In the culmination of a decade’s worth of effort, the Dark Energy Survey collaboration of scientists analyzed an unprecedented sample of nearly 1,500 supernovae classified using machine learning. They placed the strongest constraints on the expansion of the universe ever obtained with the DES supernova survey. While consistent with the current standard cosmological model, the results do not rule out a more complex theory that the density of dark energy in the universe could have varied over time.
DES scientists presented the results January 8 at the 243rd meeting of the American Astronomical Society and have submitted them for publication to the Astrophysical Journal.
The work is the output of over 400 DES scientists, including UW–Madison physics professor Keith Bechtol and former graduate student Robert Morgan, PhD ’22.
In 1998, astrophysicists discovered that the universe is expanding at an accelerating rate, attributed to a mysterious entity called dark energy that makes up about 70% of our universe. While foreshadowed by earlier measurements, the discovery was somewhat of a surprise; at the time, astrophysicists agreed that the universe’s expansion should be slowing down because of gravity.
This revolutionary discovery, which astrophysicists achieved with observations of specific kinds of exploding stars, called type Ia (read “type one-A”) supernovae, was recognized with the Nobel Prize in Physics in 2011.
In this new study, DES scientists performed analyses with four different techniques, including the supernova technique used in 1998, to understand the nature of dark energy and to measure the expansion rate of the universe.
As a graduate student in Bechtol’s group, Morgan was part of the DES supernova working group that worked to identify type Ia supernova. This group had to address two main concerns with the data to enhance detection fidelity.
“One is that there is some leakage of other types of supernovae into the sample, so you have to calibrate the rate of misclassification,” Bechtol explains. “Also, the brightness of the supernova gives us a way of estimating its distance, but there is a distribution of how bright the Ia supernovae are. Because we are slightly less likely to detect the intrinsically fainter supernovae, there is a small bias that needs to be accounted for.”
Bechtol has been part of the DES collaboration since its formation in 2012, serving as a co-convener of the DES’s Science Release Working Group for four years and a co-convener of the Milky Way Working Group for two years. His role in this new study was in data processing and presentation.
“We collect all of the data, process it, and then release it as a coherent set of data products, both for use by the DES collaboration and as part of public releases to the community,” Bechtol says. “One of the aspects I worked on is the photometric calibration — our ability to measure the fluxes of objects accurately and precisely. It’s an important part of the supernova analysis and something that I’ve been working on continuously over the past ten years.”
Five outstanding scholars, including Physics PhD student Jimena González, are joining the UW–Madison chapter of the national Edward Alexander Bouchet Graduate Honor Society this academic year.
The Bouchet Society commemorates the first person of African heritage to earn a PhD in the United States. Edward A. Bouchet earned a PhD in Physics from Yale University in 1876. Since then, the Bouchet Society has continued to uphold Dr. Bouchet’s legacy.
“The 2024 Bouchet inductees are making key contributions in their disciplines, as well as to the research, education, and outreach missions of our campus. They truly embody the Wisconsin Idea and are exemplary in every way,” said Abbey Thompson, assistant dean for diversity, inclusion, and funding in the Graduate School.
The Bouchet Society serves as a network for scholars that uphold the same personal and academic excellence that Dr. Bouchet demonstrated. Inductees to the UW–Madison Chapter of the Bouchet Society also join a national network with 20 chapters across the U.S. and are invited to present their work at the Bouchet Annual Conference at Yale University, where the scholars further create connections and community within the national Bouchet Society.
The UW–Madison Division of Diversity, Equity, and Educational Achievement supports each inductee with a professional development grant.
González is a physics PhD candidate specializing in observational cosmology. Her research centers on searching and characterizing strong gravitational lenses in the Dark Energy Survey. These rare astronomical systems can appear as long curved arcs of light surrounding a galaxy. Strong gravitational lenses offer a unique probe for studying dark energy, the driving force behind the universe’s accelerating expansion and, consequently, a pivotal factor in determining its ultimate fate.
