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Seeing the big cosmic picture

A new map of the universe confirms previous results, including the existence of dark matter and dark energy.
Sloan Digital Sky Survey Telescope
This 2.5-meter telescope is the main workhorse of the Sloan Digital Sky Survey. Its box-shaped structure protects it against the wind.
SDSS Collaboration
November 9, 2003
Similar to medieval maps of Earth that included both speculations of the unknown and concrete scientific observations, cosmologists today are slowly refining our understanding of the structure and evolution of the universe as a whole. Now, a team of international astronomers has announced they have created a 2-billion-light-year-deep survey map that reveals the most precise makeup of the universe to date.

Mining data from the Sloan Digital Sky Survey (SDSS), the astronomers made precise measurements of the pattern of large-scale clustering of galaxies and associated dark matter. By plotting the three-dimensional positions of 205,443 galaxies spread over six percent of the sky, a group led by Max Tegmark of the University of Pennsylvania obtained the sharpest-ever views of gravitational-clustering patterns between galaxies. While a clumpy variation in density appears on a scale of millions of light-years, a more uniform picture of the universe emerges on larger scales.

The results are slated for publication in the Astrophysical Journal and Physical Review D as two separate papers, and the authors have pegged the overall breakdown of cosmic matter at 70 percent dark energy, 25 percent dark matter, and only 5 percent ordinary matter composed of atoms.
3-d galaxy map
The Sloan Digital Sky Survey is two separate surveys in one. Galaxies are identified in 2-D images (right), then have their distance determined from their spectra to create a 2- billion-light-year-deep 3-D map (left) where each galaxy is shown as a single point. The color represents the galaxy's luminosity. This 3-D map shows only 66,976 out of 205,443 galaxies that lie near the plane of Earth's equator.
Sloan Digital Sky Survey
Relieving many astronomers' angst, these latest numbers precisely match both previous theoretical predications and findings released earlier this year from the Wilkinson Microwave Anisotropy Probe (WMAP). Images taken by the WMAP team measured the cosmic microwave background radiation left behind in the aftermath of the Big Bang. By combining the WMAP measurements with those from SDSS, the newly released galaxy-mapping data have put finer constraints on cosmological parameters like the age, structure, and expansion of the universe, more than halving the uncertainties from WMAP.

"Different galaxies, different instruments, different people, and different analysis — but the results agree," explains Tegmark. "Extraordinary claims require extraordinary evidence, but we now have extraordinary evidence for dark matter and dark energy and have to take them seriously no matter how disturbing they seem."

Verifying previous inflation models, these new results also offer up critical evidence for the existence of dark energy and its critical role in the present-day expansion of the universe. But as co-investigator David Weinburg from Ohio State University points out, "The real challenge now is to figure out what these mysterious substances actually are."
Andrew Fazekas is an astronomy columnist based in Montreal, Canada, who frequently writes for magazines, newspapers, and the Canadian Space Agency. He currently does science news commentary for both radio and television, teaches backyard astronomy at Vanier College, and is an editor at the American Association for the Advancement of Science.

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