DESI completes five-year survey; will continue through 2028 to probe dark energy

The Dark Energy Spectroscopic Instrument has completed the observations planned for its original five-year survey, delivering the largest high-resolution 3D map of the universe yet and setting up a new phase of work aimed at one of cosmology’s biggest open questions.

The DESI collaboration said April 15 that the project has mapped more than 47 million galaxies and quasars since beginning its main survey in May 2021. It also said DESI will keep observing through 2028, extending the project beyond its original schedule to probe tantalizing but unconfirmed hints that dark energy — the unknown phenomenon thought to be driving the universe’s accelerating expansion — may change over time.

That matters because the standard model of cosmology treats dark energy as a constant. If future data were to show otherwise, it would point to physics beyond that framework. For now, DESI’s earlier findings amount to a hint, not a discovery.

DESI’s new map far exceeded its original five-year target of about 34 million galaxies and quasars. The collaboration also observed more than 20 million stars for studies of the Milky Way, though the project’s headline result is its extragalactic map: a sweeping 3D record of how matter is distributed across the cosmos over billions of years.

The instrument is mounted on the National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona. It uses 5,000 robotic fiber positioners to capture light from thousands of objects at once and feeds that light into 10 spectrographs, which split it into spectra. Those spectra let researchers measure redshifts — the stretching of light caused by cosmic expansion — and turn millions of observations into a 3D map.

DESI is managed by Lawrence Berkeley National Laboratory and funded by the U.S. Department of Energy’s Office of Science, with support from the NSF and international partners. The collaboration includes more than 900 researchers from more than 70 institutions, including about 300 Ph.D. students.

The extension through 2028 reflects both the instrument’s performance and the scientific stakes. In 2025, using three years of DESI data combined with other cosmology datasets, researchers reported a preference for models in which dark energy evolves over time instead of remaining constant. But the statistical significance, depending on the data combination, was about 2.8 to 4.2 sigma — below the 5-sigma threshold typically required to claim a discovery.

That makes the completed five-year dataset especially important. The collaboration said it will begin processing those observations immediately, with the first full-survey dark-energy results expected in 2027.

The expanded campaign is expected to increase DESI’s survey footprint from about 14,000 square degrees to 17,000 square degrees, roughly a 20% increase. By the end of the extension, the project aims to reach about 63 million total extragalactic redshifts.

“DESI’s five-year survey has been spectacularly successful,” Michael Levi, DESI director, said in a Lawrence Berkeley National Laboratory release. “The instrument performed better than anticipated. The results have been incredibly exciting. And the size and scope of the map and how quickly we’ve been able to execute is phenomenal.”