Map of 226M Galaxies Provides Unprecedented Look at the Evolution of the Universe

4-Meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile

The Dark Energy Survey has imaged roughly 5,000 square degrees of southern sky. The survey has mapped hundreds of millions of galaxies to help researchers understand the accelerating expansion of our universe.

Reidar Hahn, Fermilab

An international team of scientists, including several from the University of Arizona, has completed the largest-ever map of the universe, detailing the distribution and shapes of galaxies spanning more than 7 billion light-years.

The Dark Energy Survey collaboration, which includes more than 400 scientists from 25 institutions in seven countries, today presented the results of its first three-year observation run, as part of a series of 29 scientific papers.

The new results use the largest-ever sample of galaxies over an enormous piece of the sky to produce the most precise measurements yet of the universe's composition and growth.

Scientists found that the amount of matter in the universe and its distribution are consistent with the expectations from the standard cosmological model, which describes how the universe originated with the Big Bang about 14 billion years ago and how it has evolved since. This model  predicts that the distribution of matter in the cosmos is not uniform, but contains regions where matter is concentrated into "clumps," surrounded by vast stretches of mostly empty space. 

However, data gathered previously by the Dark Energy Survey and other experiments have suggested that the distribution of matter in the universe is a little less "clumpy" than predicted by the standard model.

"It's exciting to quantify how our universe has evolved over time," said Elisabeth Krause, assistant professor of astronomy and physics at UArizona, who provides scientific leadership of the Dark Energy Survey as co-chair of its science committee. "Basically, we are looking at the spatial distribution of galaxies at different ages of the universe, and we track how this distribution has changed over time. It's like watching the universe grow up, starting from when it was only one-third of its age today."

Krause co-leads the science committee with Scott Dodelson, chair of the physics and astronomy department at Carnegie Mellon University. Faculty, students and postdoctoral researchers in the Arizona Cosmology Lab, a research group at UArizona dedicated to studying grand challenges in cosmology, have contributed to the Dark Energy Survey scientific analysis.

Over the course of six years, the collaborative project has surveyed 5,000 square degrees — almost one-eighth of the entire sky — and cataloged hundreds of millions of objects. The results announced today draw on data from the first three years — 226 million galaxies observed over 345 nights — to create the largest and most precise maps yet of the distribution of galaxies in the universe in relatively recent times.

Since the Dark Energy Survey studies nearby galaxies, as well as those billions of light-years away, its maps provide both a snapshot of the current large-scale structure of the universe and a movie of how that structure has evolved over the course of the past 7 billion years.

To test cosmologists' current model of the universe, Dark Energy Survey scientists compared their results with measurements from the European Space Agency's orbiting Planck observatory. Planck used light signals known as the cosmic microwave background to peer back in time to the early universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present.

"If DES's observations don't match this prediction, we will have discovered evidence of new physics in our universe, likely related to how dark matter and dark energy evolve," said Eduardo Rozo, associate professor of physics at UArizona and part of the Dark Energy Survey collaboration.

While there have been persistent hints from the Dark Energy Survey and several previous galaxy surveys that the distribution of matter in the current universe is less clumpy than predicted, the recently released results are consistent with the prediction made by the standard cosmological model.

Ordinary matter makes up only about 5% of the universe. Dark energy, which cosmologists hypothesize drives the accelerating expansion of the universe by counteracting the force of gravity, accounts for about 70%. The remaining 25% is dark matter, which has a gravitational influence that binds galaxies together. Both dark matter and dark energy are invisible and mysterious, but the Dark Energy Survey seeks to illuminate their natures by studying how the competition between them shapes the large-scale structure of the universe over cosmic time.

Survey scientists photographed the night sky using the 570-megapixel Dark Energy Camera on the Victor M. Blanco 4-Meter Telescope at the Cerro Tololo Inter-American Observatory in Chile, a program of the National Science Foundation's NOIRLab. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for the Dark Energy Survey and built and tested at Fermilab particle physics and accelerator laboratory in Illinois. The data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

Dark Energy Camera on the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile

The Dark Energy Survey photographed the night sky using the 570-megapixel Dark Energy Camera on the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, a division of the National Science Foundation’s NOIRLab.

Reidar Hahn, Fermilab

"This data set pushes the boundaries of cosmology to a new level," said Tim Eifler, UArizona assistant professor of astronomy, who co-led the Dark Energy Survey Theory and Combined Probes working group with Dragan Huterer, professor of physics at the University of Michigan. "The sheer number of galaxies that we observe requires a new level of precision in the data analysis methodology."

To quantify the distribution of dark matter and the effect of dark energy, the Dark Energy Survey relied on two main phenomena. First, on large scales, galaxies are not distributed randomly throughout space but rather form a web-like structure due to the gravity of dark matter. The Dark Energy Survey 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, the Dark Energy Survey 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, scientists inferred the spatial distribution — or clumpiness — of dark matter.

"This is a fantastic data set to work on," said Xiao Fang, a postdoctoral researcher at UArizona's Steward Observatory and one of the lead authors of the Dark Energy Survey paper that describes the theoretical modeling of the observables. "For junior researchers like me, it is the ideal opportunity to develop and implement new ideas and to test some of the most famous theories in cosmological physics."

The methods developed by the team have paved the way for future sky surveys to probe the mysteries of the cosmos.

"The breakthroughs that the DES has accomplished in this analysis are just the beginning," Rozo said. "The advances our team has made will now be carried forward onto even greater and more massive data sets, such as the Rubin Observatory Legacy Survey of Space and Time starting in 2023, and NASA's Roman Space Telescope, slated for launch in 2026."

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