Five-year mission maps 47 million galaxies and quasars

Scientists are using DESI’s map to study dark energy, one of the biggest mysteries in physics. Because of the instrument’s excellent performance and hints that dark energy might evolve, DESI will continue observations into 2028 and further expand the map.

Dark energy is the fundamental ingredient that makes up about 70% of our universe at present and is driving its accelerating expansion. The other ingredients, also investigated by DESI, are ordinary matter (5%) and dark matter (25%).

DESI, which sits on a mountaintop in Arizona, has 5,000 fibre-optic “eyes”, each of which can collect light from a galaxy in just 20 minutes. The five-year survey finished ahead of schedule and with vastly more spectra than expected. Spectra – light from distant objects split into its constituent colours – can tell us huge amounts about the astrophysics of the source as well as its redshift, which in turn tells us its age and distance.

DESI’s quest to understand dark energy is a global endeavour. The international experiment brings together the expertise of more than 900 researchers (including 300 PhD students) from over 70 institutions including UCL. The project is managed by the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab), 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’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (a program of NSF NOIRLab).

UCL has been engaged in DESI from the start. The UCL’s instrumentation team, led by Peter Doel, was involved in the design, construction, and testing of DESI’s optical corrector. This consists of six lenses – up to 1.1 metres in diameter – that focus light on to the fibre-optic system, enabling it to be captured for spectroscopic analysis.

This work was supported by a grant from the UK’s Science and Technology Facilities Council (STFC).

Professor Ofer Lahav (UCL Physics & Astronomy), who is a member of the DESI Executive Committee and involved in the project for over a decade, said: “This is a very exciting milestone. The data-gathering phase has far exceeded expectations. DESI observations processed so far suggest the intriguing possibility that the density of dark energy may evolve over time.

“The collaboration will now analyse the full dataset to test this result, as its confirmation would represent a paradigm shift in our understanding of the universe.

"In addition, the spectra of the 47 million galaxies and quasars form a gold mine for studying how galaxies have formed and evolved.” 

Dr Michael Levi, DESI director and a scientist at Berkeley Lab, said: “DESI’s five-year survey has been spectacularly successful. 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. We’re going to celebrate completion of the original survey and then get started on the work of churning through the data, because we’re all curious about what new surprises are waiting for us.”

To put its success in context, DESI has now measured cosmological data for six times as many galaxies and quasars as all previous measurements combined. A recent UCL-led study* (accepted for publication by Monthly Notices of the Royal Astronomical Society) has shown how to identify interesting anomalies in the big data set of DESI spectra using advanced AI methods.

Paul Nathan, a PhD researcher in cosmology at UCL’s Centre for Doctoral Training in Data Intensive Science, said: “A dataset as vast and rich as this is an astronomer’s dream.

“Beyond measuring redshifts, spectra offer huge discovery potential of galaxy properties and unusual objects. It is super exciting to be part of the DESI team trying to unlock the secrets of the Universe contained within this data.”

* https://arxiv.org/abs/2506.17376

* * *

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 (STFC) 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 honoured to be permitted to conduct scientific research on I’oligam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

Links

• Berkeley Lab story
• Dark Energy Spectroscopic Instrument (DESI)
• Professor Ofer Lahav’s academic profile
• Paul Nathan’s PhD profile
• UCL Physics & Astronomy
• UCL Mathematical & Physical Sciences
• DESI’s data release

Images

• Top: Researchers use DESI’s huge 3D map to study dark energy. Earth is at the centre of this map, and every point is a galaxy. Credit: DESI collaboration and KPNO/NOIRLab/NSF/AURA/R. Proctor
• Middle: Star trails over the Mayall Telescope that houses DESI. Credit: Luke Tyas/Berkeley Lab and KPNO/NOIRLab/NSF/AURA

Media contact

Mark Greaves

m.greaves [at] ucl.ac.uk

+44 (0)20 3108 9485