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By Michael Lucibella
Image: Dark Energy Survey
Map of total (luminous and dark matter) mass distribution produced by the Dark Energy Survey team based on gravitational lensing of light from distant galaxies. Red indicates higher mass densities and blue the lower mass density regions. Size of moon is shown in upper left for comparison with the area of sky mapped.
APS April Meeting 2015, Baltimore — Scientists working on the Dark Energy Survey (DES) released a map of a section of the southern sky charting the location of matter based on its lensing effects on light from distant galaxies. The map, presented at the April Meeting, is the first preliminary analysis of results from the survey, and the first detailed picture of the distribution of matter in the universe out to about seven billion light years.
This new information about the distribution of dark matter complements the findings of existing surveys of luminous matter. “It … [includes] not only the galaxy and stars we know and love, but also the dark matter,” said Chihway Chang of the Swiss Federal Institute of Technology.
The map covers less than one percent of the total sky, but by the end of the five-year survey, the team hopes to have maps covering about a quarter of the southern sky. “The patch of sky is more than 20 times the size of the full moon across, which is a pretty substantial chunk of the universe,” Chang explained.
To create this map, the DES team observed about two million distant galaxies to find out how their shapes are distorted by the mass of both nearer galaxies and dark matter. Then the team analyzed these distorted shapes to assemble the map.
“This distortion information in fact tells us how the matter is distributed in front of us,” Chang said. “In most cases, galaxies don’t get bent that much, they only get slightly distorted because there is some form of foreground matter in front of … [them].”
Ultimately, the team wants to use these maps of dark matter to probe how the universe’s accelerated expansion, driven by dark energy, affected the structure of galaxies and galaxy clusters as they formed. Structures are formed by gravitational coalescence of matter, so accelerated expansion of the universe slows that process of formation.
“There are very well-defined ways that we can use such maps to calculate, as a function of time, to what extent … dark energy has slowed the expansion of structure,” said Bhuvnesh Jain of the University of Pennsylvania. “So to see that structure, we need to see the dark matter, for which we use lensing.”
The blue and red spots on the otherwise green map represent areas of greater or less lensing. Using other optical data, the researchers overlaid known galaxies to highlight areas that appeared to have more or less concentrations of dark matter than expected.
The team is now in its second year of collecting data from the 4-meter Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile.
“With this data and other data that we have in hand, we’ll be able to make a very impressive catalogue … because we’re going out much further than other studies of superclusters have gone,” Jain said.
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