Alan Guth’s career was up in the air. As an ambitious postdoc at Cornell University in 1978, Guth was looking for a way to contribute to his field of particle physics, but his research had attracted little interest. He also needed to find a permanent job to support his wife, Susan, and baby, Larry. But his job search hadn’t gone well either.
That fall a trio of cosmologists, or early-universe theorists, had won the Nobel Prize in physics. Soon, discussions about the origin of the universe and especially the big bang theory-the idea that the universe began with an explosion and that all matter has been moving outward ever since-were buzzing in the halls of academe and among the general public. So when a cosmologist came to speak at Cornell, Guth ‘69, SM ‘69, PhD ‘72, went to hear what he had to say. The lecturer, Robert Dicke, spoke of, among other things, a complex cosmological problem scientists had been unable to solve. “It certainly piqued my interest,” Guth says. “Dicke’s lecture seemed to point to the conclusion that traditional big-bang theory was leaving out something important.” Two years later, in the course of his research in an entirely different area, Guth happened upon the missing piece.
He introduced his explanation in 1981, calling it “inflation.” Since then the theory has garnered much attention. In the years since its appearance, Guth’s original paper has been cited in more than 2,000 publications, and last fall its author received the prestigious Dirac Medal, which many observers consider a precursor to a Nobel Prize. Today cosmologists are calling Guth’s idea a new paradigm, one of the last half-century’s few supremely influential theories of the universe.
The Road to Discovery
Guth’s office is a rain forest of loose paper. Although it is spacious for a professor’s digs-two to three times the size of most university offices-almost every inch of floor, furniture, and windowsill is littered with books, scraps of paper, manila folders, and periodicals. Guth, whose shaggy hair frequently slips over the rims of his glasses, assembles ideas from these various sources, piecing together the bits that spark his interest and pique his imagination. As he develops his theories, he draws also on what he knows. Guth formed his first inkling of the inflation theory in just this way: he collected a host of theoretical problems and came to see connections among them.
Guth’s theory came about quite by chance as he started to weave previously unconnected research with his own work. It all started with the lecture in which Dicke described the complex cosmological puzzle related to the amount and distribution of matter in the universe. Cosmologists do not know exactly how much matter is in the universe. When they talk about the amount that exists, they compare it to a critical density. This density corresponds to the amount of matter needed for a universe to become “flat.”
At the time of Dicke’s lecture, scientific observations showed that the universe was within 10 percent of the critical density, but today cosmologists can show that the universe is actually within five percent of it. In order for today’s universe to be within five percent of critical density, the universe would have had to have been extremely close to critical one second after the big bang. But “in the context of big bang theory, there’s no known reason why the universe would have started out close to critical density,” Guth says. This conundrum had been dubbed “the flatness problem.”
Guth describes another way to think about it: The universe’s density is much like a pencil balanced on its point. When it is perfectly balanced, it is at critical density. If no force disturbs it, it stays balanced. But if it is nudged in any direction, it will fall away from its balance point. In a sense, the universe is near that balance point today. But scientists didn’t know any reason why the pencil would have started straight up. They took for granted the fact that gravity always attracts-meaning the pencil would naturally tip. Guth would later discover that there are plausible circumstances in which gravity repels.
The flatness problem started Guth thinking about the early universe, but it was his work on magnetic monopoles-a seemingly unrelated field he later combined with cosmology-that led to what would become his inflation theory. Monopoles are a theoretical type of magnetic elementary particle with a single charge. Henry Tye, a fellow postdoc at Cornell, had become interested in theories that predicted the existence of magnetic monopoles, and he approached Guth about trying to determine how many would have been created in the big bang, when all particles were thought to have been created. At first, Guth says, “I thought that was a crazy way to waste one’s time.” But still hoping to make a name for himself in physics, he agreed to help.
The pair eventually deduced that the universe should be swimming in monopoles, but it is not. They also calculated the weight of monopoles and discovered they must be very heavy. These findings were just the kind of contribution Guth had been searching for, but before he and Tye could publish their results, another young researcher beat them to the punch.
So they started searching for another innovation. They began asking how they could modify the theories predicting magnetic monopoles to explain monopoles’ complete absence. They reasoned that supercooling-lowering the temperature below the freezing point without inducing freezing-in the early universe would have prevented the monopoles from being produced. This time, they published their findings immediately.
Next, Tye suggested they calculate the effect the supercooling might have had on the expansion rate of the universe. “I went home one night and did that calculation,” Guth remembers. In fact, supercooling does “affect the expansion rate of the universe enormously,” he says, “sending the universe into this exponential expansion, which is what we now call inflation.” In this instance, Guth theorized, gravity works in reverse.
