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.
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.”