Penn Researchers Help Make Maps of the Universe’s Dark Matter

Members of the Dark Energy Survey have released the first in a series of maps that show the concentration of dark matter in the cosmos. These maps, created with one of the world’s most powerful digital cameras, are the largest contiguous maps created at this level of detail and will improve researchers’ understanding of dark matter’s role in the formation of galaxies. Analysis of the distribution of the dark matter in the maps will also allow scientists to probe the nature of dark energy, the mysterious force believed to be causing the accelerating expansion of the universe.

The analysis that produced these maps was led by Vinu Vikram, who conducted the work while a postdoctoral researcher at the University of Pennsylvania and is now at Argonne National Laboratory, along with Chihway Chang of the Swiss Federal Institute of Technology in Zurich.

The two-year long effort was centered in the research group of Bhuvnesh Jain, professor and Edmund J. and Louise W. Kahn Term Chair in the Natural Sciences in the Department of Physics and Astronomy at Penn’s School of Arts & Sciences, and involved DES collaborators at Zurich, the University of Portsmouth the University of Manchester, Stanford University and other DES institutions.

Jain and Vikram’s colleagues at Penn include Gary Bernstein, the Reese W. Flower Professor of Astronomy and Astrophysics; research scientist Mike Jarvis; graduate student Dillon Brout; and undergraduates Andrew Neil and Charles Davis. Bernstein is the Project Scientist of the survey and has designed key elements of the survey and its algorithms. Jarvis led the first stage of the galaxy image analysis; the mass maps and other lensing results rely on the elegant methods developed by his team.

The new maps were released at the April meeting of the American Physical Society. They were created using data captured by the Dark Energy Camera, a 570-megapixel imaging device that is the primary instrument for the Dark Energy Survey. The collaboration includes more than 300 scientists from 25 institutions in six countries.

Dark matter, the substance that makes up roughly a quarter of the universe, is invisible to even the most sensitive astronomical instruments because it does not emit or block light. Its effects, however, can be seen using a technique called gravitational lensing, which involves studying the distortion that occurs when the gravitational pull of dark matter bends light emitted by distant galaxies. Understanding the role of dark matter is part of the research program to quantify the role of dark energy, which is the ultimate goal of the survey.

“We measured the barely perceptible distortions in the shapes of about two million galaxies to construct these new maps,” Vikram said. “They are a testament not only to the sensitivity of the Dark Energy Camera but also to the rigorous work by our lensing team to understand its sensitivity so well that we can get exacting results from it.”

The team’s dark matter maps make use of early DES observations and cover only about 3 percent of the area of sky DES will document over its five-year mission. The survey has just completed its second year. As scientists expand their search, they’ll be able to better test current cosmological theories by comparing the amounts of dark and visible matter.

Those theories suggest that, since there is much more dark matter in the universe than visible matter, galaxies will form where there are large concentrations of dark matter, and thus stronger gravity, present. So far, the DES analysis corroborates this: the maps show large filaments of matter along which visible galaxies and galaxy clusters lie and cosmic voids where very few galaxies reside.

“The cosmic background radiation left over from the Big Bang has ‘hot’ and ‘cold’ spots over the whole sky,” Jain said. “Starting from these primordial ripples in the early universe, gravity led to the growth of filaments and the emptying out of the voids we see in the maps.”

“As the universe ages,” Bernstein said, “these structures get more prominent. Mass attracts mass, and so the rich gets richer. And because most of this mass is dark matter, its concentration tells you where galaxies are more likely to form. That we see galaxy clusters in the same places where the gravitational lensing analysis tells us the dark matter is most concentrated demonstrates that this story makes sense.”

Follow-up studies of some of the enormous filaments and voids, and the enormous volume of data, collected throughout the survey will reveal more about this interplay of mass and light.

“Our analysis so far is in line with what the current picture of the universe predicts,” Chang said. “Zooming into the maps, we have measured how dark matter envelops galaxies of different types and how together they evolve over cosmic time. We are eager to use the new data coming in to make much stricter tests of theoretical models.”

The Dark Energy Survey analysis is available here.

Penn’s work on the Dark Energy Survey is supported by the School of Arts & Sciences, the Department of Energy and the National Science Foundation.

The Dark Energy Camera was constructed and tested at the U.S. Department of Energy’s Fermi National Accelerator Laboratory and is now mounted on the 4-meter Victor M. Blanco Telescope at the National Optical Astronomy Observatory’s Cerro Tololo Inter-American Observatory in Chile. The data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

Funding for the DES Projects has been provided by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência e Tecnologia, the Deutsche Forschungsgemeinschaft and the collaborating institutions in the Dark Energy Survey. The DES participants from Spanish institutions are partially supported by MINECO under grants AYA2012-39559, ESP2013-48274, FPA2013-47986 and Centro de Excelencia Severo Ochoa SEV-2012-0234, some of which include ERDF funds from the European Union.

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