Analyzing the dark
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.
Alexandra Amon, a Kavli Fellow at SLAC and Stanford, who co-led the weak-lensing analysis, said “Our results are particularly exciting because of the methodological advancements that we developed in order to harness the cosmological information from the images. It’s an enormous feat for our team to accurately analyze a dataset with a dataset with such unprecedented power! That's where SLAC researchers played a key role – in ensuring that the results were robust in terms of data calibration, by developing state-of-the-art techniques, as well as model validation, through building cosmological simulations to test the model.”
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.
SLAC researchers, including graduate student Justin Myles, led significant advances in how to calibrate the redshift distributions. Judit Prat, a postdoc at the University of Chicago who analyzed weak gravitational lensing as captured by DES said “This was a huge effort that people put a lot of work into. We now have a method that nobody has used before, and it’s very robust.”
The deep field
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.”
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.
“The real legacy of DES will be the leaps forward we’ve had to make that were essential for this key result, and which will be critical for the next generation of cosmological experiments starting soon,” said Michael Troxel, a physicist at Duke University and the key project coordinator for the DES three-year data analysis. Upcoming experiments include both space-based imaging experiments and ground-based surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time.
“With these instruments we’ve built to stare into the dark, we are working to solve universal mysteries,” said Troxel.
The recent DES results will be presented in a scientific seminar on May 27. Twenty-nine papers are available on the arXiv online repository.
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 Funding 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 and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.
Editor’s note: This story is based on a press release from Fermilab.
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