UCL Astrophysics Group


Researchers make most precise ever measurement of expanding Universe

4 April 2024

The Dark Energy Spectroscopic Instrument (DESI) collaboration, led by the Lawrence Berkeley National Laboratory in the US and involving our own Profs Lahav and Deol, has made the most precise measurements to date of how fast the universe has expanded throughout its history.

Desi Slice

The analysis, based on the largest 3D map of the cosmos ever created with just the first year of data from DESI, confirms the basics of our current best model of the universe – with some tantalising areas to explore with more data.

The instrument, which sits on a mountaintop telescope in Arizona, in the US, contains 5,000 fibre-optic “eyes”, each of which can image a galaxy in just 20 minutes. A team at UCL Physics & Astronomy helped design, assemble and build DESI’s optical corrector – six lenses, the largest 1.1m across, that reflect light on to the “eyes”.

DESI has mapped galaxies and quasars with unprecedented detail, enabling researchers to look 11 billion years into the past and study how fast the universe has expanded over time.

This is the first time that scientists have measured the expansion history of that distant period with a precision of better than 1%, giving us our best view yet of how the universe evolved.

Researchers shared the analysis of their first year of collected data in multiple papers published today on the arXiv and in talks at the American Physical Society Meeting in the United States and the Rencontres de Moriond in Italy.

Dr Michael Levi, DESI director and a scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), which manages the project, said: “We’re incredibly proud of the data, which have produced world-leading cosmology results and are the first to come out of the new generation of dark energy experiments.

“So far, we’re seeing basic agreement with our best model of the universe, but we’re also seeing some potentially interesting differences that could indicate that dark energy is evolving with time. Those may or may not go away with more data, so we’re excited to start analysing our three-year dataset soon.”

Professor Ofer Lahav (UCL Physics & Astronomy), who has coordinated UCL’s involvement in DESI and in other cosmology surveys and is a member of the DESI Executive Committee, said: “Congratulations to the DESI team on this remarkable measurement of dark energy and dark matter properties. The results validate spectacularly that the universe’s expansion is accelerating, probably due a cosmological constant Lambda proposed by Einstein in 1917, or a variant of it. UCL has contributed, with STFC’s support, to the construction of the DESI optical corrector and to the science analysis infrastructure.

“More data from DESI and other surveys are needed to pin down the mysterious nature of dark energy and dark matter that make 95% of the present universe.”

Dirk Scholte (UCL Physics & Astronomy), a PhD student who helped create the DESI catalogue of millions of galaxies used in this study, said: “Beyond cosmology, the DESI spectra provide a very rich data set to study the physical properties of an unprecedented number of galaxies and their evolution with cosmic time.”

Professor Peter Doel, who worked on the optical corrector alongside Professor David Brooks (both UCL Physics & Astronomy) said: “I’m excited to see the publication of the key findings from the first year of the DESI survey. And I’m pleased that the high-quality survey data used was due in part due to the success of the wide field corrector optics that was assembled and tested at UCL.”

Our leading model of the universe is known as Lambda CDM. It includes both a weakly interacting type of matter (cold dark matter, or CDM) and dark energy (Lambda, in its simplest version).

Both matter and dark energy shape how the universe expands – but in opposing ways. Both ordinary matter and cold dark matter slow the expansion down, via the attractive force of gravity. While the nature of dark energy is still a mystery, its presence leads to a repulsive force, speeding up the expansion of the universe. The amount of each influences how our universe evolves. This model does a good job of describing results from previous experiments and how the universe looks throughout time.

However, when DESI’s first-year results are combined with data from other studies, there are some subtle differences with what Lambda CDM would predict. As DESI gathers more information during its five-year survey, these early results will become more precise, shedding light on whether the data are pointing to different explanations for the results we observe or the need to update our model. More data will also improve DESI’s other early results, which weigh in on the Hubble constant (a measure of how fast the universe is expanding today) and the mass of particles called neutrinos. 

Dr Nathalie Palanque-Delabrouille, a Berkeley Lab scientist and co-spokesperson for the experiment, said: “No spectroscopic experiment has had this much data before, and we’re continuing to gather data from more than a million galaxies every month.

“It’s astonishing that with only our first year of data, we can already measure the expansion history of our universe at seven different slices of cosmic time, each with a precision of 1 to 3%. The team put in a tremendous amount of work to account for instrumental and theoretical modeling intricacies, which gives us confidence in the robustness of our first results.”

DESI’s overall precision on the expansion history across all 11 billion years is 0.5%, and the most distant epoch, covering 8-11 billion years in the past, has a record-setting precision of 0.82%. That measurement of our young universe is incredibly difficult to make. Yet within one year, DESI has become twice as powerful at measuring the expansion history at these early times as its predecessor (the Sloan Digital Sky Survey’s BOSS/eBOSS), which took more than a decade.

DESI is an international collaboration of more than 900 researchers from over 70 institutions around the world. The instrument was constructed and is operated with funding from the DOE Office of Science, and sits atop the Mayall 4-Meter Telescope at Kitt Peak National Observatory, a programme of NSF’s NOIRLab.

The DESI collaboration is honored to be permitted to conduct scientific research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.



  • DESI has made the largest 3D map of our universe to date. Earth is at the center of this thin slice of the full map. In the magnified section, it is easy to see the underlying structure of matter in our universe. Credit: Claire Lamman/DESI collaboration; custom colormap package by cmastro