Dark energy is a hypothetical force used to account for whatever is accelerating the expansion rate of the Universe. It is believed to constitute approximately 75 percent of the mass and energy of the Universe. A range of competing theories for the cause of dark energy have emerged since of the discovery of the acceleration in 1998. In an effort to evaluate those theories, astronomers have endeavored to perfect the measurement of large-scale cosmic structures over the years (e.g. galaxy clusters).

        Sound waves in the extremely early Universe are thought to have left detectable imprints on the large-scale distribution of galaxies and gas in the Universe. The galaxies, separated by immense voids, form a foam-like structure known as the “cosmic web”. By measuring how such large-scale structures have changed over the last few billion years, scientists hope to gain insight which theory of dark energy is the most accurate.

        The mainstream approach to map the cosmic web involves searching for hydrogen gas in these individual, distant galaxies through the observation of their radio emission. However, this often proves to be a daunting challenge beyond the technical capabilities of current instruments, as radio waves of those distant galaxies generally are too faint to be individually detected.

        In this study, the team used an alternative method, namely measuring the aggregate radio emission  from many unresolved galaxies in the cosmic web. For their study, the researchers used the Robert C. Byrd Green Bank Telescope (GBT) located at the site of the National Radio Astronomy Observatory (NRAO) in the US, and were able to find and map the hydrogen gas in many galaxies at once.  

       More importantly, the astronomers developed new techniques that removed both man-made radio interference and radio emission from nearer astronomical sources, to leave the extremely faint radio waves coming from the very distant hydrogen gas. The result was a map of part of the “cosmic web” that correlated neatly with the structure shown by the earlier optical study.

      "Our project mapped hydrogen gas to greater cosmic distances than ever before, and showed that the techniques we developed could be used to map huge volumes of the Universe in three dimensions and to test the competing theories of dark energy,” said Dr. Chang. “These observations detected more than all the hydrogen gas previously detected in the Universe, and to distances ten times farther than any radio wave-emitting hydrogen seen before,” she added.

       The GBT is presently the world’s largest fully steerable radio telescope. The NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

       The full article entitled “Hydrogen 21-cm Intensity Mapping at Redshift 0.8” is available online at the Nature website at: http://www.nature.com/nature/journal/v466/n7305/full/nature09187.html.