November 21, 2025
UChicago astrophysicists test a new piece of the sky to probe dark matter and dark energy
In the leading model of cosmology, most of the universe is invisible: a combined 95 percent is made of dark matter and dark energy. Exactly what these dark components are remains a mystery, but they have a tremendous impact on our universe, with dark matter exerting a gravitational pull and dark energy driving the universe’s accelerating expansion. What scientists know about dark matter and dark energy comes from observing their effects on the visible universe. Astrophysicists from the University of Chicago measured those effects on a new patch of sky to illuminate the invisible cosmos.
Mapping the dark universe
From 2013 to 2019, the Dark Energy Survey (DES) collected data using the Dark Energy Camera (DECam), mounted on the 4-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, and has measured and calibrated the shapes of over 150 million galaxies across 5,000 square degrees (about an eighth) of the sky. These observations help refine our understanding of the universe’s mass distribution and the properties of dark energy. In particular, DES data has been important in helping to understand one of the main issues that has arisen recently in the leading cosmological model, the Lambda-CDM (LCDM) model. There appears to be inconsistencies between measurements in the nearby universe, derived from galaxy surveys such as DES, and those from the early universe, derived from the cosmic microwave background (CMB)—the leftover radiation from the Big Bang.
While DECam was used to collect an enormous amount of data for DES, it also observed large portions of the sky outside the DES region. Described in a new set of papers published in the Open Journal of Astrophysics, a team of UChicago astrophysicists roughly doubled the number of galaxies with measured shapes using DECam data covering thousands of square degrees of the sky outside of the DES footprint. By analyzing the extended data, which were not originally intended for weak lensing studies, the team is able to independently examine previously observed inconsistencies in LCDM.
Distortion and distance
Gravitational lensing, the bending of light by massive objects, is a key method used to probe the amount of mass and its distribution in the universe, thereby gleaning insight into the interactions between dark matter, ordinary matter, and dark energy, explained Chihway Chang, associate professor of Astronomy and Astrophysics and lead of the Dark Energy Camera All Data Everywhere (DECADE) weak lensing cosmic shear project.
With weak gravitational lensing, the shape of galaxies observed from Earth appears slightly distorted (sheared) because the light from those galaxies must travel through and around the matter in the universe in order to reach us. The effect is so subtle that it requires statistical analysis to measure.
“Weak lensing measurements are best at probing the ‘clumpiness’ of matter,” said Dhayaa Anbajagane, a PhD student in Astronomy and Astrophysics who is lead analyst and first author on the series of DECADE papers. “Quantifying this clumpiness sheds light on the origin and evolution of structures like galaxies and galaxy clusters. This is loosely akin to measuring the distribution of people (the matter) living across a region and using that to understand features such as the landscape’s topography or the location or age of urban areas (factors that influence the origin and evolution of structures).”
For this project, the team measured the shapes of more than 100 million galaxies. They also measured their distances by determining how much a galaxy’s light has shifted toward the red end of the light spectrum (redshift), which indicates how fast the galaxy is receding. From that measurement, they can calculate the distance from Earth.
Using those shapes and distances, the team fit the LCDM model to the data. This is the standard model of cosmology widely accepted to explain observations about the universe, with components related to dark energy, dark matter, ordinary matter, neutrinos, and radiation. “This is a well-tested model that has survived many, many examinations in the past decade, and our data point is going to add to that story,” said Chang.
The DECADE study finds that the growth of structure in the universe is consistent with predictions from the LCDM model, supporting the results of previous weak lensing measurements. “In addition, when comparing our constraints with those derived and extrapolated from the early universe’s CMB, we also agree well,” said Chang. “This last point has been a source of debate over the past five or so years, and with our new results, we can say that we do not see tension between weak lensing and CMB.”
“We are also able to combine the DECADE lensing measurements with those of DES, resulting in a galaxy lensing analysis that uses the largest number of galaxies (270 million) covering the widest patch of sky (13,000 square degrees) to date,” said Anbajagane. “Given this large amount of data, we can make particularly conservative choices in our analysis—such as only making or using the measurements we trust most, rather than all useful or possible measurements—and still make a measurement with enough precision to meaningfully inform our comparisons with the CMB.”
