Dark Energy Survey completes six-year mission

Scientists' effort to map a portion of the sky in unprecedented detail is coming to an end, but their work to learn more about the expansion of the universe has just begun.

January 8, 2019

The following news release is being issued today by the U.S. Department of Energy's (DOE) Fermi National Accelerator Laboratory (Fermilab) on behalf of the Dark Energy Survey collaboration. Erin Sheldon, an astrophysicist at DOE’s Brookhaven National Laboratory, has worked on DES since the planning stages in 2004.  Sheldon performed the basic measurements used to infer the gravitational lensing signal, which is the key probe of dark energy used for the DES Year 1 cosmology results. With the final DES data in hand, the collaboration is poised to make the most precise measurements of dark energy to date. These pioneering steps will lay the foundation for future dark energy experiments, including those based on observations from the Large Synoptic Survey Telescope (LSST). LSST is a next-generation dark energy experiment funded by the National Science Foundation, DOE, and others. Brookhaven scientists have developed and built sensor elements for LSST and are heavily involved in preparation for the data analysis. For more information about Brookhaven's roles in these experiments or to speak with a Brookhaven Lab scientist, contact Karen McNulty Walsh, 631 344-8350, kmcnulty@bnl.gov.

Andre Salles, Fermilab Office of Communication, 630-840-3351, media@fnal.gov

Dark Energy Survey had mapped one-eighth of the full sky enlarge

By August 2015, the Dark Energy Survey had mapped one-eighth of the full sky (red shaded region) using the Dark Energy Camera on the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile (foreground). This map has led to the discovery of 17 dwarf galaxy candidates, including several in close proximity to the two largest dwarf galaxies orbiting the Milky Way, the Large and Small Magellanic Clouds, both of which are visible to the unaided eye. By comparison, the new stellar systems are so faint that they are difficult to "see" even in the deep DES images and can be more easily visualized using maps of the stellar density (inset). Credit: Dark Energy Survey Collaboration

After scanning in depth about a quarter of the southern skies for six years and cataloguing hundreds of millions of distant galaxies, the Dark Energy Survey (DES) will finish taking data tomorrow, on Jan. 9.

The survey is an international collaboration that began mapping a 5,000-square-degree area of the sky on Aug. 31, 2013, in a quest to understand the nature of dark energy, the mysterious force that is accelerating the expansion of the universe. Using the Dark Energy Camera, a 520-megapixel digital camera funded by the U.S. Department of Energy Office of Science and mounted on the Blanco 4-meter telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory in Chile, scientists on DES took data on 758 nights over six years.

Over those nights, they recorded data from more than 300 million distant galaxies. More than 400 scientists from over 25 institutions around the world have been involved in the project, which is hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The collaboration has already produced about 200 academic papers, with more to come.

According to DES Director Rich Kron, a Fermilab and University of Chicago scientist, those results and the scientists who made them possible are where much of the real accomplishment of DES lies.

“First generations of students and postdoctoral researchers on DES are now becoming faculty at research institutions and are involved in upcoming sky surveys,” Kron said. “The number of publications and people involved are a true testament to this experiment. Helping to launch so many careers has always been part of the plan, and it’s been very successful.”

DES remains one of the most sensitive and comprehensive surveys of distant galaxies ever performed. The Dark Energy Camera is capable of seeing light from galaxies billions of light-years away and capturing it in unprecedented quality. 

Map of dark matter enlarge

Map of dark matter made from gravitational lensing measurements of 26 million galaxies in the Dark Energy Survey. The map covers about 1/30th of the entire sky and spans several billion light-years in extent. Red regions have more dark matter than average, blue regions less dark matter. Credit: Chihway Chang of the Kavli Institute for Cosmological Physics at the University of Chicago and the DES collaboration

According to Alistair Walker of the National Optical Astronomy Observatory, a DES team member and the DECam instrument scientist, equipping the telescope with the Dark Energy Camera transformed it into a state-of-the-art survey machine. 

“DECam was needed to carry out DES, but it also created a new tool for discovery, from the solar system to the distant universe,” Walker said. “For example, 12 new moons of Jupiter were recently discovered with DECam (https://phys.org/news/2018-07-jupiter-moon-tiny-oddball.html), and the detection of distant star-forming galaxies in the early universe, when the universe was only a few percent of its present age, has yielded new insights into the end of the cosmic dark ages (https://www.noao.edu/news/2017/pr1703.php).”

The survey generated 50 terabytes (that’s 50 million megabytes) of data over its six observation seasons. That data is stored and analyzed at the National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign.

“Even after observations are ended, NCSA will continue to support the scientific productivity of the collaboration by making refined data releases and serving the data well into the 2020s,” said Don Petravick, senior project manager for the Dark Energy Survey at NCSA.