Guth postulated that the entire universe started out a hundred billion times smaller than a proton. The universe was ruled by the laws of particle interaction, which scientists still don’t fully understand. The repulsive gravitational force of inflation enlarged the speck-size universe in just a tiny fraction of a fraction of a second, making the growth look like an explosion. And then inflation merged into the big bang theory.
Having postulated such a momentous occurrence, Guth says, “that same night, I realized that this exponential expansion would solve the flatness problem. And that, of course, made me tremendously excited.”
How does inflation solve the flatness problem? Guth’s idea was that the metaphorical pencil didn’t have to start out on its point; instead it could have started out in a tipped position. His theory suggests that gravity was reversed at the creation of the universe, so the metaphorical pencil would be pulled up from a horizontal position until it stopped on its point. In other words, it isn’t necessary to assume that the early universe began at critical density. In fact, it could have begun far from that because reverse gravity would cause the universe to move toward critical density.
Inflation solved another problem. The big bang should have produced a universe with radiation temperatures ranging from hot to cold. But scientists had found that on large scales the temperatures are homogenous throughout the universe. They were hard-pressed to explain this homogeneity, given that there hasn’t been enough time for the radiation to even itself out throughout the universe. Inflation accounts for this so-called horizon problem because the large-scale homogeneity would have been established when the universe was still smaller than a proton, and it has simply stretched to what we see today.
Image by John MacNeill.
Guth’s solutions started a revolution in cosmology, but his initial excitement was soon followed by anxiety. He was a young researcher with no faculty appointment, and he was unsure about his theory. “I was very nervous about it because I felt there were too many things about it that I really didn’t understand. I was afraid it was going to somehow blow up.” Despite his concerns, in early 1980 Guth explained inflation in a series of lectures for cosmologists around the country.
“Alan demonstrated unusual courage-especially for someone without tenure or even a faculty position-in putting forth inflation,” says Michael Turner, a University of Chicago astrophysicist.
Those who heard him were intrigued, and soon offers for faculty positions emerged, albeit not from MIT. Nevertheless, Guth wanted to return to his alma mater, so when a fortune cookie told him “an exciting opportunity lies just ahead if you are not too timid,” he called MIT and offered himself as a prospect. Later that year he came to the Institute as a visiting associate professor. Today, as the Victor F. Weisskopf Professor of Physics, he works just down the hall from his son Larry, a graduate student in mathematics. Larry works in the office his father occupied as a graduate student, a coincidence Guth finds “enormously cute.”
Some of Guth’s concerns about his theory’s validity were not unfounded. With help from Erick Weinberg of Columbia University, Guth discovered that his idea was slightly flawed. His explanation of the way inflation led to the big bang didn’t work. But because Guth believed in inflation’s importance, he wrote an article that described both his theory and its problems.
“He wrote a paper saying, I think this is a very important idea, but I can show it doesn’t work in the form I am proposing,’” Turner says. “He invited other scientists to think about inflation and improve it.” Three other cosmologists responded to his challenge. Russian scientist Andrei Linde and, independently, U.S. researchers Paul Steinhardt (Princeton University) and Andreas Albrecht (University of California, Davis) came up with a modification that avoided the flaw. They called it new inflation. Guth shares the 2002 Dirac Medal with Linde and Steinhardt.
Inflation “is a very exciting idea that has drawn together physicists across a range of subdisciplines and that has motivated some of the most exciting experiments in science today,” says Steinhardt. Linde, now at Stanford University, says Guth’s idea helped change modern cosmology. In fact, it has inspired about 30 variations-theories that use inflation as a base.
Shortly after new inflation was introduced, Guth and six other physicists started studying the origin of density fluctuations in the new model. They predicted a pattern of how these fluctuations would appear in the universe’s radiation temperature. Today satellites and balloon-based experiments show the pattern they predicted is remarkably accurate. Turner, who studies this radiation, says he is confident that over the next decade, these measurements will provide definite proof of inflation.
Guth, who is a member of the National Academy of Sciences, downplays his role in influencing the direction of so much new research. “Inflation would have been invented whether I’d invented it or not,” he says. “It really was pretty much just a piecing together of ideas that were already known to one physicist or another. There were just a lot of chances involved in my coming up with the idea.”
But to Turner and countless others who ponder the cosmos, it is clear that Guth’s contributions have provided a key to much recent progress, both experimental and theoretical. “Alan’s idea of inflation has revolutionized the way cosmologists think about the beginning of the universe,” Turner says. “In my opinion, it is the single most important idea since the big bang itself.”