New results from the Dark Energy Survey use the largest ever sample of galaxies over an enormous piece of the sky to produce the most precise measurements of the universe’s composition and growth to date. Scientists measured that the way matter is distributed throughout the universe is consistent with predictions in the standard cosmological model.

Over the course of six years, DES surveyed 5,000 square degrees — almost one-eighth of the entire sky — in 758 nights of observation, cataloguing 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 at relatively recent epochs.

Since DES studied 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, DES 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 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, there is possibly an undiscovered aspect to the universe. While there have been persistent hints from DES and several previous galaxy surveys that the current universe is a few percent less clumpy than predicted—an intriguing find worthy of further investigation—the recently released results are consistent with the prediction.

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 last 25% is dark matter, whose gravitational influence binds galaxies together. Both dark matter and dark energy remain invisible and mysterious, but DES seeks to illuminate their natures by studying how the competition between them shapes the large-scale structure of the universe over cosmic time.

“DES has been able to define the properties of dark matter in such a precise way that competes with the data resulting from the study of the cosmic microwave background’s radiation, and besides, it complements it”, asserts Ignacio Sevilla, tenured scientist at CIEMAT. “It’s exciting to have achieved one of the most precise measurements ever obtained of the fundamental properties of the universe”.

DES photographed the night sky using the 570-megapixel Dark Energy Camera, installed on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for DES and built and tested at Fermilab (United States). There was an important Spanish contribution to the design, building, verification and installation of the DECam.

“There was an unprecedented complexity to this challenge, which required a multidisciplinary team involving hundreds of people, an investment of millions of hours on supercomputers and the development of techniques that will determine the future of the field in almost every aspect of the analysis”, adds Martin Crocce, ICE researcher who leads the group which studies the great-scale structure at the DES international collaboration. “We’re entering a new era in our global understanding of the universe, with direct observations ranging from the early universe, 380.000 years old, to our recent universe, 13 billion years later”.

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. The pattern of these distortions depends on the amount and distribution of matter through light’s trajectory. “By analyzing the subtle distortions of 100 millions of galaxies, DES has successfully tracked the distribution of matter that produces these distorsions”, explains Marco Gatti, predoctoral researcher at IFAE (now at the University of Pennsylvania), who has led the group which elaborates maps of matter. “These are the largest maps ever created, they cover an eighth of the sky and primarily show dark matter, which doesn’t emit light and cannot be detected through traditional methods”. This analysis has been partly possible thanks to new techniques for modeling large-field maps and large simulations carried out by Spanish groups and distributed on a new Big Data platform (CosmoHub), housed in the Port d’Informació Científica (PIC), a CIEMAT and IFAE data center.

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.

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 development of new methodologies to measure the 100 millions of galaxies’ redshift, directly related to their distance, has been a key factor, which allows us to produce a 3D map of the universe”, notes Giulia Giannini, predoctoral researcher at IFAE and one of the scientists in charge of these measurements. “We have combined several independent methods and applied more sophisticated and precise statistical techniques to calibrate the relationship between colors, positions and redshifts of galaxies as accurately as possible, something crucial to obtain unbiased data.”

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.

“Such precise measurements are the result of an analysis that is carried out with an extreme attention to detail every step of the way, from the data collection in the telescope to the calculation of the final results. Among many other factors, we have corrected the impact of external elements, such as stars of atmospheric effects, in our data”, says Martín Rodríguez Monroy, predoctoral researcher at CIEMAT, and one of the people in charge of the measurement of close-by galaxies’s clustering. “Seeing how all this effort translates into such precise and robust results is a great satisfaction”.

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 dataset is unique because it allows us to test the cosmological model by studying very different phenomena”, asserts Santiago Ávila, postdoctoral researcher at IFT and scientist in charge of analysing the relationship between the initial conditions of the Universe and the observed galaxy clustering. “Larger scales reveal wavelengths generated in the initial universe (acoustic barium oscillations), as well as how the first structures were formed from quantum fluctuations during the cosmological inflation”, he adds.

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, which will double the number of galaxies used in the results made public today. 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 DES results will be presented in a scientific seminar today, May 27 at 17:30 (Madrid, GMT+2 time). It can be followed via Zoom at: https://fnal.zoom.us/j/94822142182?pwd=UnlPSzg0NXdNdlFzK3R2VWV6aEk1dz09 The 30 scientific papers related to these results will be available after the seminar on the following link: https://www.darkenergysurvey.org/des-year-3-cosmology-results-papers/ and described in the video: https://www.youtube.com/watch?v=2faGqB2UDGo

The Dark Energy Survey is a collaboration of more than 400 scientists from 25 institutions in seven countries. For more information about the survey, please visit the experiment’s webpage: https://www.darkenergysurvey.org/es/

Spain was the first international group to join the United States to create, in 2005, the DES project, and participates through three different institutions, two of them in Barcelona (the Institute of Spaces Sciences, ICE, CSIC, and the Institut de Física d’Altes Energies, IFAE) and one in Madrid (the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT), along with researchers from the Instituto de Física Teórica, IFT (CSIC-UAM).