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    DESI (Dark Energy Spectroscopic Instrument)

    Also known as DESI, Dark Energy Spectroscopic Instrument

    DESI is a survey instrument on a 4-metre telescope in Arizona that measures the 3D positions of galaxies and quasars to trace how the universe has expanded.[1][2] Its March 2025 results hinted at evolving dark energy at 2.8 to 4.2 sigma; it finished its planned five-year map in April 2026 with more than 47 million galaxies and quasars, and full results are expected in 2027.[3][4][5]

    Editor reviewedUpdated Particle physics and cosmologyPhysicsScience
    Key facts

    What it is

    The Dark Energy Spectroscopic Instrument (DESI) is mounted on the Nicholas U. Mayall 4-metre Telescope at Kitt Peak National Observatory in Arizona. It can capture light from 5,000 galaxies at once.[1] More than 900 researchers from over 70 institutions take part. Lawrence Berkeley National Laboratory manages the project with funding from the DOE Office of Science.[6] DESI began collecting data in May 2021. Light gathered by its optical fibres is split by ten spectrographs, and about 80 gigabytes of data a night flow to supercomputers at Berkeley Lab’s NERSC centre. It has now measured six times as many galaxies and quasars as all previous measurements combined.[7]

    How it works

    DESI measures baryon acoustic oscillations (BAO). These are a pattern in the distribution of galaxies that acts as a standard ruler, whose apparent size at different epochs depends on how the universe was expanding.[2] Comparing the ruler across cosmic time tests whether dark energy behaves like Einstein’s cosmological constant, as the standard Lambda-CDM model assumes.[8]

    The 2025 results

    On 19 March 2025 DESI released BAO results from three years of data, covering about 14 to 15 million galaxies and quasars. Combined with cosmic microwave background and supernova data, they preferred evolving dark energy over a cosmological constant at 2.8 to 4.2 sigma, depending on the supernova sample. That is short of the 5-sigma discovery standard.[3] The preferred models have w0 > -1 and wa < 0.[9] The BAO data alone show a mild 2.3-sigma tension with parameters inferred from the CMB.[10]

    The same data constrain neutrinos. The 95% upper limit on the sum of neutrino masses is 0.064 eV under Lambda-CDM, loosening to 0.16 eV if dark energy evolves.[11] The laboratory experiment KATRIN, which measures neutrino mass directly, has an upper limit of 0.45 eV.[12]

    Survey complete, results pending

    DESI finished surveying the full area of its originally planned map at sunrise on 15 April 2026, after one last night of observations, and Berkeley Lab announced it that day. Fermilab republished the release on 17 April, which is why some sources give that date.[13] The five-year survey finished ahead of schedule. It observed more than 47 million galaxies and quasars, against an original target of 34 million, plus more than 20 million stars.[4] It will keep observing through 2028, growing its map from 14,000 to about 17,000 square degrees. The first dark energy results from the full five-year dataset are expected in 2027.[5] Those results will show whether the evolving-dark-energy hint strengthens or fades.[5]

    Other surveys on the same question

    DESI is no longer the only new dark energy survey. On the ground, the Rubin Observatory in Chile started its 10-year Legacy Survey of Space and Time in mid-2026, and dark energy is one of its main goals.[14][15] In space, NASA’s Roman Space Telescope launched on 30 August 2026 to study dark energy with wide infrared surveys and Type Ia supernovae.[16][17]

    DESI data also feed into the Hubble constant debate. ACT’s 2025 analysis, combining CMB data with DESI, gave a Hubble constant of 68.43 ± 0.27 km/s/Mpc.[18]

    Questions readers ask

    Did DESI prove that dark energy is changing?

    No. Its 2025 analysis preferred evolving dark energy at 2.8 to 4.2 sigma, below the 5-sigma standard for a discovery.[3]

    How does DESI measure dark energy?

    It measures baryon acoustic oscillations, a pattern in galaxy positions that acts as a standard ruler. The ruler's apparent size at different times reveals how the universe expanded.[2]

    Does DESI say anything about neutrinos?

    Yes. Its 2025 data limit the sum of neutrino masses to below 0.064 eV assuming the standard model of cosmology, or 0.16 eV if dark energy evolves.[11]

    Is DESI still observing?

