DASI’s data to help explain origin of universe

The Earth is round, but the universe is probably flat.

Cosmologists predicted more than a decade ago that the geometry of the universe would turn out to be flat, meaning that it would neither expand forever nor eventually collapse but forever remain precisely balanced between the two. But no one had the instruments to prove it.

Now Chicago’s new Degree Angular Scale Interferometer and a sister instrument, the California Institute of Technology’s Cosmic Background Imager, are among the instruments with the capability to answer that question and a host of others regarding the origin and evolution of the universe.

University astronomers shipped DASI from campus in early September, and they expect it to arrive at the South Pole, via Los Angeles and New Zealand, on Nov. 13. If all goes well, DASI will blossom early next year in the clear, frigid air of the South Pole.

“First results should be in by February or March,” said John Carlstrom, Professor in Astronomy & Astrophysics, who co-heads the DASI project with Mark Dragovan at NASA’s Jet Propulsion Laboratory.

“We’ll get the best results when the sun sets and the atmosphere is very cold and very steady, and we will observe 24 hours a day. That’s going to be incredible. DASI is just so amazingly sensitive.”

Over the coming year, DASI team members will rotate operational responsibilities for the instrument, which consists of 13 small radio telescopes mounted in an 18-foot-tall structure, at the South Pole site. In addition to Carlstrom and Dragovan, the DASI team members are Bill Holzapfel, University of California-Berkeley; Chicago postdoctoral researchers Clem Pryke and Erik Leitch; graduate students Nils Halverson, John Kovac and Sam Laroque; and research assistants Gene Davidson and Ethan Schartman.

The $3 million array, funded by the National Science Foundation, will take pictures of the infant universe. DASI will record slight temperature fluctuations in the cosmic microwave background, the big bang’s afterglow. This radiation dates back at least 10 billion years, to when the universe was only 300,000 years old.

The remarkably uniform microwave glow in the sky is slightly hotter in some areas, slightly cooler in others, as discovered in 1992 by scientists working with the orbiting Cosmic Background Explorer, or COBE. “Those slight temperature fluctuations indicate where matter was clumping in the very early universe,” Halverson said. “That clumping matter was a precursor to all the structure we see in the universe today, including stars, galaxies and clusters of galaxies.”

Scientists would like to get a more focused look at that clumping. With its seven-degree resolution, COBE could clearly image phenomena in the sky covering nearly 200 times the apparent area of the full moon. DASI, CBI and the Microwave Anisotropy Probe, which will be launched late next year, are all sensitive to temperature fluctuations, the anisotropy, on scales of one degree or less.

“That’s where we believe most of the information is,” said Carlstrom, who also is a member of the CBI team.

DASI’s resolution takes the image from one degree down to a quarter of a degree, good enough to see detail as small as an area one-quarter the apparent area of the full moon. Caltech’s CBI, now en route to its viewing site in Chile’s Atacama Desert, will collect data at still finer scales.

In musical terms, COBE records the bass, DASI the midrange and CBI the treble. The difference: music is a vibration in air; the cosmic microwave background is a vibration in space. “You can take any picture and break it up into long wavelengths, which are like the bass, and also into short wavelengths, which are like the treble,” Carlstrom explained. “You need different instruments to detect those things.”

Astrophysicists have made precise predictions about the temperature fluctuations these instruments will detect. If the universe is indeed flat, the temperature fluctuations will be largest on the degree scale. If the universe will expand forever, the fluctuations will be largest on the half-degree scale. And if the universe will end in a big crunch, the fluctuations will be largest on scales larger than a degree.

“It is remarkable that by making sensitive measurements of the tiny temperature fluctuations in the cosmic microwave background, we will learn the ultimate fate of the universe,” Carlstrom said.

Other University instruments used to study the cosmic microwave background include TopHat, which will ride on top of a large balloon as it circumnavigates Antarctica, and the Python telescope, which has produced results of the large angular-scale anisotropy.

For more information, see http://astro.uchicago.edu/dasi/.