The 4-Percent Universe

by Richard Panek

  Turner talked it over with his doctoral thesis advisor at Stanford, the physicist Robert Wagoner. "That early-universe cosmology stuff? Don't do that," Wagoner told him. Wagoner himself had participated in the Big Bang revolution. As a postdoc at Stanford in the two years immediately following the discovery of the cosmic microwave background, he had worked on the same kind of primordial particle physics that Schramm had adopted in the following decade. But he had a point. By the late 1970s, the Big Bang bandwagon had stalled. It lacked the one thing that could save it from swinging back to the voodoo side of the metaphysics-physics continuum, the one thing any theory needs to be scientific: a prediction to verify or falsify.

  Whatever Feynman's liabilities as an advisor, he had taught Turner this lesson: Don't try to solve a problem until you think you have the answer. That approach was the opposite of how particle physics usually worked. In particle physics, the math came first. The math told you that a particle should exist, and that you could create that hypothetical particle from existing particles. Then you (and a thousand colleagues) commandeered an accelerator and smashed those existing particles together at velocities approaching the speed of light and waited for the hypothetical particle to pop into existence.

  Nothing wrong with that approach. It worked.

  But Feynman had taught Turner that sometimes you didn't need to do the math first. Instead, you needed to trust your intuition. To leap to a conclusion first. To imagine what the universe might be, and then go back and do the math until, with luck, it matched.

  To imagine what your life might be, and then go back and do the work until, with luck, it matched.

  "Don't do that early-universe stuff," said his thesis advisor at Stanford. "Come to Chicago and do great things!" boomed Schramm.

  Schramm made you think you could do great things. He was, in a way, the embodiment of cosmology: big and bold and fearless. His colleagues called him Schrambo or Big Dave. At 6 feet 4 inches and 230 pounds, he had the build of a former wrestler (which he was) and the bearing of an amateur pilot (ditto): king and conqueror of all he surveyed. When he decided to pursue the physics of the early universe during a period when the Big Bang was unfashionable, he not only made no apologies but claimed the field as his own. Big Bang Aviation, he named the corporation that controlled his private plane, of which he was sole proprietor. big bang read the vanity plates on his red Porsche.

  Turner might not have responded to cosmology or particle physics in isolation, but the combination proved irresistible—a balance of the loud, wildly speculative, and the quiet, "neat and simple." In particle astrophysics, Turner could reconcile the two dominant tendencies of his life. The bohemian who dropped out, the intellectual who crept back. The incautious and the careful.

  So Michael Turner would go to Chicago. And he might even get to do great things—just as long as cosmology came up with a prediction.

  In October 1981, Golden Tickets appeared in the mailboxes of cosmologists around the world, only the wonderland they would be entering at the appointed day and hour wasn't Willy Wonka's Chocolate Factory but Stephen Hawking's Nuffield workshop. The Nuffield Foundation, a charitable trust, had agreed to endow an annual workshop for three years. In the second year, Hawking and Gary W. Gibbons, also at Cambridge, decided to consolidate the remaining funds and go all out: an assault on the farthest frontier of cosmology, the "very early Universe," which the invitation defined as "< 1 sec."

  Among the three dozen or so theorists who received the letter was Turner. He figured that Hawking and Gibbons had known to invite him because one of his colleagues at the University of Chicago was one of Hawking's frequent collaborators. Not that there were all that many theorists working this particular corner of the cosmological landscape. And of course a good workshop should have a fair number of young minds to ward against the calcification of old ideas and received wisdom. But Turner would have to earn his ticket, too. He would be one of a handful of attendees who would be not just giving a talk but writing a paper.

  On his arrival at Cambridge on the first day of summer 1982, Turner presented the preliminary draft of his paper to Hawking. Hawking nodded his thanks, then motioned to an assistant, who presented Turner with Hawking's paper. A couple of other papers were circulating as well. The time had come to confront a question that had been haunting cosmology right from the day that Einstein extended general relativity to the universe: Why was the universe simple?