    Yes. It will keep observing through 2028 and expand its map to about 17,000 square degrees.[5]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      DESI is an instrument that can capture light from 5,000 galaxies at once, mounted on the Nicholas U. Mayall 4-metre Telescope at Kitt Peak National Observatory in Arizona. confirmedas of 2025-03-19

    2. [2]

      Baryon acoustic oscillations leave a pattern in the distribution of galaxies that acts as a standard ruler, whose size at different times depends on how the universe was expanding. confirmedas of 2025-03-19

    3. [3]

      On 19 March 2025 DESI released baryon acoustic oscillation results from three years of data covering about 14 to 15 million galaxies and quasars; combined with other data they preferred evolving dark energy over a cosmological constant at 2.8 to 4.2 sigma, short of the 5-sigma discovery standard. confirmedas of 2025-03-19

    4. [4]

      In April 2026 DESI completed its planned five-year survey ahead of schedule, having observed more than 47 million galaxies and quasars, against an original target of 34 million, plus more than 20 million stars. confirmedas of 2026-04-15

    5. [5]

      DESI will keep observing through 2028, expanding its map from 14,000 to about 17,000 square degrees, and its first dark energy results from the full five-year dataset are expected in 2027. confirmedas of 2026-04-15

    6. [6]

      DESI involves more than 900 researchers from over 70 institutions and is managed by Lawrence Berkeley National Laboratory with funding from the DOE Office of Science. confirmedas of 2026-04-15

    7. [7]

      DESI began collecting data in May 2021; light gathered by its fibres is split by ten spectrographs, about 80 gigabytes of data flow to Berkeley Lab's NERSC supercomputers each night, and it has measured six times as many galaxies and quasars as all previous measurements combined. confirmedas of 2026-04-15

    8. [8]

      The standard cosmological model, Lambda-CDM, combines cold dark matter with Lambda, the simplest form of dark energy, which acts as a cosmological constant. confirmedas of 2025-03-19

    9. [9]

      The DESI 2025 analysis favours dark energy models with w0 greater than -1 and wa less than 0, meaning dark energy whose strength changes over time. confirmedas of 2025-03-19

    10. [10]

      DESI's 2025 BAO results show a mild 2.3-sigma tension with parameters inferred from the cosmic microwave background under Lambda-CDM. confirmedas of 2025-03-19

    11. [11]

      DESI 2025 data set a 95% upper limit on the sum of neutrino masses of 0.064 eV assuming Lambda-CDM, loosening to 0.16 eV in an evolving-dark-energy model. confirmedas of 2025-03-19

    12. [12]

      In April 2025 the KATRIN experiment reported in Science that the neutrino mass is below 0.45 electronvolts (90% confidence), half the previous limit, using 259 days of data from 2019 to 2021 and more than 36 million electrons. confirmedas of 2025-04-16

    13. [13]

      Berkeley Lab announced on 15 April 2026 that DESI had finished surveying the full area of its originally planned map at sunrise after the previous night's observations; Fermilab republished the announcement on 17 April. confirmedas of 2026-04-17

    14. [14]

      NSF NOIRLab, which operates Rubin with SLAC, announced on June 30, 2026 that the 10-year Legacy Survey of Space and Time had officially started after a period of system optimization and an operational readiness review; Rubin Observatory's own website dates the start of the survey to July 2026. confirmedas of 2026-10-10

    15. [15]

      The NSF-DOE Vera C. Rubin Observatory on Cerro Pachón in Chile, named after the astronomer Vera Rubin and aimed at questions including dark energy and dark matter, uses the 8.4-meter Simonyi Survey Telescope and the LSST Camera, the largest digital camera ever built, and is operated by NSF NOIRLab and SLAC. confirmedas of 2026-10-10

    16. [16]

      NASA's Nancy Grace Roman Space Telescope launched on a SpaceX Falcon Heavy from Kennedy Space Center at 7:26 a.m. EDT on August 30, 2026. confirmedas of 2026-08-30

    17. [17]

      Roman is expected to find many more Type Ia supernovae, the exploding stars used to discover dark energy, to test the standard cosmological model, and its catalogues will be available to everyone rather than only to academics. reportedas of 2026-08-28

    18. [18]

      The Atacama Cosmology Telescope's final (DR6) analysis in March 2025 found the CMB described by Lambda-CDM and, combined with Planck, CMB lensing and DESI, a Hubble constant of 68.22 ± 0.36 km/s/Mpc (68.43 ± 0.27 with DESI DR2). confirmedas of 2025-03-18

    Revision history (2)
    1. Page created.
    2. Resolved the completion date (Berkeley Lab announcement 15 April 2026; Fermilab repost 17 April); added operations details and links to Rubin and Roman.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "DESI (Dark Energy Spectroscopic Instrument)." ContentLora, updated Oct 10, 2026. https://contentlora.com/wiki/desi-dark-energy-survey

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