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Monday, November 28, 2005

Supernova Survey Provides New Data on the Dark Energy that Accelerates the Universe

Dark energy is an antigravitational force that is causing space to expand at an ever-faster pace, but scientists don't know what it is. One possibility is a cosmological constant (back), like a field of springs covering every point in space, all the same length and motionless. Another possibility is quintessence (front), also like a field of springs but stretched to different lengths at different times and places.

The first results from the Supernova Legacy Survey (SNLS), a major new study using distant supernovae to measure cosmological parameters, puts strong constraints on how much the antigravitational pressure of dark energy has varied during the expansion of the universe. An article describing the results will soon be published in the journal Astronomy & Astrophysics and is now available online as a downloadable pdf file.

The SNLS is an ongoing study conducted by an international team of some 40 researchers from France, Canada, the United States, the United Kingdom, Portugal, Chile, and Sweden. Many of the team members and coauthors of the Astronomy & Astrophysics article are members of the international Supernova Cosmology Project based at Berkeley Lab and headed by Saul Perlmutter of the Physics Division, who is also a professor of physics at the University of California at Berkeley.

In the first year of a planned five-year study, the SNLS team analyzed the brightness and spectra of 71 Type Ia supernovae at distances of from two to eight billion light years to see what they indicated about the changing rate of expansion of the universe. The results are consistent, to within 10 percent, with acceleration caused by a form of dark energy resembling Albert Einstein's cosmological constant, an arbitrary term Einstein introduced in early formulations of General Relativity to keep his equations from predicting that the universe would collapse under its own gravity; he later dropped the term when the universe was found to be expanding.

"These are tantalizing results that whet our appetite for future data from this and other supernova studies," says Perlmutter.

Since the discovery that the universe is expanding at an accelerating rate, announced by the Supernova Cosmology Project and the rival High-Z Supernova Search Team in 1998, theories have multiplied about what kind of mysterious, invisible something could fill space and overcome the mutual gravitational attraction of all the matter in the universe to push the galaxies farther apart.

Most theories fall into one of two groups: either dark energy is a property of space itself and has always exerted the same pressure -- thus, a cosmological constant -- or that it has changed over time through an agency dubbed quintessence. The measurable difference between these two kinds of phenomena would be very small, and the best way to detect the difference is by comparing the distance and redshifts of very far-off objects, especially supernovae, to high accuracy.

Perlmutter says the SNLS findings kick off a dramatic new generation of cosmology work using supernovae. "The data are more beautiful than we could have imagined 10 years ago -- a real tribute to the instrument builders, the analysis teams, and the large scientific vision of the Canadian and French science communities."

The SNLS uses images from the Canada-France-Hawaii Telescope (CFHT) atop Mauna Kea in Hawaii taken with a wide-field, 340-million-pixel camera called the Megacam. Spectra of candidate supernovae are then acquired with large telescopes including the Gemini North Telescope and Keck Telescopes on Mauna Kea and the Gemini South Telescope and European Southern Observatory/Very Large Telescope in Chile. The current paper is based on about one-tenth of the imaging data that will be obtained by the end of the survey.

The SNLS first-year findings are described in press releases from the University of Toronto and the California Institute of Technology, as well as from Astronomy & Astrophysics.

-- Paul Preuss




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