Dec. 16, 2002 Using a powerful new instrument at the South Pole, a team of cosmologists has produced the most detailed images of the early Universe ever recorded. The research team, which was funded by the National Science Foundation (NSF), has made public their measurements of subtle temperature differences in the Cosmic Microwave Background (CMB) radiation.
The CMB is the remnant radiation that escaped from the rapidly cooling Universe about 400,000 years after the Big Bang. Images of the CMB provide researchers with a snapshot of the Universe in its infancy, and can be used to place strong constraints on its constituents and structure. The new results provide additional evidence to support the currently favored model of the Universe in which 30 percent of all energy is a strange form of dark matter that doesn't interact with light and 65 percent is in an even stranger form of dark energy that appears to be causing the expansion of the Universe to accelerate. Only the remaining five percent of the energy in the Universe takes the form of familiar matter like that which makes up planets and stars. The researchers developed a sensitive new instrument, the Arcminute Cosmology Bolometer Array Receiver (ACBAR), to produce high-resolution images of the CMB. ACBAR's detailed images reveal the seeds that grew to form the largest structures seen in the Universe today. These results add to the description of the early Universe provided by several previous ground-, balloon- and space-based experiments. Previous to the ACBAR results, the most sensitive, fine angular scale CMB measurements were produced by the NSF-funded Cosmic Background Investigator (CBI) experiment observing from a mountaintop in Chile.
William Holzapfel, of the University of California at Berkeley and ACBAR co-principal investigator, said it is significant that the new ACBAR results agree with those published by the CBI team despite the very different instruments, observing strategies, analysis techniques, and sources of foreground emission for the two experiments. He added that the new data provide a more rigorous test of the consistency of the new ACBAR results with theoretical predictions.
"It is amazing how precisely our theories can explain the behavior of the Universe when we know so little about the dark matter and dark energy that comprise 95 percent of it," said Holzapfel.
The dark energy inferred from the ACBAR observations may be responsible for the accelerating expansion of the Universe. "It is compelling that we find, in the ancient history of the Universe, evidence for the same dark energy that supernova observations find more recently," said Jeffrey Peterson of Carnegie Mellon University.
The construction of the ACBAR instrument and observations at the South Pole were carried out by a team of researchers from the University of California, Berkeley, Case Western Reserve University, Carnegie Mellon University, the California Institute of Technology, Jet Propulsion Laboratory (JPL), and Cardiff University in the United Kingdom. Principle investigators Holzapfel and John Ruhl at Case Western led the effort, which built and deployed the instrument in only two years.
ACBAR is specifically designed to take advantage of the unique capabilities of the 2.1-meter Viper telescope, built primarily by Jeff Peterson and collaborators at Carnegie Mellon and installed by NSF and its South Pole Station in Antarctica. The receiver is an array of 16 detectors built by Cal Tech and the JPL that create images of the sky in 3-millimeter wavelength bands near the peak in the brightness of the CMB. In order to reach the maximum possible sensitivity, the ACBAR detectors are cooled to two-tenths of a degree above absolute zero, or about –273 degrees Celsius (–459 Fahrenheit). ACBAR has just completed its second season of observations at the South Pole. Researcher Mathew Newcomb kept the telescope observing continuously during the six-month-long austral winter, despite temperatures plunging below –73 degrees Celsius (–100 Fahrenheit).
The construction of ACBAR and Viper was funded as part of the NSF Center for Astrophysical Research in Antarctica. The U.S. Antarctic Program provides continuing support for telescope maintenance, observations, and data analysis. NSF's Amundsen-Scott South Pole Station is ideally suited for astronomy, especially observations of the CMB. The station is located at an altitude of approximately 3,000 meters (10,000 feet), atop the Antarctic ice sheet. Water vapor is the principal cause of atmospheric absorption in broad portions of the electromagnetic spectrum from near infrared to microwave wavelengths. The thin atmosphere above the station is extremely cold and contains almost no water vapor. "Our atmosphere may be essential to life on Earth," said Ruhl, "but we'd love to get rid of it. For our observations, the South Pole is as close as you can get to space while having your feet planted firmly on the ground."
Papers describing the ACBAR CMB angular power spectrum and the constraints it places on cosmological parameters have been submitted to the Astrophysical Journal for publication.
For more information and drafts of the submitted papers, see: http://cosmology.berkeley.edu/group/swlh/acbar
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