Gravitational Waves and the Great Inflation

Back in 1979, puzzling over why the universe seemed so uniform in every direction, physicist Alan Guth hit on an idea that we now know as inflation: when it was less than a trillionth of a second old, the universe doubled in size about 60 times within another trillionth of a second, expanding much faster than the speed of light. (This is allowable under special relativity because space itself was moving, not some object passing through space.) Most scientists said, "intriguing" and went back to work on whatever they were doing, since it seemed highly unlikely that inflation could ever be proved or disproved. But then in 2002 it was discovered that the cosmic microwave background (CMB) radiation, the leftover glow of the Big Bang, is polarized, meaning it vibrates more in one direction than in others. This means that it could, in theory anyway, bear the imprint of the gravitational waves that should have been part of the great inflation. And now a team of scientists say they have found evidence of those waves:

Cosmologists have detected what they say is the first direct evidence of this inflation—one of the biggest discoveries in the field in 20 years. From studying the cosmic microwave background (CMB)—the leftover radiation from the big bang—they have spotted traces of gravitational waves—undulations in the fabric of space and time—that rippled through the universe in that infinitesimally short epoch following its birth. The imprint of these gravitational waves upon the CMB matches what theorists had predicted for decades. The findings, announced this morning at a scientific presentation at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, also show that gravity—at the smallest scale—follows the rules of quantum mechanics, similar to other forces such as electromagnetism.

“This is an astounding result,” says Alan Guth, a cosmologist at Massachusetts Institute of Technology in Cambridge and one of the original proponents of inflation. Guth—who was not involved in the work being highlighted today—says the researchers showed him a draft paper a week ago, after swearing him to secrecy. “The observations are at a very high level of significance,” he says. Andrei Linde, a cosmologist at Stanford University in California who developed one of the most theoretically successful models of inflation, agrees: “If these results are right, inflationary theory has passed its most difficult test ever.”

The discovery comes from observations by a small but sophisticated telescope at the South Pole dedicated to a project known as the Background Imaging of Cosmic Extragalactic Polarization (BICEP). Just like visible light and other kinds of electromagnetic radiation, a cosmic microwave’s electric and magnetic fields could be oscillating along any of an infinite number of orientations. The telescope used by the BICEP researchers is designed to map the orientation—or polarization—of the CMB as it varies in different parts of the sky. In data taken from a small patch of the sky between January 2010 and December 2012, the researchers found a random pattern of faint pinwheel-like swirls in the CMB. Such swirls, called B modes, are the hallmark of gravitational waves in the primordial universe and—many cosmologists say—are the smoking gun for inflation.

“We believe that gravitational waves could be the only way to introduce this B-mode pattern,” says John Kovac, a cosmologist at Harvard and one of the four principal investigators of BICEP.

If confirmed, this is an amazing achievement. It is also a return to the fruitful interplay of mathematical theorizing and precisely measured experiment that was the hallmark of physics during its great run, from Galileo to the 1970s. Of late theorists have mainly been predicting things that nobody can figure out how to measure, so it is great to see the old synergy working again.