DASI grows into new fields of observation after latest results

During the end of the summer season, 2001, and after outfitting the telescope with polarizers, (left to right) Ben Reddall (in shadow), Charlie Kaminski, Clem Pryke, John Kovac and Erik Leitch pose for a photo inside the DASI ground shield at the South Pole.

When University physicist Sean Carroll began planning a schedule of speakers for the COSMO-02 workshop that assembled 275 cosmologists in Chicago last month, John Carlstrom, the S. Chandrasekhar Professor in Astronomy & Astrophysics and the College, was not on the program. But Carroll gratefully made last-minute arrangements that would allow Carlstrom to announce his team’s latest experimental results from the Degree Angular Scale Interferometer.

“Our conference was not devoted primarily to new experimental results, but rather to theorists exchanging ideas; but this was an important result that everyone in attendance could immediately relate to,” said Carroll, Assistant Professor in Physics and the College. “It’s a forward-thinking crowd, and they recognize that the real significance of the DASI result is that it opens up an entire new field of observations.”

Using the DASI radio telescope at the National Science Foundation’s Amundsen-Scott South Pole Station, Carlstrom’s team measured a minute polarization of the cosmic microwave background, the sky-pervading afterglow of the big bang. The discovery, which astrophysicists have pursued with increasingly sensitive instruments for more than two decades, verifies the framework that supports modern cosmological theory.

“This beautiful framework of contemporary cosmology has many things in it we don’t understand, but we believe in the framework,” said Clem Pryke, Assistant Professor in Astronomy & Astrophysics, and a member of the team that announced the discovery. “This new result was a crucial test for the framework to pass.”

Unlike the radiation that DASI measured, most light is unpolarized, its many individual waves jumbled together, each wave flickering up and down in a different plane as it speeds toward Earth. Unpolarized light becomes polarized whenever it is reflected or scattered. This is the principle behind polarizing sunglasses that remove the glare from the hood of a car or the surface of a pool. In both cases, the sunglasses only permit waves that tend to flicker up and down in the same plane to pass.

When the scattering of cosmic light last interacted with matter nearly 14 billion years ago, the polarization of the cosmic microwave background was produced. If no polarization had been found, astrophysicists would have had to reject all their interpretations of the remarkable data they have compiled in recent years, said Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics.

“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 said. “Polarization is predicted. It’s been detected, and it’s in line with theoretical predictions. We’re stuck with this preposterous universe.”

It is a universe in which 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 vast majority of the universe, meanwhile, is made of a mysterious force that astronomers call “dark energy.” Scientists simply do not know what it is. They only know that it acts in opposition to gravity, accelerating the expansion of the universe.

Additionally, cosmic inflation theory improbably proposes that the universe underwent a gigantic growth spurt in a fraction of a second, just moments after the big bang.

Carlstrom’s other collaborators on the polarization discovery were John Kovac, graduate student in Physics, and Erik Leitch, Research Scientist in Astronomy & Astrophysics, both of Chicago; and Nils Halverson and Bill Holzapfel, of the University of California, Berkeley.

The discovery follows in the wake of another important DASI finding. Last year, Carlstrom’s team precisely measured temperature differences in the cosmic microwave background, further supporting the cosmic inflation theory.

The polarization signal is more than 10 times fainter than the temperature differences that DASI detected earlier. DASI’s first discovery came after it collected data for 92 days from 32 spots in the sky. But DASI needed to watch two spots in the sky for more than 200 days to detect the polarization.

Scientists now predict that increasingly sensitive detections of polarization will yield many more discoveries. “It’s going to triple the amount of information that we get from the cosmic microwave background,” Kovac said. “It’s like going from the picture on a black-and-white TV to color.”

The polarization is a signpost from when the universe was only 400,000 years old, when matter and light were only just beginning to separate from one another. “What’s unique about polarization is that it directly measures the dynamics in the early universe,” Carlstrom said.

Temperature differences revealed patterns of lumpy matter frozen in the early universe, but by measuring polarization, astronomers can actually see how the early universe was moving.

In the coming years, astronomers will attempt to use the CMB polarization to measure gravity waves, a form of radiation predicted by general relativity that corresponds to ripples in the fabric of space-time, said Michael Turner, the Bruce V. and Diana M. Rauner Distinguished Service Professor in Astronomy & Astrophysics.

“Detection of the polarization opens a new door to exploring the earliest moments and answering the deep questions before us,” Turner said.