the former contestants discovered that the problem of quantum-induced density perturbations that they worked so hard to untangle had already been solved—a full year before they crossed swords in Cambridge. The solution was published by two Russian physicists, Slava Mukhanov
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and Gennady Chibisov, from the Lebedev Institute in Moscow.
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They worked out perturbations for the Starobinsky version of inflation, but the calculation was essentially the same as for the scalar field models. You can often find something interesting by reading Russian physics journals!
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and Gennady Chibisov, from the Lebedev Institute in Moscow.
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They worked out perturbations for the Starobinsky version of inflation, but the calculation was essentially the same as for the scalar field models. You can often find something interesting by reading Russian physics journals!
The end point of the calculations was a formula for the magnitude of density perturbations produced by quantum jitters of the scalar field as it rolls downhill during inflation. This magnitude depends on the shape of the energy landscape and also on the size of the region where the perturbation occurs. Cosmic structures span a wide range of distance scales. The scale of stars is much smaller than that of galaxies, which is in turn smaller than the scale of galaxy clusters. The magnitude of perturbations on these vastly different scales could well be very different. But the formula says that all perturbations
are created very nearly equal. From the smallest cosmic structures to the largest, their magnitude changes by no more than 30 percent.
are created very nearly equal. From the smallest cosmic structures to the largest, their magnitude changes by no more than 30 percent.
This property of scale-independence of the inflationary perturbations is not difficult to understand. The quantum kicks initially affect the scalar field in a tiny region of space, but then the perturbation is stretched to a much greater size by the exponential expansion of the universe. Perturbations produced earlier during inflation are stretched for a longer time and encompass a larger region. But the magnitude of the perturbation is set by the initial quantum kick, which is pretty much the same for all relevant scales.
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Scale-independence of the density perturbations can be used to derive predictions for variations in the intensity of cosmic microwaves over the sky and, ultimately, to test inflation. A speculative hypothesis about the early moments of the universe has thus been transformed into a testable physical theory. But it took another decade before the theory of inflation was put to the test.
It usually takes years, if not decades, for a new theory to be widely accepted. Physicists may appreciate a beautiful idea, but they will only be convinced when predictions of the theory are confirmed by experiments or by astronomical observations. This is particularly true in cosmology, where observers have always had a hard time keeping up with the imagination of the theorists, and the big bang theory is as good an example as any. The papers by Alexander Friedmann remained unnoticed until after his death, and the work of George Gamow was all but ignored for more than a decade. What a contrast to how inflation was received!
Nearly forty papers were published on the new theory in the first year after Guth’s original paper. In a couple of years, this number climbed to two hundred and remained more or less steady at about two hundred papers a year for the following decade. It looked as if people dropped whatever they were doing and started working on the theory of inflation.
Why was inflation such an instant success? In part, this was due to sociological
reasons. Particle physicists had just finished developing theories of strong and electroweak interactions. There was a small army of them, and suddenly they found themselves with little to do. New ideas in particle physics were all related to extremely high energies. There was no way to test these theories in the existing particle accelerators, so progress had stalled. The only accelerator that could boost particles to the required energies appeared to be the big bang, and particle physicists were increasingly turning their sights to cosmology as a testing ground for new ideas. By the early 1980s, a mass conversion was under way from particle physics to cosmology. The converts were new to the field and were looking for interesting problems to solve.
reasons. Particle physicists had just finished developing theories of strong and electroweak interactions. There was a small army of them, and suddenly they found themselves with little to do. New ideas in particle physics were all related to extremely high energies. There was no way to test these theories in the existing particle accelerators, so progress had stalled. The only accelerator that could boost particles to the required energies appeared to be the big bang, and particle physicists were increasingly turning their sights to cosmology as a testing ground for new ideas. By the early 1980s, a mass conversion was under way from particle physics to cosmology. The converts were new to the field and were looking for interesting problems to solve.
It was on this background that Guth suggested his idea of inflation. He gave physicists exactly what they were looking for. It really helped that Guth’s theory was incomplete. If you fully solve an important problem, your work may be admired, but you do not create an industry. Inflation, on the other hand, was just an outline of a theory, with many blanks to be filled. It offered plenty of problems to work on and to give to your graduate students.
But, apart from sociology, the long-term popularity of inflation is due to the appeal and the power of the idea itself. In some ways, inflation is similar to Darwin’s theory of evolution. Both theories proposed an explanation for something that was previously believed to be impossible to explain. The realm of scientific inquiry was thus substantially expanded. In both cases, the explanation was very compelling, and no plausible alternatives have ever been suggested.
Another parallel with Darwin is that the idea of inflation was already in the air at the time when Guth proposed it.
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Guth’s key contribution was that he clearly realized what inflation was good for, providing the motivation to solve the graceful exit and other problems of inflation.
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Guth’s key contribution was that he clearly realized what inflation was good for, providing the motivation to solve the graceful exit and other problems of inflation.
BOOK: Many Worlds in One: The Search for Other Universes
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