Astrophysicists map the invisible universe using warped galaxies

Between 2013 and 2019, the Dark Energy Survey (DES) gathered observations with the Dark Energy Camera (DECam) on the 4-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. Over that period, DES measured and calibrated the shapes of more than 150 million galaxies across 5,000 square degrees (about an eighth) of the sky. Those galaxy shape measurements help scientists refine estimates of how mass is spread through the universe and how dark energy behaves.

DES has also played a role in a major recent puzzle involving the Lambda-CDM (LCDM) model, the standard framework used to describe the universe. Some studies of the nearby universe using galaxy surveys such as DES have seemed to disagree with predictions based on the early universe, which are inferred from the cosmic microwave background (CMB) — the leftover radiation from the Big Bang.

Although DECam was built to support DES, it also collected many images outside the main DES footprint. In a new set of papers in the Open Journal of Astrophysics, UChicago astrophysicists used those additional observations and nearly doubled the number of galaxies with measured shapes by adding data from thousands of square degrees beyond the DES region. Because these images were not originally captured for weak lensing work, the expanded data set offers an independent way to recheck the earlier LCDM inconsistencies.

Weak gravitational lensing and why galaxy shapes matter

Gravitational lensing happens when mass bends light, and it is one of the most powerful tools for studying where mass sits in the universe. That includes ordinary matter as well as dark matter, and it can also shed light on dark energy’s role, said Chihway Chang, associate professor of Astronomy and Astrophysics and lead of the Dark Energy Camera All Data Everywhere (DECADE) weak lensing cosmic shear project.

In weak gravitational lensing, galaxies do not look dramatically stretched. Instead, their shapes appear just slightly distorted (sheared) because their light passes through and around matter on its way to Earth. The signal is extremely small, so researchers rely on statistical methods to detect it.

“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).”

Measuring galaxy distances and testing the standard cosmology model

For the DECADE work, the researchers measured the shapes of more than 100 million galaxies. They also estimated how far away those galaxies are by analyzing how much each galaxy’s light shifts toward red wavelengths (redshift). That shift shows how quickly a galaxy is moving away, and it can be used to calculate its distance from Earth.

With galaxy shapes and distances in hand, the team fit the LCDM model to the observations. LCDM is the widely used cosmology model that accounts for 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 results show that cosmic structure growth matches what LCDM predicts, aligning with earlier weak lensing studies. “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.”

An unconventional survey built from archival telescope images

DECADE provides an independent check on whether weak lensing results agree with CMB-based expectations, using a different part of the sky than DES but at a comparable scale. Alex Drlica-Wagner, Scientist at Fermilab and UChicago associate professor in Astronomy and Astrophysics who led the DECADE observing campaign, noted that success was not guaranteed at the outset. “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 said.

A standout feature of the project involved image quality decisions, Anbajagane explained. Traditional weak lensing surveys collect close to a hundred thousand purpose-built images over many years, and many frames are rejected when they fail strict standards. “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.

That approach could influence how researchers handle future weak lensing studies, including work based on theVera C. Rubin Legacy Survey of Space and Time (Rubin LSST) survey. Using a larger share of available images could boost the precision of cosmological measurements. The team’s ability to use archival images also depended heavily on careful image inspection, led by Chin Yi Tan, a PhD student in Physics.

A massive public galaxy catalog and global collaboration

Combined with DES, the final catalog covers about one-third of the sky (13,000 square degrees) and includes 270 million galaxies. The catalog was released to the scientific community this fall, and researchers have already begun using the images for other studies, including work on dwarf galaxies and new maps of the universe’s mass. “We’re actively working on applying other analysis methods to our data alongside experts at theKavli Institute for Cosmological Physics,” said Anbajagane.

The DECADE analysis brought together scientists from UChicago, Fermilab, and NCSA at UIUC, along with collaborators from Argonne, UW-Madison, and many other institutions worldwide. “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.”