  As the letter from Hawking and Gibbons had said, Big Bang cosmology "assumes certain initial conditions." Those assumptions, however, were notoriously ad hoc, from the Latin for for this. As in: For this purpose—the creation of a cosmological model from the general theory of relativity—Einstein assumed homogeneity, that the universe looked the same on the largest scale. For this purpose—the creation of a cosmological model that wasn't static—other theorists had added the assumption of isotropy, that the universe looked the same in every direction.

  And the universe did seem to be homogeneous and isotropic. The discovery of the cosmic microwave background seventeen years earlier had satisfied most cosmologists that they now had the answer to the question of whether the universe was simple: Yes. On the largest scale it would look the same no matter where you were in it. And they had answered the question of how simple the universe was: Very. The cosmic microwave background was extremely smooth, just as theory had predicted.

  But assuming that something is the way it is—even if those assumptions turn out to be correct, as the Big Bang theory's apparently were—is no substitute for understanding how it got that way. Why would a universe be, of all things a universe could be, simple—and not just simple, but so simple? On reflection, maybe the answer to the question of how simple the universe was shouldn't have been the satisfying "Very" but a suspect "Too."

  Now, however, cosmology had a possible answer to the question of how the universe became so simple. Late in the evening of December 6, 1979, a no-longer-young academic with a boyish mop of hair, a boyish smile, and a grown-up worry about meeting the monthly rent sat down at the desk in his study, as he often did at that hour of the day. On this occasion, however, Alan Guth received a midnight visit from the Muse of Math. The next morning he bicycled to his office at the Stanford Linear Accelerator Center (in the process establishing a new personal best of nine minutes and thirty-two seconds) and immediately sat down with his notebook to summarize his long night's work.

  "spectacular realization," he wrote near the top of a fresh page, and then he did something he'd never done before with a notebook entry. He drew two boxes around it.

  By the time of the Nuffield workshop, two and a half years later, the story was already the stuff of scientific legend. Guth had experienced a genuine "Eureka!" moment. His was the kind of insight that causes colleagues to slap their foreheads and groan, "Of course!" The day after Guth gave his first seminar on his spectacular realization, in January 1980, he received calls from seven institutions either inviting him to give the same seminar or asking if he would consider a faculty position. By then Guth had given his idea the name that stuck: inflation, a pun that accommodated both the defining physical property of his discovery and the dominant economic worry of the era.*

  According to his calculations, the universe had gone through a monumental expansion in its first moment of existence. At the age of a trillionth of a trillionth of a trillionth of one second—or 1/1,000, 000,000,000,000,000,000,000,000,000,000,000th of a second—the universe had expanded ten septillion-fold—or to 10,000,000, 000,000,000,000,000,000 times its previous size.

  The suggestion followed an idea that another physicist, Edward P. Tryon, had put forward several years earlier, in a 1973 article in Nature. Like Gamow with "Rotating Universe?"—the Nature article that had partly inspired Vera Rubin's master's thesis—Tryon couched the counterintuitive in the form of a question: "Is the Universe a Quantum Fluctuation?" According to the laws of quantum mechanics, virtual particles can arise out of the emptiness of space—and
actually do, as experiments since the middle of the century had shown again and again. Tryon wondered if the universe might be the result of one such quantum pop.

  The argument became less sensational if you kept in mind that in quantum theory everything was a matter of probabilities. Therefore, anything was possible. Perhaps specific events were vanishingly unlikely—the creation of a universe from the nothingness of the vacuum, for instance. But they weren't impossible. And over the course of eternity, why shouldn't one or another of those vanishingly unlikely events come to pass? The universe, Tryon wrote, "is simply one of those things which happen from time to time." Or, as Guth liked to say, "the universe is the ultimate free lunch."

