Volume 4, Issue 29 July 2007
The Telescope at the South Pole
Assembling the South Pole Telescope, December 2006. Antarctica's arid atmosphere provides the best viewing conditions available on earth. Credit: William Holzapfel
This past December, while children across the nation were concentrating on the North Pole, UC Berkeley Professor of Physics Bill Holzapfel was focused in the opposite direction. On Christmas Day, Holzapfel and his colleagues were mobilizing to assemble an ambitious new instrument at the coldest astronomy observing station in the world: the South Pole Telescope.
Holzapfel recalls the holiday as his most hectic to date. "Everyone opened their presents at home, came in, and started packing up the equipment." Their luggage ranged from long undies to supersensitive photon detectors and a cryogenic receiver the size of a Volkswagen bug.
By operating this gear at 90° south, Holzapfel and an international team of scientists from eight other institutions hope to gain insight into the recipe for the universe itself.
That recipe has three primary ingredients. The most familiar is regular matter, the kind that makes up plants and animals, planets and stars. But scientists have also deduced the existence of two more shadowy elements: dark matter, which has mass but cannot be seen; and dark energy, which is forcing the universe apart at an ever-increasing pace. The South Pole Telescope will help determine the relative proportions and properties of these ingredients.
Professor Holzapfel and colleagues were responsible for designing and building the detector system on the South Pole Telescope. Here, Holzapfel and his team are attaching the receiver to a large cryostat. The cryostat cools a secondary mirror that focuses light from the 10 meter reflector onto the detectors. Credit: William Holzapfel
To decode this cosmic formula, scientists are studying the radiation that flooded the universe after the Big Bang. These photons are known as the cosmic microwave background, or CMB. They have sailed through the cosmos since virtually the beginning of time. Today, they form a remarkably uniform backdrop throughout space. But every once in a great while, a few CMB photons will pile headlong into an aggregation of galaxies called a galaxy cluster. Among the most gargantuan structures in the universe, the mass of a single cluster may rival that of a million-billion of Suns.
Within that cluster, about one in a hundred CMB photons will slam into a superheated electron. The collision transfers energy to the low-energy CMB photon, boosting it to a higher frequency.
When the South Pole Telescope observes low-energy photons from that direction, the effect is like looking at x-rays that have been absorbed by bones: the background dims, leaving a hole in the CMB .
"If we can see these holes in the CMB, it tells us where the clusters of galaxies are," Holzapfel says. "We can use the CMB as a backlight to illuminate the universe."
The larger the photon hole, the more massive the galaxy cluster behind it. The South Pole Telescope will allow scientists to assemble a catalog of massive the galaxy clusters. Even the oldest and farthest clusters can be counted because, unlike the light from stars, CMB hole visibility is the same for clusters near and far.
The relative proportions of matter, dark matter, and dark energy "radically affects the formation of structure in the universe. And the thing that's most sensitive is the galaxy clusters," Holzapfel says.
The team will compare the results of their galaxy cluster survey with computer simulations of the evolving universe. The new data will help identify the abundances and properties of the ingredients that could have produced the universe we see today. In particular, they hope to shed new light on the yet mysterious dark energy that appears to dominate the dynamics of the universe.
The massive metal eye of the completed South Pole Telescope stares out over hundreds of miles of snow and ice. The receiver Holzapfel's team developed is installed at the end of the boxy scaffolding bolted to the dish. The dish can be rotated so that the receiver cabin "mates" with the control cabin below, allowing scientists to work on the equipment without being exposed to Antarctica's harsh winter weather. Credit: William Holzapfel
Holzapfel and his Berkeley colleagues designed and built the digital eyes that allow the South Pole Telescope to see the CMB. The heart of the system consists of a shiny silicon wafer roughly size of a Frisbee. On it are etched 960 bolometers, supersensitive heat detectors capable of finding dim spots where the CMB is a few millionths of a degree colder.
The telescope was built in Antarctica because it has some of the driest skies on the planet. Water vapor in the atmosphere obscures CMB radiation. "If you were to take the atmosphere above Berkeley on a good day, and squeeze it dry, you would get a layer of liquid water that's at least 1 cm deep. At the South Pole, in the dead of winter, you'll get 50 times less," Holzapfel says. The total darkness of the six month austral winter also minimizes the daily weather cycle , providing long stretches of dry, clear skies.
Upon landing at the bottom of the world, Holzapfel and colleagues got right to work installing, testing, and troubleshooting every piece of equipment they had brought with them. It turned out they hadn't a moment to waste. "We got on the sky, were able to observe a planet...and the next day they told us we had to leave," Holzapfel says. Temperatures were growing too cold to fly, forcing temporary visitors to leave or risk spending the winter in Antarctica. "We had all of an hour and a half to pack up our stuff and get on the plane," he says.
Now in the hands of two overwintering scientists, the telescope is online and making observations. The South Pole Telescope should produce preliminary results over the next few months and complete the planned cluster survey over the next several years. If all goes well, says Holzapfel, "we'll gain new insights into what's arguably the most interesting question in physics today."