麻豆村

The Dark Energy Spectroscopic Instrument, or DESI, has finished its planned 3D map of the universe 鈥� but the work is far from over. 麻豆村 researchers are already pushing the survey into new scientific territory, redefining what DESI can uncover about how the universe grows, changes and evolves.

Since May 17, 2021, roughly every 20 minutes, DESI鈥檚 5,000 fiber optic eyes locked onto distant pinpricks of light and gathered photons that traveled toward Earth for billions of years.

The five-year survey finished ahead of schedule and with vastly more data than expected. To date, the work has resulted in the largest high-resolution 3D map of the universe ever made. Researchers use that map to explore dark energy, the fundamental ingredient that makes up about 70% of our universe and is driving its accelerating expansion.

By comparing how galaxies clustered in the past with their distribution today, researchers have traced dark energy鈥檚 influence over 11 billion years of cosmic history. Surprising results using DESI鈥檚 first three years of data hinted that dark energy, once thought to be a 鈥渃osmological constant,鈥� . With the full set of five years of data, researchers will have significantly more information to test whether that hint disappears or grows. If confirmed, it would mark a major shift in how we think about our universe and its potential fate, which hinges on the balance between matter and dark energy.

DESI鈥檚 quest to understand dark energy is a global endeavor. The international experiment brings together the expertise of more than 900 researchers (including 300 PhD students) from over 70 institutions. The project is managed by the Department of Energy鈥檚 Lawrence Berkeley National Laboratory (), and the instrument was constructed and is operated with funding from the DOE Office of Science. DESI is mounted on the U.S. National Science Foundation鈥檚 Nicholas U. Mayall 4-meter Telescope at (a program of NSF NOIRLab) in Arizona.

鈥淒ESI鈥檚 five-year survey has been spectacularly successful,鈥� said Michael Levi, DESI director and a scientist at Berkeley Lab. 鈥淭he instrument performed better than anticipated. The results have been incredibly exciting. And the size and scope of the map and how quickly we鈥檝e been able to execute is phenomenal. We鈥檙e going to celebrate completion of the original survey and then get started on the work of churning through the data, because we鈥檙e all curious about what new surprises are waiting for us.鈥�

This visualization shows how DESI鈥檚 map of the universe accumulated over five years. It begins with DESI鈥檚 tiles on the night sky and transitions to the 3D map. Earth is at the center of the wedges, and every dot is a galaxy. Credit: DESI collaboration and KPNO/NOIRLab/NSF/AURA/R. Proctor

DESI has now measured cosmological data for six times as many galaxies and quasars as all previous measurements combined. The collaboration will immediately begin processing the completed dataset, with the first dark energy results from DESI鈥檚 full five-year survey expected in 2027. In the meantime, DESI scientists continue to analyze the survey鈥檚 first three years of data, refining dark energy measurements and producing additional results on the structure and evolution of the universe, with several papers planned later this year.

鈥淭he Dark Energy Spectroscopic Instrument has truly exceeded all expectations, delivering an unprecedented 3D map of the universe that will revolutionize our understanding of dark energy,鈥� said Kathy Turner, Program Manager for the Cosmic Frontier in the Office of High Energy Physics at the Department of Energy. 鈥淔rom its inception, we envisioned a project that would push the boundaries of cosmology, and to see it come to such a spectacularly successful completion for its initial survey, ahead of schedule and with such rich data, is incredibly rewarding. The dedication and ingenuity of the entire DESI collaboration have made this world-leading science a reality, and I am immensely proud of the groundbreaking results we are already seeing and the discoveries yet to come as we continue to explore the mysteries of our cosmos.鈥�

Carnegie Mellon contributions

Antonella Palmese co鈥慶hairs DESI鈥檚 Transients and Low-Redshift Cosmology Working Group, which studies supernovae, kilonovae, tidal disruption events and active galactic nuclei variability, phenomena collectively known as transient events. Her leadership is helping transform DESI 鈥� originally built for galaxy redshift mapping 鈥� into a powerful tool for transient science and multimessenger cosmology, improving measurements of how fast the universe is expanding.

