At the Kitt Peak National Observatory in
Arizona, an instrument with 5,000 tiny robotic eyes scans the night sky. Every 20 minutes, the instrument and the telescope it’s attached to observe a new set of 5,000 galaxies. The instrument is called DESI—Dark Energy Survey Instrument—and once it’s completed its five-year mission, it’ll create the largest 3D map of the Universe ever created. But scientists are getting access to DESI’s first data release and it suggests that dark energy may be evolving. DESI is the most powerful multi-object survey spectrograph in the world, according to their website. It’s gathering the spectra for tens of millions of galaxies and quasars. The goal is a 3D map of the Universe that extends out to 11 billion light-years. That map will help explain how dark energy has driven the Universe’s expansion. DESI began in 2021 and is a five-year mission. The first year of data has been released, and scientists with the project say that DESI has successfully measured the expansion of the Universe over the last 11 billion years with extreme precision. “The DESI team has set a new standard for studies of large-scale structure in the Universe.” DESI collects light from 5,000 objects at once with its 5,000 robotic eyes. It observes a new set of 5,000 objects every 20 minutes, which means it observes 100,000 objects—galaxies and quasars—each night, given the right observing conditions. DESI’s data creates a map of the large-scale structure of the Universe. The map will help scientists unravel the history of the Universe’s expansion and the role dark energy plays. We don’t know what dark energy is, but we know some force is causing the Universe’s expansion to accelerate. “The DESI instrument has transformed the Mayall Telescope into the world’s premier cosmic cartography machine,” said Pat McCarthy, Director of NOIRLab, the organization behind DESI. “The DESI team has set a new standard for studies of large-scale structure in the Universe. These first-year data are only the beginning of DESI’s quest to unravel the expansion history of the Universe, and they hint at the extraordinary science to come.” DESI measures dark energy by relying on baryonic acoustic oscillations (BAO.) Baryonic matter is “normal” matter: atoms and everything made of atoms. The acoustic oscillations are density fluctuations in normal matter that date back to the Universe’s beginnings. BAO are the imprint of those fluctuations, or pressure waves, that moved through the Universe when it was all hot, dense plasma. As the Universe cooled and expanded, the density waves froze their ripples in place, and where density was high, galaxies eventually formed. The ripple pattern of the BAO is visible in the DESI leading image. It shows strands of galaxies, or galaxy filaments , clustered together. They’re separated by voids where density is much lower. The deeper DESI looks, the fainter the galaxies are. They don’t provide enough light to detect the BAO. That’s where quasars come in. Quasars are extremely bright galaxy cores, and the light from distant quasars creates a shadow of the BAO pattern. As the light travels through space, it interacts with and gets absorbed by clouds of matter. That lets astronomers map dense pockets of matter, but it took over 450,000 quasars. That’s the most quasars ever observed in a survey like this. Because the BAO pattern is gathered in such detail and across such vast distances, it can act as a cosmic ruler. By combining the measurements of nearby galaxies and distant quasars, astronomers can measure the ripples across different periods of the Universe’s history. That allows them to see how dark energy has stretched the scale over time. It’s all aimed at understanding the expansion of the Universe. In the Universe’s first three billion years, radiation dominated it. The Cosmic Microwave Background is evidence of that. For the next several billion years, matter dominated the Universe. It was still expanding, but the expansion was slowing because of the gravitational force from matter. But since then, the expansion has accelerated again, and we give the name dark energy to the force behind that acceleration. So far, DESI’s data supports cosmologists’ best model of the Universe. But there are some twists. “We’re incredibly proud of the data, which have produced world-leading cosmology results,” said DESI director and LBNL scientist Michael Levi. “So far, we’re seeing basic agreement with our best model of the Universe, but we’re also seeing some potentially interesting differences that could indicate dark energy is evolving with time.” Levi is referring to Lambda Cold Dark Matter (Lambda CDM), also known as the standard model of Big Bang Cosmology. Lambda CDM includes cold dark matter—a weakly interacting type of matter—and dark energy. They both shape how the Universe expands but in opposite ways. Dark energy accelerates the expansion, and regular matter and dark matter slow it down. The Universe evolves based on the contributions from all three. The Lambda CDM does a good
Job of describing what other experiments and observations find. It also assumes that dark energy is constant and spread evenly throughout the Universe. This data is just the first release, so confirmation of dark energy evolution must wait. By the time DESI has completed its five-year run, it will have mapped over three million quasars and 37 million galaxies. That massive trove of data should help scientists understand if dark energy is changing. Whatever the eventual answer, the question is vital to understanding the Universe. “This project is addressing some of the biggest questions in astronomy, like the nature of the mysterious dark energy that drives the expansion of the Universe,” says Chris Davis, NSF program director for NOIRLab. “The exceptional and continuing results yielded by the NSF Mayall telescope with DOE DESI will undoubtedly drive cosmology research for many years to come.” DESI isn’t the only effort to understand dark energy. The ESA’s Euclid
spacecraft is already taking its own measurements to help cosmologists answer their dark energy questions. In a few years, DESI will have some more powerful allies in the quest to understand dark energy. The Vera Rubin Observatory and Nancy Grace Roman Space Telescope will both contribute to our understanding of the elusive dark energy. They’ll perform surveys of their own, and by combining data from all three, cosmologists are poised to generate some long-sought answers. But for now, scientists are celebrating DESI’s first data release. “We are delighted to see cosmology results from DESI’s first year of operations,” said Gina Rameika, associate director for High Energy Physics at the Department of Energy. “DESI continues to amaze us with its stellar performance and how it is shaping our understanding of dark energy in the Universe.”