The Enigma of Cosmic Expansion
The nature of the force driving the expansion of the universe remains one of the most profound mysteries in astronomy. According to the standard model, the ordinary matter observable to humankind constitutes merely 5% of the total universe, with the remaining 95% thought to consist of unknown entities known as “dark matter” and “dark energy.” This intriguing paradigm raises questions about the fundamental components of our cosmos.
Unveiling Dark Energy with DESI
The “Spectrograph for Dark Energy Survey” (DESI) was developed with the mission to unravel the mysteries surrounding dark energy. By April 2026, DESI had completed five years of observations, culminating in the creation of the largest three-dimensional map of the universe ever produced. In this context, the latest release of two-dimensional imaging data underpinning DESI, titled “DESI Legacy Imaging Surveys,” has recently been published.
The Legacy Imaging Surveys initiative began with the selection of galaxies and quasars intended for observation by DESI and has evolved over the course of 13 years, incorporating data incrementally. This project is based on an impressive 263,407 exposures captured by the 570-megapixel Dark Energy Camera (DECam) mounted on the Blanco telescope, as well as data from the Mayall telescope at the Kitt Peak National Observatory and the Bok telescope at the Steward Observatory of the University of Arizona. Additionally, it integrates publicly available data, including observations from NASA’s Wide-field Infrared Survey Explorer (WISE) and the Dark Energy Survey (DES).
A Collective Effort Over Thirteen Years
Over 160 scientists have contributed to the data collection process, with a dedicated team of 20 individuals finalising the definitive dataset. To date, more than 1,800 scientific papers have been published based on this data, which has become an indispensable pillar of astronomical research.
The recently published map boasts an astonishing volume of information, encompassing 5.6 trillion pixels and documenting around 4 billion celestial objects, including stars, galaxies, black holes, and asteroids. It covers approximately 75% of the night sky using both visible and near-infrared light, allowing for observations deep into the cosmos (the exoplanetary universe) without interference from dust or stars in the Milky Way. This wealth of data is accessible to the public via the “Legacy Survey Sky Viewer.”
Significance in Astronomical Research
“This is already an integral part of astronomical research,” explains David Schlegel, a member of the Lawrence Berkeley National Laboratory under the U.S. Department of Energy and co-director of the Legacy Surveys. “Researchers working with celestial objects typically start by opening the Legacy Imaging Viewer to identify their target of study.”
This latest map starkly contrasts with the three-dimensional DESI map released in April 2026, which reconstructs the universe’s history over 11 billion years and examines the impact of dark energy based on spectroscopic observations of over 47 million galaxies and quasars to ascertain their distances.
Insights into Dark Energy’s Nature
The analysis of data collected during the first three years, in conjunction with other observations—such as the oldest light arriving from all directions in the universe (the cosmic microwave background radiation) and supernovae—has indicated that dark energy may not be a constant force. There are suggestions that it could be weakening over time. Comprehensive results from the five-year data analysis are anticipated to be published in 2027.
The Legacy Imaging Surveys, in essence, serve as precise photographs of the night sky, providing two-dimensional data that records the position and brightness of celestial bodies. While they do not include information about distances or velocities, it is precisely these imaging data that have enabled DESI to select observation targets and construct three-dimensional maps. They lay the groundwork for creating such maps and simultaneously serve as a valuable independent observational resource.
Setting the Stage for Next-Generation Telescopes
Looking ahead, the data from the Legacy Surveys is expected to become a vital reference for next-generation observational facilities. The Vera C. Rubin Observatory, jointly funded by the NSF and the U.S. Department of Energy, has commenced a decade-long observational programme, while NASA’s Nancy Grace Roman Space Telescope has already been launched and is preparing to begin its observations. With the integration of this new observational data, researchers will be able to compare the most comprehensive and exhaustive night sky datasets to date with the fresh results from upcoming observations. Furthermore, these datasets are set to facilitate the training of artificial intelligence tools, enhancing the analysis of astronomical data on a petabyte scale.
“Exploring the universe always begins with images of the night sky,” states Arjun Day, co-director of Legacy Surveys and an astronomer at NSF’s NOIRLab. “These data form the foundation for studying the history of the universe’s expansion and the formation of the Milky Way. However, the sky belongs to all of us. These data provide everyone with the opportunity to uncover its wonders.”
Mapping the Cosmos
If the three-dimensional DESI map acts as the compass guiding us towards the enigma of dark energy, the Legacy Imaging Surveys serve as the detailed roadmap supporting that compass. The synergy between these two achievements is allowing us to outline the past and present of the universe with unprecedented clarity.