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Image courtesy NRAO/AUI  The GBT is the world's largest fully steerable radio telescope. It is located in Green Bank, West Virginia.

New maps made with the Green Bank Telescope (pictured) may help test dark energy theories.

Photograph courtesy NRAO/AUI

Ker Than

for National Geographic News

Published July 21, 2010

A newly developed technique could one day help astronomers use giant sound waves to test theories of dark energy, the mysterious force thought to be causing the universe to fly apart faster over time.

Called intensity mapping, the technique looks for unique radio emissions of hydrogen gas in galaxies and galaxy clusters to map the large-scale structure of the universe. Hydrogen is the lightest and most abundant element in the universe, and it tends to cluster around galaxies because of their strong gravitational pull.

Until now, astronomers have been mapping large cosmic structures by identifying the galaxies and clusters themselves—a process akin to mapping forests on Earth by counting individual trees. The new method is more like mapping forests by looking for large patches of green.

By speeding up large-scale mapping efforts, the method should reveal how structures in the universe have evolved since the big bang.

In particular, the new maps might help astronomers detect variations in matter distribution caused by enormous sound waves called baryon acoustic oscillations, or BAOs, which were created shortly after the big bang.

Using the cosmic sound waves as rulers to measure the size of the universe over time can help scientists understand how dark energy has affected the cosmos.

(Related: "Most Direct Evidence of Dark Energy Detected.")

Sound Waves as Cosmic Rulers

In the new proof-of-concept study, Tzu-Ching Chang of the Canadian Institute for Theoretical Astrophysics in Toronto and colleagues used the Robert C. Byrd Green Bank Telescope in West Virginia to pick up hydrogen radio emissions in or near thousands of galaxies at once.

The team was also able to measure signals emitted when the universe was only about seven billion years old. (Related: "Universe 20 Million Years Older Than Thought.")

"These observations detected more hydrogen gas than all the previously detected hydrogen in the universe, and at distances ten times farther than any radio wave-emitting hydrogen seen before," study co-author Ue-Li Pen, of the University of Toronto, said in a statement.

The team hopes the method, with refinement, will detect hydrogen emissions from even further back in time, perhaps before the formation of the very first galaxies.

And by scanning large enough regions of the sky, the method may uncover the imprints of BAOs propagating through the universe, the scientists say.

As BAOs traveled through the primordial gases of the early universe, they affected the matter they passed through, causing it to clump up in some places more than in others. This pattern should show up in the distribution of today's galaxies and galaxy clusters.

(Related: "New Proof Unknown "Structures" Tug at Our Universe.")

Because scientists know BAOs repeat every 450 million light-years, "one can measure the BAO signature at different [ages of the universe] and use it as a standard ruler to determine the size of the universe at that time," study leader Chang said.

"And because at later times the expansion of the universe is mostly driven by dark energy, BAO measurements at one or several [time points] can thus help study the properties of dark energy."

Current theories of dark energy, for example, state that the expansion rate is dependent on the density of matter in the universe. Since the early universe was smaller and more tightly packed, scientists hope the BAO data will show that the early universe's expansion rate was slower than it is now.

(Also see: "Dark Energy's Demise? New Theory Doesn't Use the Force.")

The intensity mapping research is detailed in this week's issue of the journal Nature.

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