During her graduate program, Jimena has received the Albert R. Erwin, Jr. & Casey Durandet Award and the Firminhac Fellowship from the Department of Physics. Additionally, she was honored with the 2023 Open Science Grid David Swanson Award for her outstanding implementation of High-Throughput Computing to advance her research. Jimena has contributed as a co-author to multiple publications within the field of strong gravitational lensing and has presented her work at various conferences. In addition to her academic achievements, Jimena has actively engaged in outreach programs. Notably, she was selected as a finalist at the 2021 UW–Madison Three Minute Thesis Competition and secured a winning entry in the 2023 Cool Science Image Contest. Her commitment to science communication extends to a contribution in a Cosmology chapter in the book AI for Physics. Jimena has also led a citizen science project that invites individuals from all around the world to inspect astronomical images to identify strong gravitational lenses. Jimena obtained her bachelor’s degree in physics at the Universidad de los Andes, where she was awarded the “Quiero Estudiar” scholarship.
Dark Energy Survey releases most precise look at the universe’s evolution
The Dark Energy Survey collaboration has created the largest ever maps of the distribution and shapes of galaxies, tracing both ordinary and dark matter in the universe out to a distance of more than 7 billion light years. The analysis, which includes the first three years of data from the survey, is consistent with predictions from the current best model of the universe, the standard cosmological model. Nevertheless, there remain hints from DES and other experiments that matter in the current universe is a few percent less clumpy than predicted.
New results from the Dark Energy Survey — a large international team that includes researchers from the University of Wisconsin–Madison — use the largest ever sample of galaxies over an enormous piece of the sky to produce the most precise measurements of the universe’s composition and growth to date. Scientists measured that the way matter is distributed throughout the universe is consistent with predictions in the standard cosmological model, the best current model of the universe.
Over the course of six years, DES surveyed 5,000 square degrees — almost one-eighth of the entire sky — in 758 nights of observation, cataloguing hundreds of millions of objects. The results, announced May 27, draw on data from the first three years — 226 million galaxies observed over 345 nights — to create the largest and most precise maps yet of the distribution of galaxies in the universe at relatively recent epochs.
Since DES studied nearby galaxies as well as those billions of light-years away, its maps provide both a snapshot of the current large-scale structure of the universe and a movie of how that structure has evolved over the course of the past 7 billion years.
“This a stringent test of the current standard cosmological paradigm, a model proposing that 95% of the universe is dark matter and dark energy that we do not yet understand,” explains UW–Madison physics professor Keith Bechtol. “By measuring the apparent positions and shapes of hundreds of millions of galaxies in our survey, we test whether the cosmic structures that have formed in the universe today match the predictions based on structures observed in the early universe.”
To test cosmologists’ current model of the universe, DES scientists compared their results with measurements from the European Space Agency’s orbiting Planck observatory. Planck used light signals known as the cosmic microwave background to peer back to the early universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present. If DES’s observations don’t match this prediction, there is possibly an undiscovered aspect to the universe. While there have been persistent hints from DES and several previous galaxy surveys that the current universe is a few percent less clumpy than predicted—an intriguing find worthy of further investigation—the recently released results are consistent with the prediction.
“In the area of constraining what we know about the distribution and structure of matter on large scales as driven by dark matter and dark energy, DES has obtained limits that rival and complement those from the cosmic microwave background,” said Brian Yanny, a Fermilab scientist who coordinated DES data processing and management. “It’s exciting to have precise measurements of what’s out there and a better understanding of how the universe has changed from its infancy through to today.”
Ordinary matter makes up only about 5% of the universe. Dark energy, which cosmologists hypothesize drives the accelerating expansion of the universe by counteracting the force of gravity, accounts for about 70%. The last 25% is dark matter, whose gravitational influence binds galaxies together. Both dark matter and dark energy remain invisible and mysterious, but DES seeks to illuminate their natures by studying how the competition between them shapes the large-scale structure of the universe over cosmic time.
DES photographed the night sky using the 570-megapixel Dark Energy Camera on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile, a Program of the National Science Foundation’s NOIRLab. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for DES and built and tested at Fermilab. The DES data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
“These analyses are truly state-of-the-art, requiring artificial intelligence and high-performance computing super-charged by the smartest young scientists around,” said Scott Dodelson, a physicist at Carnegie Mellon University who co-leads the DES Science Committee with Elisabeth Krause of the University of Arizona. “What an honor to be part of this team.”
To quantify the distribution of dark matter and the effect of dark energy, DES relied on two main phenomena. First, on large scales, galaxies are not distributed randomly throughout space but rather form a weblike structure due to the gravity of dark matter. DES measured how this cosmic web has evolved over the history of the universe. The galaxy clustering that forms the cosmic web, in turn, revealed regions with a higher density of dark matter.