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As a postdoctoral fellow, Chang was lead illustrator for the DES outreach program, Dark Bites. This illustration is a cartoon rendition of a well-known figure (Figure 2 in this paper) illustrating the different effects that are involved in each galaxy image (represented by sheep here) from which we measure the weak lensing signal. The original image of the galaxies (or sheep) is distorted (sheared) through weak lensing, convolved with the atmosphere and instrumental effects (PSF convolved), imaged by the pixelated detectors, combined with noise, and blended with the light from other galaxies (or sheep). (Credit: Chihway Chang) |
“An unconventional weak lensing survey”
The DECADE project independently checks consistency between CMB and weak lensing measurements on a completely different but similarly sized patch of sky from DES. However, this outcome was not a given at the start of the project, notes Alex Drlica-Wagner, associate professor in Astronomy and Astrophysics and Scientist at Fermilab, who led the DECADE observing campaign. “It was not clear that the DECADE dataset would be of sufficient quality to perform a cosmological analysis, but we have shown that it can indeed produce robust results,” he added.
“One unique result from this work has to do with choices we make on image quality,” said Anbajagane. A conventional weak lensing–focused survey takes nearly a hundred thousand dedicated images over many years, yet many are discarded because, for example, the image quality failed to meet the set criteria. “The DECADE project is unique as it repurposes archival data—images originally taken by the astronomy community for a wide variety of science goals, from studying dwarf galaxies to stars to distant galaxy clusters—and uses significantly more permissive criteria for image quality. Our work shows robust lensing analyses can be done even if we do not have lensing-dedicated imaging campaigns,” he said.
This changes how astronomers might view future lensing analyses, such as those from the Vera C. Rubin Legacy Survey of Space and Time (Rubin LSST) survey. Such analyses could use more of their images than they may have otherwise, improving how precisely astronomers measure cosmological properties. The DECADE project’s ability to use such archival image data was also enabled in large part by meticulous inspection of the images, a task led by Chin Yi Tan, a PhD student in Physics.
The completed catalog combined with DES covers approximately one-third of the sky (13,000 square degrees) and contains 270 million galaxies. This catalog was released to the scientific community this fall and has already caught the interest of cosmologists and astronomers. For instance, the team’s imaging data has been used to study dwarf galaxies and make maps of the mass in the universe. “We’re actively working on applying other analysis methods to our data alongside experts at the Kavli Institute for Cosmological Physics,” said Anbajagane.
Scientists from UChicago, Fermilab, and NCSA at UIUC joined forces with researchers at Argonne, UW-Madison, and many other institutions around the world to carry out the DECADE analysis. “It was quite special to have these different components all sitting in the hallway,” said Chang. “It also allowed us to learn from each other—and resulted in an unexpected but wonderful outcome of this project.”
Citations:
- Anbajagane, D., Z. Zhang, C. Chang, C. Y. Tan, M. Adamow, L. F. Secco, M. R. Becker, et al. 2025. “The DECADE Cosmic Shear Project I: A New Weak Lensing Shape Catalog of 107 Million Galaxies.” The Open Journal of Astrophysics 8 (October).
- Anbajagane, D., A. Alarcon, R. Teixeira, C. Chang, L. F. Secco, C. Y. Tan, A. Drlica-Wagner, et al. 2025. “The DECADE Cosmic Shear Project II: Photometric Redshift Calibration of the Source Galaxy Sample.” The Open Journal of Astrophysics 8 (October).
- Anbajagane, D., C. Chang, N. Chicoine, L. F. Secco, C. Y. Tan, P. S. Ferguson, A. Drlica-Wagner, et al. 2025. “The DECADE Cosmic Shear Project III: Validation of Analysis Pipeline Using Spatially Inhomogeneous Data.” The Open Journal of Astrophysics 8 (October).
- Anbajagane, D., C. Chang, A. Drlica-Wagner, C. Y. Tan, M. Adamow, R. A. Gruendl, L. F. Secco, et al. 2025. “The DECADE Cosmic Shear Project IV: Cosmological Constraints from 107 Million Galaxies across 5,400 Deg2 of the Sky.” The Open Journal of Astrophysics 8 (October).
- Anbajagane, D., C. Chang, A. Drlica-Wagner, C. Y. Tan, M. Adamow, R. A. Gruendl, L. F. Secco, et al. “The Dark Energy Camera All Data Everywhere cosmic shear project V: Constraints on cosmology and astrophysics from 270 million galaxies across 13,000 deg2 of the sky.” arXiv:2509.03582v2 [astro-ph.CO]