  The problem with Tryon's idea was that it couldn't account for the size of our universe. Inflation, however, could. Guth realized that the infant universe could have gone through a process that physicists call a "phase transition" and everybody else calls "the thing that happens when water turns into ice or vice versa." When the temperature of water changes, the transformation doesn't happen all at once. It's not as if the word goes out and suddenly every molecule of H2O in the lake has melted into liquid or hardened into ice. Instead, the transformation happens piecemeal. Even within small sections of the pond the ice isn't freezing or melting uniformly. Cracks and fissures appear faintly, then harden, leaving a veined appearance. Guth found that if you apply that transformation mathematically to the conditions of the early universe, the phase transition would have produced a temporary vacuum. That vacuum, in turn, would have produced a negative pressure—a strong gravitational repulsion—that would have expanded space exponentially. The universe would have doubled in size, then doubled in size again, then doubled in size yet again. It would have done this at least a hundred times, and it would have done so over the course of 10−35 seconds (or 1/1035). After that, the vacuum would have decayed, the exponential expansion would have stopped, and the standard expansion of the universe—the one in the Big Bang theory that we can see for ourselves in the redshifting of the light from distant galaxies—would have begun.

  At once Guth recalled a lecture by Bob Dicke that he had attended the previous year, one of a series that Dicke and Peebles had been delivering on a topic they called the "flatness problem." They would explain to their audiences that the fate of the universe depended on how much matter was in the universe: enough to reverse the expansion, not enough, or just right. The designation that scientists had given to the measure determining the fate of the universe was, aptly, the final letter in the Greek alphabet, omega. If the universe contained half the mass necessary to halt the expansion, then you would say omega equaled 0.5, or if it contained three-quarters of the necessary mass, you would say omega equaled 0.75. If the universe contained more than enough mass to halt the expansion, then omega equaled more than 1—1.5 times, maybe, or 2 times, or 100 times. And if the universe contained just the right amount—precisely the critical density to stop the universe from expanding but keep it from collapsing back on itself—then omega equaled 1.

  Astronomers would even be able to measure omega, if they had a standard candle that they could trace far enough across the universe. But you might not need observations to know omega, Dicke argued. Theory alone might be enough.

  According to Dicke, any significant deviation from 1 in the earliest universe would have led, effectively and almost immediately, to the end of the universe: either an exponential expansion toward infinity or a collapse. Calculating backward, the closer and closer you got to the Big Bang, the closer and closer omega must have been to 1. At three minutes after the Big Bang, omega would have been within a hundred-trillionth of 1. At one second after the Big Bang, omega would have been within a quadrillionth of 1—that is, between 0.999999999999999 and 1.000000000000001. The earlier in the universe you calculated, the more decimal places you added. At some point in the calculations you simply conceded: Omega as good as equaled 1.

  And if omega equaled 1 then, it had to equal 1 now, because the value of omega depended on the measure of matter, and whatever matter the universe had then, it would have now and forever.

  But for Big Bang theorists like Dicke and Peebles, a flat universe posed a problem similar to the one Newton and Einstein faced: Why would a universe that was full of matter not be collapsing through the effects of gravity? Newton had to invoke a universe of evenly spaced stars—plus God. Einstein had to invoke a universe of randomly spaced stars—plus lambda. Evidence for an expanding universe had allowed Einstein to abandon lambda and prompted future generations to try to figure out how to measure the rate at which the expansion was slowing. But now Dicke and Peebles were arguing that in a Big Bang universe, omega had to equal 1. The expansion had to slow to a virtual stop and stay there forever. All the matter in the universe had to reach a state of gravitational equilibrium—an eventuality with the same likelihood as a pencil standing on its point forever. Not impossible, according to the laws of classical physics, but not likely either.

  But that December night in 1979, Guth realized that if inflation did occur and the entire universe was actually ten septillion times the volume of what we see, then what was flat wasn't the universe but only our part of it—the part we'd always assumed to be the universe in its entirety. Our part of the universe would appear flat to us, just as a football field appears flat even though the Earth is round. The universe as a whole could have any value of omega; the universe that we see, however, has a value close enough to 1 that, for all practical purposes, it may as well be 1.