Second, DES detected the signature of dark matter through weak gravitational lensing. As light from a distant galaxy travels through space, the gravity of both ordinary and dark matter can bend it, resulting in a distorted image of the galaxy as seen from Earth. By studying how the apparent shapes of distant galaxies are aligned with each other and with the positions of nearby galaxies along the line of sight, DES scientists inferred the spatial distribution (or clumpiness) of the dark matter in the universe.
Analyzing the massive amounts of data collected by DES was a formidable undertaking. The team began by analyzing just the first year of data, which was released in 2017. That process prepared the researchers to use more sophisticated techniques for analyzing the larger data set, which includes the largest sample of galaxies ever used to study weak gravitational lensing.
For example, calculating the redshift of a galaxy — the change in light’s wavelength due to the expansion of the universe — is a key step toward measuring how both galaxy clustering and weak gravitational lensing change over cosmic history. The redshift of a galaxy is related to its distance, which allows the clustering to be characterized in both space and time.
“Redshift calibration is one topic where we significantly improved upon our year-1 data analysis,” said Ross Cawthon, a UW-Madison physics postdoc who led the redshift calibration efforts for two of the main galaxy samples. “We developed new methods and refined old ones. It has been a huge effort by DES members from all over the world.”
Ten regions of the sky were chosen as “deep fields” that the Dark Energy Camera imaged repeatedly throughout the survey. Stacking those images together allowed the scientists to glimpse more distant galaxies. The team then used the redshift information from the deep fields to calibrate measurements of redshift in the rest of the survey region. This and other advancements in measurements and modeling, coupled with a threefold increase in data compared to the first year, enabled the team to pin down the density and clumpiness of the universe with unprecedented precision.
Along with the analysis of the weak-lensing signals, DES also precisely measures other probes that constrain the cosmological model in independent ways: galaxy clustering on larger scales (baryon acoustic oscillations), the frequency of massive clusters of galaxies, and high-precision measurements of the brightnesses and redshifts of Type Ia supernovae. These additional measurements will be combined with the current weak-lensing analysis to yield even more stringent constraints on the standard model.
“DES has delivered cost-effective, leading-edge science results directly related to Fermilab’s mission of pursuing the fundamental nature of matter, energy, space and time,” said Fermilab Director Nigel Lockyer. “A dedicated team of scientists, engineers and technicians from institutions around the world brought DES to fruition.”
The DES collaboration consists of over 400 scientists from 25 institutions in seven countries.
“The collaboration is remarkably young. It’s tilted strongly in the direction of postdocs and graduate students who are doing a huge amount of this work,” said DES Director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “That’s really gratifying. A new generation of cosmologists are being trained using the Dark Energy Survey.”
UW–Madison physics graduate student Megan Tabbutt was one of the many significant contributors to this work, developing new methods that contributed to an independent validation of the galaxy clustering analysis.
DES concluded observations of the night sky in 2019. With the experience of analyzing the first half of the data, the team is now prepared to handle the complete data set. The final DES analysis is expected to paint an even more precise picture of the dark matter and dark energy in the universe. And the methods developed by the team have paved the way for future sky surveys to probe the mysteries of the cosmos.
“This work represents a ‘big statement’ from the Dark Energy Survey. DES data combined with other observations provide world-leading constraints on the nature of dark energy,” Bechtol says. “At the same time, we are training a new generation of cosmologists, and pioneering advanced methodologies that will be essential to realize the full potential of upcoming galaxy surveys, including the Vera C. Rubin Observatory Legacy Survey of Space and Time.”
The recent DES results were presented in a scientific seminar on May 27. Twenty-nine papers are available on the arXiv online repository.
The second data release from the Dark Energy Survey, or DES, is the culmination of over a half-decade of astronomical data collection and analysis with the ultimate goal of understanding the accelerating expansion of the universe and the phenomenon of dark energy, which is thought to be responsible for this accelerated expansion. It is one of the largest astronomical catalogs released to date. Keith Bechtol, assistant professor of physics at UW–Madison, has served as the DES Science Release co-coordinator since 2017, guiding the effort to assemble, scientifically validate, and document data releases for both cosmology analysis by the DES Collaboration and exploration by the broad astronomical community.
