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Essay: DASI Does It

Plunked on a platform a little over half a mile (one km) away from the the NSF Amudsen-Scott South Pole Station, DASI looks a bit like a giant PC tilted hopefully at the vast Antarctic sky. Pronounced "daisy," the name stands for Degree Angular Scale Interferometer, which is an instrument composed of many smaller telescopes. First assembled in Chicago, this one was flown to the South Pole in three 25,000-pound (almost 11,365 kg) plane loads. It's studded with an array of 13 small, cylindrical receivers, each a miniature telescope. These work in unison to produce an image of the Cosmic Microwave Background (CMB), a vast sea of light from when the Universe was very young, some 14 billion years ago.

Observing a sea of faint and ancient light

No longer visible to the eye, CMB radiation can only be detected by antennas and receivers specially designed to pick up light with long wavelengths, called microwaves. DASI is made up of 13 small telescopes that work in unison to create very sharp images. Since each telescope sees the same spot in the sky from a slightly different position, the microwaves from that spot reach each telescope at slightly different times. "So comparing the timing of all the signals coming in allows astrophysicists to map where the radiation is coming from," explains John Carlstrom of the University of Chicago, director of the Center for Astrophysical Research in Antarctica (CARA) and leader of the team that operates the three-million-dollar telescope.

Sitting atop a 30-foot-high (around nine meters) platform, DASI can be rotated along three axes, allowing astrophysicists to generate fine-resolution images of structures billions of light-years away. "DASI is really a mapping machine. It stares at a spot in the sky about three degrees across at once, but it maps all the structure within that part of the sky." says Carlstrom. "And it does that very well." The process takes about 24 hours, after which time the instrument moves on to another part of the sky. More telescopes would be even better, but each additional pair geometrically increases the electronics as well as the amount of data that must be stored and transmitted. "We'd love to have more telescopes," Carlstrom admits. "You'd have to give us a lot more money to do it. But it'd be that much more sensitive. It'd be great."

Why put a telescope at the South Pole?

A unique aspect of positioning a telescope at the South Pole is that the earth essentially rotates underneath DASI while it looks upward. "We can look at the same part of [the Pole's] incredible, dry, very high-transparency sky for days on end, and just keep looking," Carlstrom comments. That sky is one of the major charms the South Pole holds for astrophysicists. Antarctica is essentially an ice desert, so the air is free of the water vapor that absorbs radiation and emits "noise" elsewhere on Earth. The Pole also sits on any icy plateau some two miles (about three km) thick, an elevation that puts it above much of Earth's atmosphere-another advantage when observing very, very faint cosmic signals. Another bonus is six months of darkness-from mid-March until the sun rises again in mid-September. "If you have the sun coming up every single day, it dwarfs your signals," explains Carlstrom. "At the South Pole you actually have six months where the sun is down and the Earth acts as a shield." DASI's collar and "petals" also act as a shield, blocking radiation from the ground that could get confused with radiation from the sky.

Why study the Cosmic Microwave Background?

DASI is the most recent in a series of experiments that have mapped the unique pattern of the Cosmic Microwave Background with ever-increasing accuracy. Some of the antennas used in these experiments-interferometers in particular-are very, very sensitive to tiny differences in temperature. This is significant because minute differences in the temperature of the CMB show how matter in the early Universe was distributed. "We're seeing the very, very early seeds of all the structure that formed in the Universe," explains Carlstrom. Data from DASI—several hundred megabytes a day after compression at the South Pole—is sent via satellite to computers in Chicago, where scientists use it to generate sky maps of an infant Universe just 400,000 years old.

New technology gives rise to new cosmology

The DASI team includes scientists at the California Institute of Technology and the University of California at Berkeley. DASI began operating in 1999, and the team presented its first results in April 2001. These findings supported the predictions of inflation, a theory about the formation of the early Universe that was first introduced in the early 1980s. This support is thrilling to cosmologists—scientists who study the origin and evolution of the Universe—because data from instruments like DASI are turning their field from a largely theoretical science into an observational one. Powering this New Cosmology, says Michael Turner, a colleague of Carlstrom's at the University of Chicago, are "the technology and instrumentation that allow us to test these ideas [about the Big Bang and inflation]. And without the ability to test them, it isn't science."

More data will emerge from DASI and its sister instrument, the California Institute of Technology's Cosmic Background Explorer, as well as Chicago's TopHat experiment and NASA's Microwave Anisotropy Project. Says Turner, "This is just the beginning."

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