鈥淭his has been a state-of-the-art cosmology survey,鈥� said Palmese, an assistant professor of physics, member of the McWilliams Center for Astrophysics and Cosmology and part of the DESI team since 2018. 鈥淒ESI has been very efficient and because we were able to finish the main survey early, we鈥檝e been able to make some very special contributions.鈥�

Three Carnegie Mellon doctoral students, Xander Hall, Ariel Amsellem and Ekaterine Dadiani, have been key contributors to DESI work.

Hall leads efforts linking DESI galaxy data with gravitational鈥憌ave detections, enabling new ways to measure the universe鈥檚 expansion. Their work broadens DESI鈥檚 impact beyond dark energy into the physics of stellar explosions, black holes, and transient cosmology. Because DESI鈥檚 5,000 optical fibers are always fully assigned to tasks, Hall takes advantage of unused fibers to capture serendipitous discoveries without adding to the cost of the survey.

鈥淏y looking through the millions of objects that DESI observes, we are looking for discoveries that may not have been obvious from other imaging surveys,鈥� Palmese said. 鈥淴ander is running algorithms to identify and classify objects that will be potential targets for future study.鈥�

Palmese said Hall鈥檚 leadership as first author of the 鈥淒ESI Transient Survey: Legacy Classifications and Methodology鈥� has been instrumental, and a version of this project will be a major component of DESI-II, beginning in 2029.

鈥淭he contribution Xander had to this has been very important, and I don鈥檛 know if it would have happened without them,鈥� Palmese said.

Amsellem is finding more precise ways to measure how fast the universe is expanding by combining DESI鈥檚 galaxy data with distance measurements from gravitational waves produced when black holes and neutron stars merge. His work helps address longstanding discrepancies in Hubble constant measurements. He also creates mock data sets to test simulations for a DESI project related to low-redshift galaxies.

Dadiani is searching for massive black hole binaries, which occur when two black holes orbit each other and produce powerful gravitational-wave signals. Her work will be essential when LISA (the Laser Interferometer Space Antenna) launches in the mid-2030s. The space-based observatory will detect low-frequency gravitational waves from merging supermassive black hole binaries and thousands of compact binary star systems.

鈥淓ka is producing what will likely become one of the largest catalogs of LISA source progenitors,鈥� Palmese said.

In addition to these efforts, DESI鈥檚 secure classifications of supernovae, DESI are supporting the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory, which will create the most detailed timelapse of the university.

鈥淭his will become an important part of DESI-II,鈥� Palmese said.

An observing machine

DESI in May 2021. Since then, the instrument has far surpassed the collaboration鈥檚 original goals. The plan was to capture light from 34 million galaxies and quasars (extremely distant yet bright objects with black holes at their cores) over the five-year sky survey. DESI instead observed more than 47 million galaxies and quasars and 20 million stars.

The project鈥檚 success is even more impressive in light of several challenges. DESI is a complicated machine with thousands of parts to maintain. In 2020, final tests of the instrument were interrupted by the COVID-19 pandemic. In 2022, the but, through the efforts of firefighters and staff, did not damage the telescope. Recovery efforts were slowed by monsoons and mudslides.

These accelerated time-lapses show how the machinery holding DESI moves the instrument into position. Credit: Marilyn Sargent/Berkeley Lab

鈥淒ESI is a complicated but wonderfully robust system, and it鈥檚 been a huge amount of fun to see it come together and work so well for such a long time,鈥� said Connie Rockosi, co-instrument scientist for DESI and a professor at UC Santa Cruz and UC Observatories. 鈥淲e鈥檝e learned about the instrument over five years, and we know its personality and behavior pretty well. That鈥檚 important because having the instrument be so efficient is why we鈥檙e here at the end of DESI鈥檚 original survey with such great data and so much science coming out.鈥�

To map objects, researchers use specially-designed software to optimize DESI observations and decide where to point the telescope. Robotic positioners precisely line up optical fibers that are accurate to within 10 microns, or less than the width of a hair. Ten spectrographs then measure and split the light into its separate colors to determine each object鈥檚 position, velocity, and chemical composition. Each night, roughly 80 gigabytes of data streams through ESnet, DOE鈥檚 high-speed science network, to supercomputers at Berkeley Lab鈥檚 National Energy Research Scientific Computing Center (NERSC). Initial processing lets researchers do quality assurance and make any adjustments needed for the next night of observations.