Including a catalog of nearly 700 million astronomical objects, DR2 builds on the 400 million objects cataloged with the survey’s prior data release, or DR1, and also improves on it by refining calibration techniques, which, with the deeper combined images of DR2, lead to improved estimates of the amount and distribution of matter in the universe.
Astronomical researchers around the world can access these unprecedented data and mine them to make new discoveries about the universe, complementary to the studies being carried out by the Dark Energy Survey collaboration. The full data release is online and available to the public to explore and gain their own insights as well.
“Most of the nearly 700 million objects visible in DES DR2 images had never been seen by humans before the past few years,” Bechtol says. “If you take a moment to look at even a small patch of sky in the DES images, you can see asteroids of our Solar System, stars out to the edge of the Milky Way, and distant galaxies as they were billions of years ago. We look forward to see how our colleagues use this enormous new dataset for research and education.”
DES was designed to map hundreds of millions of galaxies and to discover thousands of supernovae in order to measure the history of cosmic expansion and the growth of large-scale structure in the universe, both of which reflect the nature and amount of dark energy in the universe. DES has produced the largest and most accurate dark matter map from galaxy weak lensing to date, as well as a new map, three times larger, that will be released in the near future.
One early result relates to the construction of a catalog of a type of pulsating star known as “RR Lyrae,” which tells scientists about the region of outer space beyond the edge of our Milky Way. In this area nearly devoid of stars, the motion of the RR Lyrae hints at the presence of an enormous “halo” of invisible dark matter, which may provide clues on how our galaxy was assembled over the last 12 billion years. In another result, DES scientists used the extensive DR2 galaxy catalog, along with data from the LIGO experiment, to estimate the location of a black hole merger and, independent of other techniques, infer the value of the Hubble constant, a key cosmological parameter. Combining their data with other surveys, DES scientists have also been able to generate a complete map of the Milky Way’s dwarf satellites, giving researchers insight into how our own galaxy was assembled and how it compares with cosmologists’ predictions.
Covering 5,000 square degrees of the southern sky (one-eighth of the entire sky) and spanning billions of light-years, the survey data enables many other investigations in addition to those targeting dark energy, covering a vast range of cosmic distances — from discovering new nearby solar system objects to investigating the nature of the first star-forming galaxies in the early universe.
“This is a momentous milestone. For six years, the Dark Energy Survey collaboration took pictures of distant celestial objects in the night sky. Now, after carefully checking the quality and calibration of the images captured by the Dark Energy Camera, we are releasing this second batch of data to the public,” said DES Director Rich Kron of Fermilab and the University of Chicago. “We invite professional and amateur scientists alike to dig into what we consider a rich mine of gems waiting to be discovered.”
Once captured, these images (and the large amount of data surrounding them) are transferred to the National Center for Supercomputing Applications for processing via the DES Data Management project. Using the Blue Waters supercomputer at NCSA, the Illinois Campus Cluster and computing systems at Fermilab, NCSA prepares calibrated data products for public and research consumption. It takes approximately four months to process one year’s worth of data into a searchable, usable catalog.
The detailed precision cosmology constraints based on the full six-year DES data set will come out over the next two years.
NCSA, NOIRLab and the LIneA Science Server collectively provide the tools and interfaces that enable access to DR2.
“Because astronomical data sets today are so vast, the cost to handle them is prohibitive for individual researchers or most organizations. CSDC provides open access to big astronomical data sets like DES DR2 and the necessary tools to explore and exploit them — then all it takes is someone from the community with a clever idea to discover new and exciting science,” said Robert Nikutta, project scientist for Astro Data Lab at CSDC.
“With information on the positions, shapes, sizes, colors and brightnesses of over 690 million stars, galaxies and quasars, the release promises to be a valuable source for astronomers and scientists worldwide to continue their explorations of the universe, including studies of matter (light and dark) surrounding our home Milky Way galaxy, as well as pushing further to examine groups and clusters of distant galaxies, which hold precise evidence about how the size of the expanding universe changes over time,” said Dark Energy Survey Data Management Project Scientist Brian Yanny of Fermilab.
This work is supported in part by the U.S. Department of Energy Office of Science.
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, 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, Funding Authority for Studies and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science, Technology and Innovation, the German Research Foundation and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.