The Dark Energy Survey collaboration is releasing results that, for the first time, combine all of the data from an intensive six-year mapping of galaxies in the universe.
The new analysis, of millions of galaxies mapped with a telescope located in the Chilean Andes, yielded new, tighter constraints that narrow down the possible models for how the universe behaves. These constraints are more than twice as strong as those from past analyses, while remaining consistent with previous Dark Energy Survey results, the scientists said.
The collaboration, which includes more than 400 scientists from 35 institutions including the University of Chicago, released a set of papers that includes a summary of their first results found by combining four different approaches to measuring the universe: baryon acoustic oscillations, type-Ia supernovae, galaxy clusters and weak gravitational lensing.
It also sheds more light on a major question in the field about whether the force known as dark energy has stayed constant throughout the age of the universe, or has evolved over time.
“There’s something very exciting about pulling the different cosmological probes together,” said Chihway Chang, associate professor at UChicago and co-chair of the Dark Energy Survey science committee. “It’s quite unique to the Dark Energy Survey that we have the expertise to do this.”
How to measure dark energy
A century ago, astronomers noticed that distant galaxies appeared to be moving away from us. Then, in 1998, two independent teams of cosmologists used distant supernovae to discover that the universe’s expansion is accelerating rather than slowing.
To explain these observations, they proposed a new kind of energy that is responsible for driving the universe’s accelerated expansion: dark energy. Astrophysicists now believe dark energy makes up about 70% of the mass-energy density of the universe—yet we still know very little about it.
In the following years, scientists began devising experiments to study dark energy, including the Dark Energy Survey. Today, the survey is an international collaboration of hundreds of scientists from 35 institutions in seven countries. Led by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, the DES collaboration also includes scientists from US universities, NSF NOIRLab and DOE national laboratories Argonne, Lawrence Berkeley and SLAC.
To study dark energy, the collaboration carried out a deep, wide-area survey of the sky from 2013 to 2019. Fermilab built an extremely sensitive 570-megapixel digital camera, DECam, and installed it on the U.S. National Science Foundation Víctor M. Blanco 4-meter telescope at NSF Cerro Tololo Inter-American Observatory, a program of NSF NOIRLab, in the Chilean Andes. For 758 nights over six years, the collaboration recorded information from 669 million galaxies that are billions of light-years from Earth, covering an eighth of the sky.
Scientists tease out information from this data by examining four different parameters. One is baryon acoustic oscillations, sound waves from the early age of the universe that are imprinted in the distribution of modern-day galaxies; type-Ia supernovae, exploding stars that scientists use as a measuring stick to gauge how fast the universe is expanding; galaxy clusters, which offer hints about the forces that cause galaxies to clump together; and weak gravitational lensing, which catalogues the distribution of matter in the universe.
Constant or evolving dark energy model
In this analysis, the Dark Energy Survey tested their data against two models of the universe: the currently accepted standard model of cosmology, known as Lambda-Cold Dark Matter (ΛCDM)—in which the dark energy density is constant, as well as an extended model in which the dark energy density evolves over time—known as wCDM.
They found that their data mostly aligned with the standard model of cosmology. The data also fit the evolving dark energy model, but no better than they fit the standard model.
However, one parameter is still off. Based on measurements of the early universe, both the standard and evolving dark energy models predict how matter in the universe clusters at later times—times probed by surveys like the Dark Energy Survey. In previous analyses, galaxy clustering was found to be different from what was predicted. When DES added the most recent data, that gap widened, but not yet to the point of certainty that the standard model of cosmology is incorrect. The difference persisted even when DES combined their data with those of other experiments.
“What we are finding is that both the standard model and evolving dark energy model fit the early and late universe observations well, but not perfectly,” said Judit Prat, co-lead of the Dark Energy Survey weak lensing working group and the Nordita Fellow at the Stockholm University and the KTH Royal Institute of Technology in Sweden.
Paving the way for more dark energy experiments
Next, the Dark Energy Survey will combine this work with the most recent constraints from other dark energy experiments to investigate alternative gravity and dark energy models. This analysis is also important because it paves the way for the new NSF-DOE Vera C. Rubin Observatory, funded by the U.S. National Science Foundation and the U.S. Department of Energy's Office of Science, to do similar work with its Legacy Survey of Space and Time (LSST).
“The measurements will get tighter and tighter in only a few years,” said Anna Porredon, co-lead of the Dark Energy Survey Large Scale Structure working group and senior fellow at the Center for Energy, Environmental and Technological Research (CIEMAT) in Madrid. “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.
“It’s exciting that we will probably have some of the answers about dark energy in the next 10 years.”
The Dark Energy Survey is jointly supported by the U.S. Department of Energy's Office of Science and the U.S. National Science Foundation. More information on the collaboration and the funding for this project can be found here.
—Adapted from a longer release published by Fermilab.
