Cosmologists have produced an enormous map of the distribution of dark matter in our Universe, tracing the invisible substance by monitoring its gravitational effects on light.
The picture, which maps clumps and voids of dark matter in a patch of sky covering around two million galaxies and showing features hundreds of millions of light years across, was presented by Chihway Chang of the Swiss Federal Institute of Technology in Zurich on April 13 at a meeting of the American Physical Society in Baltimore, Maryland. The researchers also describe their results in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
Dark matter is about five times more abundant than the matter we can see. Although invisible, its presence can be detected because it curves space-time. Large concentrations of dark matter in the foreground bend the light coming from background galaxies, skewing our images of them. This effect, known as 'weak gravitational lensing', has so far been studied for its applications to understanding dark matter in single clusters of galaxies, but the latest survey has upped the scale.
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Using a 570-megapixel camera at the Victor M. Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, the researchers photographed about two million galaxies in a large patch of southern sky (from the perspective of a viewer on Earth, the patch is about the area of 700 Suns). They pinpointed small distortions in the shapes of the most distant galaxies in order to map the mass of dark matter in the foreground.
The observations fit the standard picture of cosmology strikingly well, as dark matter is thought to be the main driver in the formation of large-scale cosmic structures, says Chang. “We identify some truly massive structures, about 100 million light years across,” she adds, referring to the map's giant voids (blue regions) and superclusters of dark matter (red regions). Follow-up studies will be able to confirm if those are genuine structures, she says.
The map constitutes only 3% of the region of the sky that the project, known as the Dark Energy Survey (DES), will cover by the time that it is concluded in 2018. It is named after dark energy – not dark matter – because its ultimate goal is to chart the distribution of matter well enough to measure dark energy, the enigmatic force that is pushing the Universe to expand at an accelerating pace.
In the past, weak gravitational lensing has produced some of the most convincing evidence of the existence of dark matter. In a landmark 2006 study of two colliding clusters of galaxies – known collectively as the ‘bullet cluster’ -- a team found that the region with the strongest lensing was markedly offset from the bulk of visible matter, evidence that dark matter was bending space-time. In a paper published in Science last month, David Harvey of the Swiss Federal Institute of Technology of Lausanne (EPFL) and his collaborators published similar evidence for 72 clusters of galaxies, the largest collection so far. “Surveys such as DES will ultimately find many more colliding clusters similar to those in our study,” he says.
The effects of gravitational lensing caused by dark matter can be spotted not only by looking at light from distant galaxies, but also in the Universe's most ancient light - the cosmic microwave background. In a study published on April 13, researchers describe using the Atacama Cosmology Telescope in Chile to measure these distortions to map the distributions of dark matter around 12,000 galaxies.
This article is reproduced with permission and was first published on April 13, 2015.