The international astronomical community has taken a giant leap forward with the completion of the main survey by the Dark Energy Spectroscopic Instrument (DESI) , which has succeeded in creating the largest three-dimensional map of the universe ever constructed. The survey, planned for five years, has finished ahead of schedule and with a much greater amount of data than expected.
Thanks to this coordinated effort, researchers now have a high-resolution 3D map spanning billions of years of cosmic history, which will be used to study dark energy, dark matter, and galaxy evolution in detail . Much of this work involved significant participation from European institutions, particularly Spanish centers such as the Instituto de Astrofísica de Canarias (IAC) and the Instituto de Astrofísica de Andalucía (IAA-CSIC).
A milestone: the largest high-resolution 3D map of the cosmos
On one of the final nights of the main survey, DESI's 5.000 fiber-optic "eyes" were pointed toward a region of the sky near Ursa Minor. Every twenty minutes or so, the system would lock onto tiny points of light extremely far away, capturing photons that had traveled billions of years to reach the telescope at Kitt Peak, Arizona.
At dawn, the international collaboration celebrated a historic moment: the entire area planned for the original DESI map had been successfully completed. This goal, conceived as a five-year program, has been achieved ahead of schedule and with far more data than planned, resulting in the largest and most detailed three-dimensional map of the universe currently available.
This mapping allows us to compare how galaxies were distributed in the past with their current distribution, something essential for reconstructing the influence of dark energy over some 11.000 billion years of cosmic history . The initial results from the first three years already suggested that dark energy, traditionally treated as a “cosmological constant,” could vary over time.
With the complete set of five years' worth of observations, cosmologists now have a much more robust database to test whether these indications persist or fade away. If dark energy is confirmed to be evolving , it would be necessary to revise many current theories and rethink the scenario regarding the ultimate fate of the universe, which depends on the balance between matter and dark energy.
A global collaboration with a strong European and Spanish presence
The DESI science program is the result of a global effort involving more than 900 researchers , including approximately 300 doctoral students, at over 70 institutions worldwide. The project is managed by the Lawrence Berkeley National Laboratory (Berkeley Lab), part of the U.S. Department of Energy (DOE), and is funded by the DOE Office of Science, the U.S. National Science Foundation (NSF), and numerous international agencies and foundations.
Europe, and Spain in particular, plays a leading role in the collaboration. At the Instituto de Astrofísica de Canarias (IAC) , two teams are focused on exploiting cosmological and stellar data. One group, with researchers such as Francisco Kitaura, Ginevra Favole, Aurelio Carnero, and Francesco Sinigaglia, is dedicated to deciphering the information encoded in the galaxy distribution measured by DESI using advanced numerical simulations.
Another team from the IAC, led by astronomers such as David Aguado, Guillaume Thomas, and Carlos Allende Prieto, is working alongside doctoral students like Andrea Ruiz del Pozo, Daniel García Jiménez, and Ruizhi Zhang in the search for the first stars that formed in the Milky Way . To do this, they are analyzing the massive collection of stellar spectra obtained by DESI, an unprecedented database for studying the early history of our galaxy.
Also from Spain, the Institute of Astrophysics of Andalusia (IAA-CSIC) has made a key contribution to the instrument's technological development. IAA researchers participated in the design and construction of the robotic fiber optic positioners located on the telescope's focal plate. After more than a decade of work, these components have become the mechanical heart of the system, allowing for the precise placement of thousands of fibers over specific galaxies and quasars.
The IAA-CSIC is also leading the generation of virtual galaxy maps based on cosmological simulations . These simulations are fundamental for rigorously quantifying the uncertainties of DESI measurements and extracting the maximum scientific yield from the project, a key aspect for precision cosmology.
How the DESI “observing machine” works
DESI was installed on the 4-meter Nicholas U. Mayall Telescope at Kitt Peak National Observatory in Arizona and formally began collecting scientific data in May 2021. Since then, the instrument has far exceeded its initial goals: the collaboration's plan was to capture light from about 34 million galaxies and quasars , but ultimately more than 47 million of these objects have been observed, in addition to approximately 20 million stars.
Despite its enormous complexity, the instrument has proven remarkably robust. According to team members, integrating all its subsystems and verifying their reliable operation over years has been a major challenge. DESI uses specialized software designed to optimize observations and determine which region of the sky to point towards at any given time, taking into account observing conditions and scientific priorities.
The telescope's focal plate houses thousands of robotic positioners, each associated with an optical fiber. These positioners adjust the fibers' location with an accuracy on the order of 10 microns, less than the thickness of a human hair . In this way, each fiber aligns with a specific galaxy, quasar, or star, collecting its light with great efficiency.
This light is carried through fibers to an array of ten virtually identical spectrographs. There, it is broken down into its different wavelengths, allowing scientists to determine the distance, recession velocity, and chemical composition of each observed object. With millions of such measurements, astronomers can reconstruct the large-scale structure of the universe in three dimensions.
The result of this entire process is that DESI has managed to gather cosmological data for a number of galaxies and quasars that is approximately six times greater than the total obtained by all previous projects combined . The collaboration has already begun the full processing of the dataset, with the goal of publishing the first results on dark energy based on five years of observations around 2027.
Challenges overcome: pandemic, fires and recovery
The success of DESI is all the more remarkable considering the obstacles encountered along the way. In 2020, the COVID-19 pandemic forced the interruption of final testing of the instrument , delaying the project's launch and complicating the coordinated work between the international teams.
