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New Insights Into Dark Energy and Neutrinos by South Pole Telescope

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Analysis of data from the National Science Foundation- (NSF) funded 10-meter South Pole Telescope (SPT) in Antarctica provides new support for the most widely accepted explanation of dark energy, the source of the mysterious force that is responsible for the accelerating expansion of the universe.

The results begin to hone in on the tiny mass of the neutrinos, the most abundant particles in the universe, which until recently were thought to be without mass.

The SPT data strongly support Albert Einstein's cosmological constant--the leading model for dark energy--even though researchers base the analysis on only a fraction of the SPT data collected and only 100 of the over 500 galaxy clusters detected so far.

"With the full SPT data set we will be able to place extremely tight constraints on dark energy and possibly determine the mass of the neutrinos," said Bradford Benson, an NSF-funded postdoctoral scientist at the University of Chicago's Kavli Institute for Cosmological Physics.

Benson presented the SPT collaboration's latest findings, Sunday, April 1, at the American Physical Society meeting in Atlanta.

These most recent SPT findings are only the latest scientifically significant results produced by NSF-funded researchers using the telescope in the five years since it became active, noted Vladimir Papitashvili, Antarctic Astrophysics and Geospace Sciences program director in NSF's Office of Polar Programs.

"The South Pole Telescope has proven to be a crown jewel of astrophysical research carried out by NSF in the Antarctic," he said. "It has produced about two dozen peer-reviewed science publications since the telescope received its 'first light' on Feb. 17, 2007. SPT is a very focused, well-managed and amazing project."

The 280-ton SPT stands 75 feet tall and is the largest astronomical telescope ever built in the clear and dry air of Antarctica. Sited at NSF's Amundsen-Scott South Pole station at the geographic South Pole, it stands at an elevation of 9,300 feet on the polar plateau. Because of its location at the Earth's axis, it can conduct long-term observations.

NSF manages the U.S. Antarctic Program through which it coordinates all U.S. scientific research on the southernmost continent and aboard ships in the Southern Ocean as well as providing the necessary related logistics support.

An international research collaboration led by the University of Chicago manages the South Pole Telescope. The collaboration includes research groups at Argonne National Laboratory; Cardiff University in Wales; Case Western Reserve University; Harvard University; Ludwig-Maximilians-Universität in Germany; the Smithsonian Astrophysical Observatory; McGill University in Canada; the University of California, Berkeley; the University of California, Davis; the University of Colorado Boulder; and the University of Michigan, as well as individual scientists at several other institutions.

SPT specifically was designed to tackle the dark-energy mystery. The 10-meter telescope operates at millimeter wavelengths to make high-resolution images of Cosmic Microwave Background (CMB) radiation, the light left over from the big bang.

Scientists use the CMB to search for distant, massive galaxy clusters that can be used to pinpoint the properties of dark energy and also help define the mass of the neutrino.

"The CMB is literally an image of the universe when it was only 400,000 years old, from a time before the first planets, stars and galaxies formed in the universe," Benson said. "The CMB has travelled across the entire observable universe, for almost 14 billion years, and during its journey is imprinted with information regarding both the content and evolution of the universe."

The new SPT results are based on a new method that combines measurements taken by the telescope and by NASA and European Space Agency X-ray satellites, and extends these measurements to larger distances than previously achieved.

The most widely accepted property of dark energy is that it leads to a pervasive force acting everywhere and at all times in the universe. This force could be the manifestation of Einstein's cosmological constant that assigns energy to space, even when it is free of matter and radiation.

Einstein considered the cosmological constant to be one of his greatest blunders after learning that the universe is not static, but expanding.

In the late 1990s, astronomers discovered the universe's expansion appears to be accelerating according to cosmic distance measurements based on the relatively uniform luminosity of exploding stars. The finding was a surprise because gravity should have been slowing the expansion, which followed the big bang.

Einstein introduced the cosmological constant into his theory of general relativity to accommodate a stationary universe, the dominant idea of his day. But his constant fits nicely into the context of an accelerating universe, now supported by countless astronomical observations.

Others hypothesize that gravity could operate differently on the largest scales of the universe. In either case, the astronomical measurements point to new physics that have yet to be understood.

As the CMB passes through galaxy clusters, the clusters effectively leave "shadows" that allow astronomers to identify the most massive clusters in the universe, nearly independent of their distance.

"Clusters of galaxies are the most massive, rare objects in the universe, and therefore they can be effective probes to study physics on the largest scales of the universe," said John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics, who heads the SPT collaboration.

"The unsurpassed sensitivity and resolution of the CMB maps produced with the South Pole Telescope provides the most detailed view of the young universe and allows us to find all the massive clusters in the distant universe," said Christian Reichardt, a postdoctoral researcher at the University of California, Berkeley and lead author of the new SPT cluster catalog paper.

The number of clusters that formed over the history of the universe is sensitive to the mass of the neutrinos and the influence of dark energy on the growth of cosmic structures.

"Neutrinos are amongst the most abundant particles in the universe," Benson said. "About one trillion neutrinos pass through us each second, though you would hardly notice them because they rarely interact with 'normal' matter."

The existence of neutrinos was proposed in 1930. They were first detected 25 years later, but their exact mass remains unknown. If they are too massive they would significantly affect the formation of galaxies and galaxy clusters, Benson said.

The SPT team has been able to improve estimates of neutrino masses, yielding a value that approaches predictions stemming from particle physics measurements.

"It is astounding how SPT measurements of the largest structures in the universe lead to new insights on the evasive neutrinos," said Lloyd Knox, professor of physics at the University of California at Davis and member of the SPT collaboration. Knox will also highlight the neutrino results in his presentation on Neutrinos in Cosmology at a special session of the APS on Tuesday, April 3.

NSF's Office of Polar Programs primarily funds the SPT. The NSF-funded Physics Frontier Center of the Kavli Institute for Cosmological Physics, the Kavli Foundation and the Gordon and Betty Moore Foundation provide partial support.

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