  So much for the flatness problem.

  A few weeks after inventing inflation, Guth was listening to some colleagues at lunch and learned about another apparent contradiction in a homogeneous and isotropic universe, the "horizon problem." Look into the universe in one direction, then look into the universe in the opposite direction. This is essentially what antennas measuring the cosmic microwave background do. The light from one direction will just be reaching you, and the light from the other direction will just be reaching you, but the light from the first source will not yet have had time to reach the second source, and vice versa. Yet the cosmic microwave background reveals a similarity in temperature to within one part in 100,000. How did one part of the universe "know" the temperature of the other and match it if the two had never "communicated" with each other?

  "Yeah," Guth thought, "inflation could solve that, too." If inflation did occur, then two distant parts of the universe would have been in contact with each other when the universe was less than 10 seconds old. Guth thought a little more. Then he told himself, "This really might be a good idea after all."

  Guth's paper "The Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems" appeared in early 1981. While the Nuffield workshop was officially called "The Very Early Universe," it quickly became a referendum on inflation. Seventeen of the thirty-six sessions addressed the topic directly, and many of the others mentioned it.

  The question wasn't so much whether inflation made sense. Inflation explained two ad hoc assumptions—homogeneity and isotropy. It solved two problems—flatness and the horizon. It was too good not to be true—or at least that's how many of the theorists at Nuffield felt. The question instead was whether they could fix its flaws.

  Guth's original idea was plagued by a problem that he himself hadn't identified. Once his version of inflation started, it couldn't stop. Other theorists—Andrei Linde, at the Lebedev Physical Institute in Moscow, and, independently, Paul Steinhardt and Andreas Albrecht, at the University of Pennsylvania—identified the problem and found the solution. They reconceived the inflationary period to be, as Guth came to think of it, less like the bubbling of boiling water than the congealing of a single Jell-O bubble. The problem with the one-bubble inflationary model, however, was that it still had to account for the visible universe—homogeneous and isotropic, but not too homogeneous and isotropic, or else we wouldn't be here.

  They were all borrowing from Hawking. In 1973 Hawking had
redefined the study of the early universe with his work on black holes; he found that, owing to a combination of quantum and gravitational effects, they weren't one-way tickets to a singularity. At the edge of the event horizon—the black hole's ring of no return—quantum effects dictated that particles and antiparticles would be popping into existence, while gravitational effects dictated that one partner would disappear into the black hole but not the other. Rather than annihilating each other "immediately," one would slip over the edge, into the black hole, but the other would escape into space and the universe as we know it. Black holes, Hawking contended, aren't black after all. They leak radiation—Hawking radiation, as it came to be called.

  In effect, Hawking had begun to bridge the two seemingly irreconcilable theories of the twentieth century, quantum mechanics and general relativity, a necessary step if science was ever going to describe the earliest, foamiest time after the singularity, or perhaps even the singularity itself. Two years later, Hawking and Gibbons extended the concept of quantum gravity to the universe as a whole and found that it would fill with thermal fluctuations. In early 1982, in the months leading up to Nuffield, Turner and Paul Steinhardt had begun working on the idea that those fluctuations could have been present during the inflationary period.

  For Guth and Turner and some of the other attendees, Nuffield was the latest stop on what they'd come to regard as a "traveling circus" of cosmologists. In early 1982 they had attended conferences in London, the French Alps, and Switzerland. In April, Steinhardt and Hawking happened to be visiting the University of Chicago at the same time; head-scratching with Turner inevitably commenced. In May, Steinhardt visited Harvard; Guth biked over from MIT. In June, just two weeks before Nuffield, Hawking gave a lecture at Princeton; Steinhardt drove over from Philadelphia, then called Guth and Turner with the latest update on new inflation.