In 2022, the so-called Contreras Fire severely affected the area of the Kitt Peak National Observatory. The advancing flames threatened the facilities, but ultimately, thanks to the efforts of the firefighters and observatory staff, the Mayall Telescope and the DESI system itself did not suffer direct damage.
Following the fire, recovery efforts on the mountain were slowed by monsoon rains and subsequent mudslides, requiring very careful planning to safely resume operations. Despite these challenges, the collaborative effort managed to keep the project on track and meet, and even exceed, the initially planned scientific timeline.
This degree of resilience and adaptability of the team has been decisive so that, despite logistical and meteorological difficulties, DESI has been able to complete its main sounding ahead of schedule and with a volume of data higher than expected when the instrument was conceived.
The map expands: scientific objectives until 2028
Although the initial map that gave rise to the five-year scientific program is now complete, DESI's work does not end here. The instrument will continue observing the sky until 2028 with the aim of expanding its mapping by approximately 20%, increasing from 14.000 to about 17.000 square degrees of the observable sky.
To put these figures into context, the entire sky covers just over 41.000 square degrees, while the full moon occupies only about 0,2 square degrees. With this additional coverage, DESI will encompass regions of the sky that are particularly difficult to study , such as areas near the plane of the Milky Way, where the high density of bright stars makes it hard to see more distant objects, or more southerly regions, where observation is done through a greater portion of the atmosphere.
In addition to expanding the area covered, the team plans to re-observe previously mapped regions to obtain data on new types of galaxies, particularly so-called luminous red galaxies. These are very bright objects, but more distant and fainter from our perspective, allowing scientists to probe even deeper volumes of the universe and fill in gaps in the three-dimensional map.
Repeating these surveys will allow for the creation of an even denser and more detailed map , improving the accuracy of dark energy measurements and refining cosmological models. This "second pass" approach to previously studied areas is especially valuable for reducing systematic errors and confirming possible subtle signals detected in the initial analyses.
Alongside its purely cosmological aspects, DESI will also exploit its ability to study nearby dwarf galaxies and stellar streams —that is, structures of stars that have been pulled from smaller galaxies by the Milky Way's gravity. These gravitational trails offer very useful clues about the distribution of dark matter in our galactic neighborhood.
Dark energy, dark matter, and the fate of the universe
The project's main objective is to shed light on the nature of dark energy, which represents about 70% of the universe's energy content and is responsible for its accelerated expansion. By comparing its large-scale structure across different cosmic epochs, DESI aims to determine whether this component behaves as a constant over time or, conversely, shows signs of evolution.
Preliminary results based on the first three years of data suggested that dark energy might deviate slightly from the standard model, which assumes it remains constant over time. The new, complete map will allow us to verify whether these clues are confirmed or refuted . The answer will directly influence scenarios for the future expansion of the cosmos.
If dark energy remains constant or weakens, the universe could expand ever more slowly or tend toward some kind of long-term equilibrium. If, on the other hand, dark energy strengthens over time , the accelerated expansion would continue to increase and, in certain models, could even fragment gravitationally bound structures in the extremely distant future.
In addition to dark energy, DESI offers a wealth of information about dark matter , the invisible form of matter that makes up most of the mass of the universe and is only manifested through its gravitational influence. The study of dwarf galaxies, stellar streams, and cosmic voids will help reconstruct how this non-luminous matter is distributed.
In this context, some lines of research focus on cosmic voids , large regions of space with a galaxy density far below average. Analyzing their shape, size, and evolution with DESI data provides a complementary and highly sensitive way to test dark energy models and alternative theories of gravity on cosmological scales.
The scientific contribution from Spain and Europe
European participation in DESI is channeled through multiple research centers and funding agencies, but in the case of Spain, the impact is particularly noticeable in the combination of technological development, numerical simulations, and scientific exploitation . Both the IAC and the IAA-CSIC have positioned themselves at the forefront within their respective fields.
For years, the IAA-CSIC has worked on the design and manufacture of the robotic fiber optic positioners and the focal plate , a set of essential devices that allow each of DESI's 5.000 fibers to be positioned exactly where needed on the sky plane. This technological advancement has been key to achieving the instrument's enormous efficiency.
In parallel, the IAC has promoted studies focused on the cosmological interpretation of the galaxy distribution observed by DESI through numerical simulations, as well as the detailed analysis of stellar spectra to understand the formation history of the Milky Way. This dual focus, cosmological and galactic, places the Spanish community in a very favorable position with regard to the scientific results that will be published.
The Spanish funding comes largely from the Ministry of Science, Innovation and Universities , which participates in the consortium along with similar bodies from other countries. International support also includes the UK's Science and Technology Facilities Council, the French Alternative Energies and Atomic Energy Commission (CEA), Mexico's Secretariat of Science, Humanities, Technology and Innovation, and several private foundations.
All of this positions DESI as a clear example of how international collaboration, combined with regional specialization , allows us to address scientific questions that would be impossible for a single country or institution to tackle. For Europe and Spain, this type of project reinforces their role at the forefront of precision cosmology.
After completing the largest three-dimensional map of the universe and overcoming top-level technical and logistical challenges, DESI now enters a phase in which the detailed analysis of the data will be as important as the observations themselves; with millions of galaxies, quasars and stars already registered, and with outstanding participation from European and Spanish teams, the upcoming publications promise to redefine our vision of dark energy, dark matter and the long-term destiny of the cosmos , consolidating this project as one of the pillars of modern cosmology.