This animation shows a 3D model of DESI's focal plane. The movement of the 5,000 robotic positioners is coordinated so that they don鈥檛 bump into one another. Credit: David Kirkby/DESI collaboration

Collaborators across the project found ways to make DESI more efficient. Efforts spanned telescope operations, tweaks to the instrument hardware, updates to software, observing protocols, methods to reduce the data, and more.

鈥淭here鈥檚 been constant monitoring and intervention to make the whole thing tick,鈥� said Adam Myers, co-manager for DESI鈥檚 survey operations and professor at the University of Wyoming. 鈥淎nd the DESI team is remarkable. This huge group of people have all been working on whether they could save one or two or three percent in their particular area, and when you add it all up, it results in these amazing gains in efficiency.鈥�

DESI is designed to make several overlapping passes of the sky to observe its full footprint (and sometimes make repeated observations of faint objects). The survey was so efficient, the team completed an entire additional pass over the sky for the 鈥淏right-Time Survey,鈥� which is carried out when reflected light from the moon hinders observations of faint and distant objects. All told, DESI made five passes during the Bright-Time Survey and seven during the Dark-Time Survey, covering about two-thirds of the northern night sky.

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This time-lapse shows DESI鈥檚 observations accumulating to fill out the survey鈥檚 map during the 鈥淒ark-Time Survey,鈥� which focuses on the faintest and most distant objects. Each 鈥渢ile鈥� is one telescope pointing where DESI records spectra from thousands of objects at once. The tiles overlap to add density to the map, with most areas observed multiple times. Credit: DESI data: Anand Raichoor/DESI collaboration; Sky map: Axel Mellinger, A Color All-Sky Panorama Image of the Milky Way, Publ. Astron. Soc. Pacific 121, 1180-1187 (2009)听

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The sky鈥檚 the limit

DESI will continue observations through 2028 and grow its map by about 20%, from 14,000 square degrees to 17,000 square degrees. (For comparison, the moon covers approximately 0.2 square degrees, and the full sky has over 41,000 square degrees). The extended map will cover parts of the sky that are more challenging to observe: areas that are closer to the plane of the Milky Way, where bright nearby stars can make it harder to see more distant objects, or further to the south, where the telescope must account for peering through more of Earth鈥檚 atmosphere.

The experiment will also revisit the existing area of the map to collect data from a new set of galaxies: more distant and faint 鈥渓uminous red galaxies.鈥� These will provide an even denser and more detailed map in the regions DESI has already covered, giving researchers a clearer picture of the universe鈥檚 history.

This time-lapse shows the sky over Kitt Peak on February 12-13, 2024. On that night, DESI observed a record 41 tiles for the Dark-Time Survey. DESI鈥檚 planned five-year survey map is outlined in red, and the moving circle shows where the instrument pointed throughout the night. The Mayall Telescope that houses DESI is visible at the top of the image, just left of center. Credit: DESI data: Anand Raichoor/DESI collaboration; Spacewatch All-Sky Camera footage: SPACEWATCH庐, Lunar and Planetary Laboratory, University of Arizona

Researchers will also study nearby dwarf galaxies and stellar streams, bands of stars torn from smaller galaxies by the Milky Way鈥檚 gravity. The hope is to better understand dark matter, the invisible form of matter that accounts for most of the mass in the universe but has never been directly detected.

The extended map is already underway. When it became clear that DESI would operate beyond its original survey plan, researchers began interleaving the new observations with the ongoing DESI survey to optimize the use of telescope time and keep the instrument from sitting idle.

鈥淲e鈥檝e built a remarkable piece of equipment that met all our expectations and then some,鈥� Levi said. 鈥淣ow we鈥檙e pushing beyond our original plan. We don鈥檛 know what we鈥檒l find, but we think it鈥檒l be pretty exciting.鈥�

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science national user facility. Additional support for DESI is provided by the U.S. National Science Foundation; the Science and Technology Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission (CEA); the Secretariat of Science, Humanities, Technology and Innovation (SECIHTI) of Mexico; the Ministry of Science and Innovation of Spain; and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct scientific research on I鈥檕ligam Du鈥檃g (Kitt Peak), a mountain with particular significance to the Tohono O鈥檕dham Nation.

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