Scientists have released the most detailed account yet of how the Universe has expanded over the past six billion years, deepening understanding of dark energy and offering one of the strongest tests so far of the prevailing model of cosmology.
The findings draw on six years of observations from the Dark Energy Survey (DES) and deliver roughly twice the precision of its earlier analyses. For the first time, the project’s key measurement techniques have been brought together in a single, unified study – a goal set when DES was first conceived a quarter of a century ago. While the results largely support the standard picture of the Universe, they also bring renewed focus to a persistent discrepancy in how matter appears to clump across cosmic scales.
Led by the US Department of Energy’s Fermi National Accelerator Laboratory, the international collaboration includes UK support from the Science and Technology Facilities Council and six universities, among them the University of Portsmouth.
Over its lifetime, DES mapped 669 million galaxies, many billions of light-years away, covering roughly one-eighth of the sky. From this immense dataset, researchers reconstructed how matter has been distributed through space and time, revealing how dark matter and dark energy have together shaped the Universe’s evolving structure.
The new release combines results from 18 studies and, for the first time, integrates four of the most powerful techniques used to study dark energy within a single experiment, allowing scientists to cross-check their measurements and reduce uncertainty.
Professor David Bacon, Director of the Institute of Cosmology and Gravitation at the University of Portsmouth, said the survey’s strength lies in its collaborative design.
“The beauty of the Dark Energy Survey has been bringing together several different types of observations of the cosmos to learn about how the Universe expands,” he said.
“These results don’t happen automatically – they come from the sustained effort of hundreds of scientists working together over many years.”
Central to the analysis are techniques such as weak gravitational lensing, which captures the subtle distortion of distant galaxies by intervening matter, and galaxy clustering, which traces how galaxies group together across space. Used in combination, they offer a more reliable picture of the Universe’s fundamental behaviour than any single method alone.
Professor Michele Dougherty, Executive Chair of the Science and Technology Facilities Council, said the research demonstrated the value of long-term international investment in fundamental science.
“Dark energy remains one of the great unanswered questions,” she said.
“By bringing multiple approaches together, studies like this give us a clearer view of the Universe and help point the way to future discoveries.”
Following the expansion
Between 2013 and 2019, DES carried out a deep survey of the southern sky using a specially built 570-megapixel camera mounted on the Blanco four-metre telescope at the Cerro Tololo Inter-American Observatory in Chile.
By tracking the distribution of matter across six billion years of cosmic history, the data show how dark energy – thought to account for around 70 per cent of the Universe – has driven the accelerating expansion of space itself.
When tested against competing theories, the results remain broadly consistent with the standard model in which dark energy does not change over time. Yet the full dataset also sharpens a long-standing tension in measurements of how matter clusters, suggesting that the story may not yet be complete.
A century ago, astronomers first realised that the Universe is expanding; in 1998, the discovery that this expansion is accelerating transformed modern cosmology. Dark energy was proposed to explain the phenomenon, but its nature remains elusive.
With more than 400 researchers from over 35 institutions across seven countries, DES now hands its legacy to the next generation of surveys, including the Vera C. Rubin Observatory in Chile, which will probe the cosmos with even greater depth and precision.
The Universe is expanding faster than ever, and the effort to understand why is entering a sharper, more exacting phase.
