Penn Astronomers Contribute to the Most Accurate Measurement of Dark Matter Structure in the Universe

For the past four years, as part of a project called the Dark Energy Survey, a team of scientists from around the globe has aimed one of the world’s most powerful digital cameras towards the sky with the hopes of answering fundamental questions about the universe.

“One of the things that the Dark Energy Survey is investigating is the 20-year-old discovery that the expansion of the universe is accelerating, which wouldn't happen in a ‘normal’ universe,” said Gary Bernstein, Reese W. Flower Professor of Astronomy and Astrophysics in the School of Arts & Sciences at the University of Pennsylvania and Project Scientist for DES. “There's something in the universe, which we call dark energy, that's causing this space to basically repel itself and grow faster and faster.”

Penn astronomers are playing an integral role in this team and its mission. The survey officially began in August of 2013, the culmination of 10 years of planning, building and testing by more than 400 scientists from 26 institutions in seven countries.

In a presentation today at the American Physical Society Division of Particles and Fields meeting at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, DES scientists will unveil the most accurate measurement ever made of the present large-scale structure of the universe.

Most notably, this result supports the theory that 26 percent of the universe is in the form of mysterious dark matter and that space is filled with an also-unseen dark energy, which is causing the accelerating expansion of the universe and makes up 70 percent.

Dark matter is invisible to even the most sensitive astronomical instruments because it does not emit or block light. DES scientists used two methods to measure dark matter. First, they created maps of galaxy positions as tracers, and, second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of dark matter over billions of light years using a technique called gravitational lensing.

“As light travels from a distant galaxy towards the telescope,” said Lucas Secco, a Ph.D. student at Penn, “it gets bent and changes its trajectory based on the matter that it finds on its way to us. And that matter is not only the luminous stuff that we see but also includes a lot of dark matter. The gravitational lensing is measured by the distortions it causes to the galaxy’s appearance.”

The DES team developed new ways to detect the tiny lensing distortions of galaxy images, an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals.

Bhuvnesh Jain, Walter H. and Leonore C. Annenberg Professor in the Natural Sciences in the Department of Physics and Astronomy and Secco's advisor, served as co-coordinator of the gravitational lensing group. Research-staff member Mike Jarvis is a lead developer of the Survey’s “lensing pipeline,” which reduces each of the galaxies imaged with the Dark Energy Camera to its vital stats via computer algorithms, revealing the gravitational lensing signatures.

“A lot of the basic ideas that go into making these weak gravitational lensing measurements,” Bernstein said, “were developed at Penn. That’s one of our specialties.”

Secco contributed to the calculations combining the gravitational lensing data with the clustering of galaxies to reach a set of cosmological parameters.

“My part in this paper,” he said, “was to do the proper statistical combination of the parameters that DES found with the parameters that people have found using other completely different experiments, such as measurements of the cosmic microwave background and sets of supernovae from previous surveys, to reach the final parameter constraints for all of these probes.”

The measurements of the amount and “clumpiness” (or distribution) of dark matter in the present-day cosmos were made with a precision that, for the first time, rivals that of inferences from the early universe by the European Space Agency’s orbiting Planck observatory. The new DES result is close to “forecasts” made from the Planck measurements of the distant, allowing scientists to understand more about the ways the universe has evolved during 14 billion years.

“This result is beyond exciting,” said Scott Dodelson of Fermilab, one of the lead scientists on this result. “For the first time, we’re able to see the current structure of the universe with the same clarity that we can see its infancy, and we can follow the threads from one to the other, confirming many predictions along the way.”

Paradoxically, it is easier to measure the large-scale clumpiness of the universe in the distant past than it is to measure it today. In the first 400,000 years following the Big Bang, the universe was filled with a glowing gas, the light from which survives to this day. Planck’s map of this cosmic microwave background radiation gives us a snapshot of the universe at that very early time. Since then, the gravity of dark matter has pulled mass together and made the universe clumpier over time. But dark energy has been fighting back, pushing matter apart. Using the Planck map as a start, cosmologists can calculate precisely how this battle plays out over 14 billion years.

“The DES measurements, when compared with the Planck map, support the simplest version of the dark matter/dark energy theory,” said Joe Zuntz of the University of Edinburgh, who worked on the analysis. “The moment we realized that our measurement matched the Planck result within 7 percent was thrilling for the entire collaboration.”

The primary instrument for DES is the 570-megapixel Dark Energy Camera, one of the most powerful in existence, able to capture digital images of light from galaxies eight billion light years from Earth. The camera was built and tested at Fermilab, the lead laboratory on the Dark Energy Survey, and is mounted on the National Science Foundation’s 4-meter Blanco telescope, part of the Cerro Tololo Inter-American Observatory in Chile, a division of the National Optical Astronomy Observatory. The DES data are processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

The Dark Energy Camera is able to see light from more than 100,000 galaxies as far as 8 billion light years away in each snapshot. Every week, the camera images several million distant galaxies, systematically mapping one-eighth of the sky in unprecedented detail.

The fifth and final year of observation for DES will begin this month. The new results released today draw only from data collected during the survey’s first year, which covers one-thirtieth of the sky.

“These results come from just the first year of what is to be a five-year survey, and we're about to start the fifth year next month,” Bernstein said, “so we've already started analyzing data that is more than three times as much as we released today. And these experiments get more accurate the more sky you use.”

In the process of obtaining these results, the scientists created the largest guide to spotting dark matter in the cosmos ever drawn. The new dark matter map is 10 times the size of the one DES released in 2015 and will eventually be three times larger than it is now.

According to Masao Sako, an associate professor at Penn who is working on a different aspect of DES that uses supernovae, stellar explosions, to probe dark energy, these results tell us that we understand the universe but we really know nothing about it.

“We understand how the universe was expanding pretty well,” he said, “but we still don't know what makes up this mysterious thing that's pushing the galaxies apart, and we also don't know what dark matter is, even though we understand what it's doing and we're pretty sure it's there.”

For Bernstein, one of the most exciting aspects of these results is seeing that the project, which is the culmination of years of development and collaboration, actually works and does what it’s supposed to do.

“We've been talking about doing this project for 20 years,” he said, “and this is the first time that I feel like we're really pushing the frontier of knowledge with this weak gravitational lensing technique. This result is a warm-up for the full power that we're going to get from DES.”

The Dark Energy Survey is supported by funding from the U.S. Department of Energy Office of Science; the National Science Foundation; funding agencies in the United Kingdom, Spain, Brazil, Germany and Switzerland; and the participating institutions.

These results and others from the first year of the Dark Energy Survey will be released today online at and announced during a talk by Daniel Gruen, NASA Einstein Fellow at the Kavli Institute for Particle Astrophysics and Cosmology at DOE’s SLAC National Accelerator Laboratory, at 5 p.m. Central time. The talk is part of the APS Division of Particles and Fields meeting at Fermilab and will be live-streamed at

The results will also be presented by Kavli Fellow Elisabeth Krause at the TeV Particle Astrophysics Conference in Columbus, Ohio, on Aug. 9, and by Michael Troxel, postdoctoral fellow at the Center for Cosmology and Astro-Particle Physics at Ohio State University, at the International Symposium on Lepton Photon Interactions at High Energies in Guanzhou, China, on Aug. 10. All three of these speakers are coordinators of DES science working groups and make key contributions to the analysis.

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