Fixed on the future
South Pole Telescope repair ensures experiment will continue to probe mysteries of the universe
Posted May 27, 2011
The South Pole Telescope is a bold experiment attempting to answer some of the big questions in cosmology, such as the nature of dark energy, a mysterious force that pervades the universe.
The 10-meter telescope, built during the 2006-07 field season at the U.S. Antarctic Program’s South Pole Station , has already made some big discoveries, such as detecting the most massive high-redshift galaxy cluster to date.
But the latest feat involving the project is one researchers have no wish to repeat.
For a few tense days during the 2010-11 season, about half of the instrument was jacked up more than a meter in the air to repair the azimuth bearing, which allows the instrument to turn side to side — an important function for an experiment scoping the southern sky.
About two years ago, the bearing was showing signs of failure, disconcertingly spitting out small metal parts. The temporary solution was to keep the bearing packed with grease. About half a million pounds of metal sits on the bearing.
“If that lets go, we have a major problem. It could potentially wreck the whole thing if it came to a screeching halt,” said Erik Nichols, project manager with the University of Chicago , one of one of nine institutions collaborating on the project.
Normally, such an operation would use a large crane to lift the instrument. Another alternative would have been to disassemble the entire telescope.
In this case, neither solution was an option. There’s not a crane that big at the South Pole Station, where most cargo arrives in the belly of an LC-130 airplane. And the summer season at South Pole is too short for the time required to take apart and rebuild the telescope.
Instead, the plan called for using four heavy-duty hydraulic jacks from a Dutch company called Mammoet that specializes in lifting really big stuff. The operation involved building a steel frame around the bottom of the telescope for the hydraulic jacks to lift. A skidding system with rails allowed the team to slide the bearing out and replace it with a new one, working akin to a DVD tray that pops in and out of the machine.
“This is a technical project, and as far as we know, it’s not been done before,” said Nichols a few days after the telescope had been set back down and secured.
That was perhaps the most difficult part of the whole project, he added, because the 3-meter-wide bearing had to be reset within a fraction of a millimeter of its original position.
“The tolerances we’re dealing with are so low; it’s not like slapping bolts and steel together. If something doesn’t fit right, you can’t break out the torches and make it fit,” said Nichols, who had worked as an ironworker at South Pole, among other jobs, in previous seasons.
“You need the whole structure to be very rigid for pointing accuracy, for looking out into space,” he added. “If you’re off a little bit down here, you’re on a whole wrong piece of the real estate up there.”
Stephan Meyer was certainly breathing a sigh of relief after the operation.
“It was pretty worrisome,” said Meyer, a co-principal investigator (PI) on the SPT project and professor from the University of Chicago. John Carlstrom , also from the University of Chicago, is the lead PI.
Fortunately, neither the trouble with the azimuth nor the subsequent repair work affected the collection of data. Each summer the team schedules down time on the telescope for regular maintenance, along with upgrades to its sensors and instrumentation.
This winter the SPT will continue its millimeter-wave survey of a huge swath of sky covering 2,500-square degrees. Its primary goal is to find a large sample of massive galaxy clusters to learn more about dark energy , which physicists estimate makes up 70 percent of the universe.
The SPT hunts for giant galaxy clusters using the Sunyaev-Zel’dovich (SZ) effect — a small distortion in the cosmic microwave background (CMB), a “glow” left over from the Big Bang . Such distortions are created as background radiation passes through a large galaxy cluster. In 2008, the telescope detected its first galaxy clusters using the SZ effect.
SPT researchers hope to detect thousands of clusters with the telescope. “The reason we need many clusters is for the statistics,” Meyer explained.
Dark energy may explain the current expansion and acceleration of the universe. Dark energy appears to exert a pressure, counteracting the gravitational attraction between galaxies. In a younger, smaller universe, gravity had a greater influence, allowing galaxy clusters to grow.
Now dark energy dominates. “It stops clusters from condensing anymore,” Meyer said.
Next year, the SPT will switch gears and join the race at South Pole to detect gravity waves on the surface of the CMB — a goal shared by other telescopes at the station. [See previous article: Inflation at the South Pole.]
The most widely accepted theory about the evolution of the universe holds that a fraction after the Big Bang it expanded exponentially — a high-energy growth spurt called inflation. Theorists say inflation should leave gravity waves, referred to as B-mode, on the CMB.
Next summer, a special instrument called a polarimeter will be attached to the SPT to search for these gravity waves.
Meyer said he expects the search will take several years, with the first dedicated to tweaking the instrument’s sensitivity.
“It’s hard to predict how many years it will go forward because the theory will evolve with the observations, which will evolve with the theory. We’re driving in the dark. You can’t tell exactly how interesting this experiment will be three years hence,” he said.
The South Pole is one of the preferred locations for these types of experiments for a variety of reasons. Its thin atmosphere due to high altitude and extreme cold means there is little water vapor in the air, which can interfere with incoming signals in the millimeter wavelengths detected by the telescope. The atmosphere is also particularly stable because the sun does not rise and set daily.
That such experiments are even possible still amazes Meyer, who said that not long ago cosmology was mostly theory with little hope of making meaningful observations. Now experiments like the SPT and others can help confirm major components of the standard model of cosmology, from the Big Bang to the CMB to dark energy.
“Who knows what will come next. It’s very exciting,” he said.
NSF-funded research in this story: John Carlstrom, John Ruhl, Joseph Mohr, William Holzapfel and Nils Halverson, University of Chicago, Award No. 0638937 ; and John Carlstrom, Stephan Meyer, John Ruhl, William Holzapfel, and Nils Halverson, Award No. 0959620 .