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South Pole Telescope data shedding light on dark energy

Published: 2 Apr 2012

Results provide fresh support for Einstein’s cosmological constant

Results provide fresh support for Einstein's cosmological constant

Analysis of data from the 10-meter South Pole Telescope is providing 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 data strongly support Albert Einstein's cosmological constant - the leading model for dark energy.

The results also are beginning to hone in on the masses of the neutrinos, the most abundant particles in the universe, which until recently were thought to be without mass. A series of papers detailing the SPT findings have been submitted to the Astrophysical Journal.

"The results released to date are just the beginning of what we'll be able to accomplish with the South Pole Telescope - the present analyses are based on only 100 of the over 500 galaxy clusters we've detected so far. We can expect much tighter constraints on dark energy and the neutrino masses with the full data set," said McGill University physics professor Gil Holder.

McGill Prof. Matt Dobbs, postdoctoral scientist Keith Vanderlinde, and graduate student Tijmen de Haan recently returned from the geographic South Pole after having installed on the telescope a new detector readout system, developed and built at McGill, the only Canadian university partner in the project. This electronics system, together with new detector technology, will allow the telescope to search for signatures produced a fraction of a second after the big bang, and refine the measurements of matter and neutrino properties.

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 result of space having energy, even when it is free of matter and radiation.  This energy of empty space, called the cosmological constant, was originally hypothesized by Einstein in order to explain why the Universe was static and not collapsing; he later considered this to be one of his greatest blunders after learning that the universe is not static, but expanding.

In the late 1990s, astronomers discovered that the expansion of the universe appeared to be accelerating according to cosmic-distance measurements based on the relatively uniform brightness of exploding stars. 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 are pointing to new physics that has yet to be understood.

The SPT was specifically designed to tackle the dark energy mystery. The 10-meter telescope operates at millimeter wavelengths to make high-resolution images of the cosmic microwave background (CMB), the light left over from the big bang. Scientists use the CMB in their search for distant, massive galaxy clusters that can be used to pinpoint the mass of the neutrino and the properties of dark energy.

"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," said Bradford Benson, a postdoctoral scientist at the University of Chicago's Kavli Institute for Cosmological Physics. "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." Benson presented the SPT collaboration's latest findings, Sunday, April 1, at the American Physical Society meeting in Atlanta.

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. McGill graduate student Tijmen de Haan was one of the lead authors on the paper (http://arxiv.org/abs/1112.5435) submitted to the Astrophysical Journal analysing galaxy clusters with a combination of SPT data and images recorded by x-ray satellites. de Haan explains, "These measurements reveal how many clusters formed throughout the history of the universe. These are the largest gravitationally collapsed objects in the universe. Their growth rate is sensitive to the mass of the neutrinos and the influence dark energy has on the growth of cosmic structures. They reveal the constituent building blocks of the universe."

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.

The SPT team has now placed tight limits on the neutrino masses, yielding a value that approaches predictions stemming from particle physics measurements.

The SPT survey is also being used to make maps of the distribution of matter in the universe by measuring subtle shifts in apparent position on the sky of the cosmic microwave background with unprecedented accuracy. McGill graduate student Alex van Engelen, who was the lead author on a recent paper (http://arxiv.org/abs/1202.0546) submitted to the Astrophysical Journal presenting the most precise measurement of this effect to date, explains that "the shifts are caused by the gravitational force from these mass fluctuations, which are primarily made of dark matter."

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

McGill researchers participating in the South Pole Telescope collaboration include faculty members Matt Dobbs and Gil Holder; postdoctoral scientists Amy Bender and Keith Vanderlinde; and graduate students Tijmen de Haan, Jon Dudley, Alex van Engelen, and James Kennedy.

The SPT is funded primarily by the United States National Science Foundation's Office of Polar Programs. Partial support is also provided by the NSF-funded Physics Frontier Center of the KICP, the Kavli Foundation and the Gordon and Betty Moore Foundation. The Canadian team receives support from Natural Sciences and Engineering Research Council, Canadian Institute for Advanced Research, and Canada Research Chairs program.


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