Investigations

Brighter evidence of the Big Bang
As University of Chicago physicist Sean Carroll planned speakers for September's COSMO-02 workshop in Chicago, John Carlstrom, the S. Chandrasekhar professor in astronomy & astrophysics and the College, was not on the agenda. But nine days before the event, Carroll shifted the schedule so that Carlstrom could announce the latest results from the Degree Angular Scale Interferometer (DASI).

Even then, Carroll did not tip his hand. He wanted Carlstrom's announcement to come as a surprise to the 275 cosmologists at the meeting. Carlstrom's news: his team had succeeded in measuring the polarization of the cosmic microwave background (CMB), or the sky-pervading afterglow of the Big Bang.

Journalists also took note, to Carlstrom's surprise. The New York Times, Chicago Tribune, Reuters, and a host of other outlets covered the announcement. "We assumed that we would make a big splash in our own community," Carlstrom says, "but I was not sure that the press would pick up on it."

DASI's view of the CMB's polarization (black lines) and intensity, shown in hot (yellow) and cold (red).

Despite the discovery's esoteric nature, the news media were fairly accurate in defining polarization and explaining its cosmological significance. Unlike the radiation that DASI measured, most light is unpolarized, its many individual waves jumbled together, each wave flickering up and down on a different plane as it speeds toward Earth. When unpolarized light from the Big Bang last interacted with matter nearly 14 billion years ago, it became polarized, and the waves began flickering along the same plane.

Astrophysicists had suspected the polarization theory but had been unable to prove it, searching for a signal with increasingly sensitive instruments for more than two decades. Even with DASI, the Chicago team—working at the National Science Foundation's Amundsen-Scott South Pole Station—had to spend 271 days watching two spots in the sky to measure the extremely weak signal with a high degree of certainty.

But the painstaking effort paid off. The framework that supports the entire edifice of modern cosmological theory states that polarized light is an inescapable characteristic of the Big Bang, which sent a series of sound waves reverberating through the early universe. As electrons sloshed back and forth within these sound waves, they polarized the CMB. Polarization, therefore, directly measures these movements. Given Carlstrom's findings last year—in which DASI measured precise temperature differences in the CMB, hot and cold spots indicating denser and less dense areas—"the polarization should show the material sloshing about at just the right level to make the hot and cold spots," Carlstrom says. And this is exactly what his recent findings confirm.

The polarization is thus a signpost from when the universe was only 400,000 years old, when matter and light were just beginning to separate from one another. "What's unique about polarization is that it directly measures the dynamics in the early universe," Carlstrom says. This means that when DASI calculates the polarization, it actually measures the velocities of matter as it formed into clumps for the first time.

The theoretical framework that polarization supports encompasses a variety of unlikely interpretations that cosmologists have pulled from their data in recent years, Carlstrom says. If no polarization had been found, cosmologists would have had to reject these interpretations. "Instead of stating that we think we really understand the origin and evolution of the universe with high confidence, we would be saying that we just don't know," Carlstrom says. "Polarization is predicted. It's been detected, and it's in line with theoretical predictions. We're stuck with this preposterous universe."

What makes it so preposterous is that ordinary matter-the stuff of which humans, stars, and galaxies are made-accounts for less than 5 percent of the universe's total mass and energy. The rest is made of a mysterious force called dark energy. Scientists simply do not know what the force is. They only know that it acts in opposition to gravity, accelerating the expansion of the universe. DASI's polarization reinforces this paradoxical interpretation, and more besides.

This includes cosmic inflation, an extension of the Big Bang theory, which improbably proposes that just moments after the Big Bang the universe underwent a gigantic growth spurt in a fraction of a second. Although inflation forms the basis for much of contemporary cosmology, scientists have yet to reconcile it with prevailing theories of fundamental physics.

Carlstrom next expects to use DASI for more precise polarization measurement. He'll also use a newly funded South Pole telescope "to measure the polarization with very high precision," he says, "and move toward the longer-term goal of measuring the primordial gravitation waves from inflation through their subtle effect on the CMB polarization."

Gravity waves are a form of radiation predicted by general relativity that correspond to ripples in the fabric of space-time, says Michael Turner, the Bruce V. and Diana M. Rauner distinguished service professor in astronomy & astrophysics. "Detection of the polarization," Turner says, "opens a new door to exploring the earliest moments and answering the deep questions before us."

— Steve Koppes