Now the job of analyzing that data takes center stage. DES has already released a full range of papers based on its first year of data, and scientists are now diving into the rich seam of catalogued images from the first several years of data, looking for clues to the nature of dark energy.

The first step in that process, according to Fermilab and University of Chicago scientist Josh Frieman, former director of DES, is to find the signal in all the noise.

“We’re trying to tease out the signal of dark energy against a background of all sorts of noncosmological stuff that gets imprinted on the data,” Frieman said. “It’s a massive ongoing effort from many different people around the world.” 

The DES collaboration continues to release scientific results from their storehouse of data, and scientists will discuss recent results at a special session at the American Astronomical Society winter meeting in Seattle today, Jan. 8. DES will also host an interactive event from 2:30 to 3:30 p.m. Pacific time on Wednesday, Jan. 9, at the NOAO booth in the main exhibit hall of the AAS meeting, featuring a live connection to the observatory in Chile as scientists prepare for the final night of data-taking.

Scientific highlights from previous years include:

DES scientists also spotted the first visible counterpart of gravitational waves ever detected, a collision of two neutron stars that occurred 130 million years ago. (http://news.fnal.gov/2017/10/scientists-spot-explosive-counterpart-ligovirgos-latest-gravitational-waves) DES was one of several sky surveys that detected this gravitational wave source, opening the door to a new kind of astronomy.

Recently DES issued its first cosmology results based on supernovae (207 of them taken from the first three years of DES data) using a method that provided the first evidence for cosmic acceleration 20 years ago. (http://adsabs.harvard.edu/abs/2018arXiv181102374D) More comprehensive results on dark energy are expected within the next few years.

The task of amassing such a comprehensive survey was no small feat. Over the course of the survey, hundreds of scientists were called on to work the camera in nightly shifts supported by the staff of the observatory. To organize that effort, DES adopted some of the principles of high-energy physics experiments, in which everyone working on the experiment is involved in its operation in some way.

“This mode of operation also afforded DES an educational opportunity,” said Fermilab scientist Tom Diehl, who managed the DES operations. “Senior DES scientists were paired with inexperienced ones for training, and in time would pass that knowledge on to more junior observers.”

The organizational structure of DES was also designed to give early-career scientists valuable opportunities for advancement, from workshops on writing research proposals to mentors who helped review and edit grant and job applications.

Antonella Palmese, a postdoctoral researcher associate at Fermilab, arrived at Cerro Tololo as a graduate student from University College London in 2015. She quickly came up to speed and returned in 2017 and 2018 as an experienced observer. She also served as a representative for early-career scientists, helping to assist those first making their mark with DES.

“Working with DES has put me in contact with many remarkable scientists from all over the world,” Palmese said. “It’s a special collaboration because you always feel like you are a necessary part of the experiment. There is always something useful you can do for the collaboration and for your own research.”

The Dark Energy Camera will remain mounted on the Blanco telescope at Cerro Tololo for another five to 10 years and will continue to be a useful instrument for scientific collaborations around the world. Cerro Tololo Inter-American Observatory Director Steve Heathcote foresees a bright future for DECam. 

"Although the data-taking for DES is coming to an end, DECam will continue its exploration of the universe from the Blanco telescope and is expected remain a front-line engine of discovery for many years,” Heathcote said.

The DES collaboration will now focus on generating new results from its six years of data, including new insights into dark energy. With one era at an end, the next era of the Dark Energy Survey is just beginning.

Follow the Dark Energy Survey online at www.darkenergysurvey.org and connect with the survey on Facebook at www.facebook.com/darkenergysurvey, on Twitter at www.twitter.com/theDESurvey and on Instagram at www.instagram.com/darkenergysurvey.

The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funding for the DES Projects has been provided by the U.S. Department of Energy Office of Science, U.S. National Science Foundation, Ministry of Science, Innovation and Universities of Spain, Science and Technology Facilities Council of the United Kingdom, Higher Education Funding Council for England, ETH Zurich for Switzerland, National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, Kavli Institute of Cosmological Physics at the University of Chicago, Center for Cosmology and AstroParticle Physics at Ohio State University, Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, 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 Ministério da Ciência e Tecnologia, Deutsche Forschungsgemeinschaft, and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration. 

Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. NSF is an independent federal agency created by Congress in 1950 to promote the progress of science. NSF supports basic research and people to create knowledge that transforms the future.

NCSA at the University of Illinois at Urbana-Champaign provides supercomputing and advanced digital resources for the nation’s science enterprise. At NCSA, University of Illinois faculty, staff, students and collaborators from around the globe use advanced digital resources to address research grand challenges for the benefit of science and society. NCSA has been advancing one third of the Fortune 50® for more than 30 years by bringing industry, researchers and students together to solve grand challenges at rapid speed and scale. For more information, please visit www.ncsa.illinois.edu.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC, a joint partnership between the University of Chicago and the Universities Research Association Inc. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

2019-14314  |  INT/EXT  |  Newsroom