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Dark Energy Theory

Closer to Unraveling the Mystery

Dark Energy

A team including Carnegie Mellon researchers has developed a tool for unlocking the mysteries of the universe. A new tool for mapping large cosmic structures has been validated by pioneering observations made by researchers — from Academia Sinica in Taiwan, Carnegie Mellon University and the University of Toronto.

The observations promise to provide valuable clues about the nature of the mysterious "dark energy" believed to constitute nearly three-fourths of the mass and energy of the universe. The findings will be published in the July 22 issue of Nature.

"Dark energy" is what is causing the universe to expand at an accelerating rate. While the acceleration was discovered in 1998, its cause remains unknown. Physicists have advanced competing theories to explain the acceleration and believe the best way to test those theories is to precisely measure large-scale cosmic structures.

Sound waves in the matter-energy soup of the extremely early universe are thought to have left detectable imprints on the large-scale distribution of galaxies in the universe. The researchers developed a way to measure such imprints by observing the radio emission of hydrogen gas.

When applied to greater areas of the universe, their technique — called intensity mapping — could reveal how such large-scale structure has changed over the last few billion years. Ultimately, it could help answer which theory of dark energy is the most accurate.

"Since the early part of the 20th century, astronomers have traced the expansion of the universe by observing galaxies," said Jeffrey Peterson, of CMU's Bruce and Astrid McWilliams Center for Cosmology. "Our new technique allows us to skip the galaxy-detection step and gather radio emissions from a thousand galaxies at a time, as well as all the dimly-glowing material between them."

The team made the observations using the National Science Foundation's giant Robert C. Byrd Green Bank Telescope (GBT).

To get their results, the researchers used the GBT to study a region of sky that previously had been surveyed in detail in visible light by the Keck II telescope in Hawaii. This optical survey used spectroscopy to map the locations of thousands of galaxies in three dimensions.

In the GBT survey, instead of looking for hydrogen gas in these individual, distant galaxies — a daunting challenge beyond the technical capabilities of current instruments — the team used their intensity-mapping technique to accumulate the radio waves emitted by the hydrogen gas in large volumes of space including many galaxies.

"Our project mapped hydrogen gas to greater cosmic distances than ever before, and shows that the techniques we developed can be used to map huge volumes of the universe in three dimensions and to test the competing theories of dark energy," said Tzu-Ching Chang, of the Academia Sinica in Taiwan and the University of Toronto.

In addition to Chang, Peterson and Ue-Li Pen of the University of Toronto, the research team also included Kevin Bandura, a graduate student at Carnegie Mellon.

Related Links: Bruce and Astrid McWilliams Center for Cosmology  |  Physics  |  Mellon College of Science

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