Scientists at the Dark Energy Survey have published their most detailed explanation yet of how the universe has expanded over the last six billion years, thanks to an unprecedented combination of cosmic measurements.
The international group of researchers, including researchers from the University of Cambridge, is led by the US Department of Energy's Fermi National Accelerator Laboratory. Cambridge and the other five UK universities are supported by the Science and Technology Facilities Council (STFC).
Combining multiple, independent measurements of the cosmos, the research doubles the precision of previous Dark Energy Survey (DES) studies, while remaining broadly consistent with the standard model of cosmology, the most widely accepted theory of the universe.
The findings combine results from 18 separate studies and, for the first time, bring together four major techniques for studying dark energy within a single experiment, a milestone envisioned when DES was conceived 25 years ago.
The combination of these techniques - weak gravitational lensing (distortions in galaxy shapes), galaxy clustering, supernovae and galaxy clusters - enabled scientists to cross-check their measurements and gain a more robust understanding of how the universe behaves.
Around 100 years ago, scientists discovered that distant galaxies appeared to be moving away from Earth. They found that the further away a galaxy is, the faster it recedes, providing the first key evidence that the universe is expanding.
Researchers initially expected that this expansion would slow down over time due to gravity. However, in 1998, observations of distant supernovae revealed that the universe's expansion is accelerating rather than slowing down.
To explain this result, scientists proposed the idea of dark energy, which is now thought to drive the universe's accelerated expansion.
Astrophysicists believe dark energy makes up about 70% of the mass-energy content of the universe, yet we still know very little about it.
The Dark Energy Survey is an international collaboration of more than 400 astrophysicists, astronomers and cosmologists from over 35 institutions, including several from the UK. It is led by the US Department of Energy's Fermi National Accelerator Laboratory.
The other UK universities involved in the collaboration are University College London, University of Edinburgh, University of Nottingham, University of Portsmouth and University of Sussex.
Through STFC, the UK is also supporting research programmes that will advance the work of the DES collaboration in the next generation of astronomical surveys, including Vera C. Rubin Observatory, currently under construction in Chile.
"This research shows the power of long-term international collaboration and UK investment in world-leading science," said Professor Michele Dougherty, Executive Chair, STFC. "Dark energy remains one of the great unanswered questions in science. Studies like this demonstrate how bringing together different approaches can give us a clearer picture of our universe and where future discoveries may lie."
To study dark energy, the DES collaboration carried out a deep, wide-area survey of the sky between 2013 and 2019, using a specially constructed 570-megapixel Dark Energy Camera mounted on a telescope at the US National Science Foundation's Cerro Tololo Inter-American Observatory in Chile.
"This release squeezes an enormous amount of information out of subtle distortions in galaxy images, turning tiny signals into a powerful test of how the Universe works," said DES team member Calvin Preston, from Cambridge's Institute of Astronomy. "My role focused on baryonic feedback-understanding and modelling how processes like star formation and energy from supermassive black holes redistribute matter and subtly reshape the large-scale structure we measure. The results are so exciting as we've been able to learn so much about the universe and about galaxies themselves."
Over six years, scientists collected images and data from hundreds of millions of distant galaxies, billions of light-years from Earth, mapping about one-eighth of the sky.
For the latest results, scientists refined how they use subtle distortions in galaxy shapes, known as weak gravitational lensing, to reconstruct the distribution of matter in the universe over six billion years. They did this by measuring both how galaxies cluster together and how similarly their shapes are distorted by gravity.
By reconstructing the universe's matter distribution across six billion years, these measurements reveal how dark energy and dark matter have influenced the universe's evolution.
The team compared their observations with two main theories, one in which dark energy remains constant over time (the standard model of cosmology), and another in which dark energy changes as the universe evolves.
DES found that although the data mostly align with the standard model, broadly agreeing with the most widely accepted theory of the universe, there remains a long-standing discrepancy in how matter clusters in the universe, and this has become more pronounced with the inclusion of the full dataset.
Looking ahead, DES will combine these latest findings with results from other dark energy experiments to explore and test alternative ideas about gravity and dark energy.
The work also helps prepare the ground for future breakthroughs at the upcoming Vera C. Rubin Observatory in Chile for similar work with its Legacy Survey of Space and Time (LSST).
Calvin Preston is a Member of Robinson College, Cambridge.
Adapted from an STFC media release.
