Six years worth of data on nearly 670 million galaxies is helping scientists tighten the constraints on the dark energy driving the expansion of the universe
Study: Dark Energy Survey Year 6 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing (DOI: 10.48550/arXiv.2601.14559)
Although scientists can't yet directly observe or characterize what's driving the mysterious expansion of our universe, they do have a name for it: dark energy.
The Dark Energy Survey, or DES, is an international collaboration including researchers from the University of Michigan that surveyed the cosmos for six years to better understand this mysterious energy.
Now, for the first time, the collaboration has released results from all six years of observations, combining all four probes the team used to analyze data from 669 million galaxies.
"DES has ushered in a new era of observational cosmology, especially in terms of the rigor of its analysis and the diverse nature of cosmological probes that it used to constrain dark energy and dark matter," said Dragan Huterer, U-M professor of physics.
Huterer served as co-leader of the theory and combined probes working group at DES, which is jointly supported by federal funding from the U.S. Department of Energy's Office of Science and the U.S. National Science Foundation.
The analysis yielded tighter new constraints that narrow down the possible models for how the universe behaves. These constraints are more than twice as strong as those from past DES analyses, while remaining consistent with previous DES results.
Recently, another dark energy project-the Dark Energy Spectroscopic Instrument, or DESI-has made headlines with reports that its data suggest the density of dark energy in the universe is evolving. This is in contrast to a fixed density, as predicted by the currently favored standard model of cosmology, which uses the "cosmological constant" term in its equations that was first proposed by Albert Einstein.
Both the constant and time-evolving dark-energy scenarios are consistent with the new DES analysis, so it doesn't resolve that issue, but it may lean ever so slightly to the side of a universe with evolving dark energy.
"We see a mild preference for the departure from the standard model in the final DES data," Huterer said. "So there's a hint for departure, but that's a very exciting development to my mind because it's independent from DESI."
Probing dark energy
Between 2013 and 2019, DES's telescope covered an eighth of the sky from its perch in the Chilean Andes, observing millions of galaxies billions of light-years from Earth. Researchers were looking at four cosmological phenomena within the observations that they could use as indirect probes of dark energy. These probes are baryon acoustic oscillations, Type Ia supernovae, galaxy clusters and weak gravitational lensing.
"DES really showcases how we can use multiple different measurements from the same sky images. I think that's very powerful," said Martin Crocce, research associate professor at the Institute for Space Science in Barcelona and co-coordinator of the analysis. "This is the only time it has been done in the current generation of dark energy experiments."
This new analysis, which has been published as a preprint on arXiv, is also the first to combine galaxy clusters and weak gravitational lensing. The upcoming publication represents a summary of 18 supporting studies. This and all of DES's analyses have required a small army of scientists working in parallel, Huterer said.
Led by the DOE's Fermi National Accelerator Laboratory, DES is an international collaboration of more than 400 astrophysicists and scientists from 35 institutions in seven countries. In the U.S. that collaboration includes universities, NSF NOIRLab and DOE national laboratories Argonne, Lawrence Berkeley and SLAC.
Thanks to U-M's involvement with DES, an entire generation of students and postdoctoral researchers has gained hands-on experience working with data and instrumentation on one of the world's premiere dark energy studies. Numerous U-M experts have also contributed, including:
- Gus Evrard, a professor of physics who co-led the DES simulations working group and also contributed to the galaxy cluster working group
- Greg Tarlé, an emeritus professor of physics whose team made key contributions to the instrumentation
- Michael Schubnell, a research scientist who made many contributions to instrumentation, and carried out observations
- Curtis Weaverdyck, a senior engineering technician whose efforts helped ensure continued successful operation of the telescope
Two former U-M faculty members, David Gerdes and Marcelle Soares-Santos, also made significant discoveries with DES, Huterer said.
The new analysis is also important because it paves the way for future studies. The new NSF-DOE Vera C. Rubin Observatory will do similar work, but with a much larger and more powerful telescope, the Legacy Survey of Space and Time, or LSST.
"We have added a significant step in precision, but all these measurements are going to improve much more with new observations from Rubin Observatory and other telescopes," said Anna Porredon, co-lead of the DES Large Scale Structure working group and senior fellow at the Center for Energy, Environmental and Technological Research in Madrid. "It's exciting that we will probably have some of the answers about dark energy in the next 10 years."
