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Issue 5, Volume 1
May 2008

Professor Bill Holzapfel Leads The South Pole Telescope Project In A Search For Cosmological Clues

The South Pole is arguably the most remote and inhospitable place on earth. Frigid temperatures that can reach -120 F and six months of continual darkness make living and working at the South Pole a challenge. Interestingly, those same conditions allow remarkably clear views of the cosmic microwave background (CMB) radiation produced shortly after the Big Bang. The South Pole Telescope (SPT) is an ambitious new project designed to utilize novel detector technology and take advantage of these unique observing conditions to address a compelling cosmological puzzle.

The 10 meter diameter SPT secondary mirror being lifted into place.

Observations of distant supernovae indicate that the expansion of the Universe is accelerating, apparently driven by a mysterious form of energy that only interacts gravitationally.  From observations of supernovae and the CMB, scientists estimate that this Dark Energy makes up ~70% of the universe’s combined mass and energy.  Apart from the inferred density, little is known about Dark Energy. By studying its influence on the growth of structure in the Universe, we hope to learn more about its properties, such as the relationship between the pressure that it exerts to drive the acceleration of the Universe and its density. The formation of clusters depends sensitively on the properties of Dark Energy. Precise measurements of the number of clusters as a function of distance can be used to constrain the properties of Dark energy.

When CMB photons pass though a cluster, they can be scattered by energetic electrons creating slight distortions in the otherwise perfect blackbody spectrum. In this way, the CMB can be used as a backlight to study the entire observable Universe. In order to resolve distant clusters, a large aperture, and therefore a ground-based telescope, is needed. Unfortunately, water vapor is very effective at absorbing the millimeter wavelength radiation where the intensity of the CMB peaks. The cold atmosphere at the South Pole is incredibly dry and stable, making for transparent skies and excellent observing conditions.

The SPT project involves researchers from Berkeley, University of Chicago, Case Western Reserve University, Harvard-Smithsonian, University of Illinois, and LBNL. This team worked together to install the 75-foot-tall, 280-ton telescope at the South Pole in just three months during the Austral summer of 2006-07.  A team of researchers from the University of California, Berkeley have just returned from the South Pole, where they installed an upgraded array of millimeter wavelength detectors.  The heart of the system is a cryogenic receiver that cools an array of 960 detectors to temperatures near absolute zero. The receiver team is led by physics professor William Holzapfel whose group has previously published the most sensitive measurements of fine-scale CMB anisotropies. According to him, “the success of this season was due to the extraordinary efforts of all the team members”. For example, physics graduate student Erik Shirokoff worked long hours in the in Berkeley microfabrication facility producing the detectors for the final array. These detectors operate at a temperature near absolute zero in a cryostat designed and built by Berkeley postdoctoral researcher Bradford Benson, and are biased and read out with a superconducting amplifier system to which graduate students Martin Lueker and Tom Plagge have made critical contributions.

The Sun has recently set and the telescope is beginning its scheduled program of cosmological observations. Eventually, the SPT will survey about a fifth of the entire southern sky and is expected to detect thousands of previously unknown galaxy clusters. Combining the SPT cluster survey with follow-up optical observations, the scientists will determine the mass, distance and age of the clusters. These measurements should shed light on the growth of structure in the Universe and, with some luck, new constraints on the nature of the elusive Dark Energy.

The SPT is funded primarily by the NSF, along with additional support from the Kavli Foundation and the Gordon and Betty Moore Foundation of San Francisco.

The image to the left is one of six 160 element detectors arrays that together make up the SPT receiver focal plane.


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