The latest results from the Dark Energy Survey (DES) collaboration, which involves UCL researchers, combined four methods for measuring the expansion of the universe for the first time.
Dark energy is the mysterious force that is accelerating the expansion of the universe and represents about 70% of the total content of the universe. By tracking how the universe's expansion rate changes over time, scientists can narrow down possibilities for what dark energy is and how it behaves.
The results combine all six years of data relating to weak lensing (looking at how gravity distorts galaxy shapes) and galaxy clustering.
In the paper, which represents a summary of 18 supporting papers, they also presented their first results found by combining all four probes - baryon acoustic oscillations (BAO), type-Ia supernovae, galaxy clusters, and weak gravitational lensing - as proposed at the inception of DES 25 years ago.
The analysis yielded new, tighter constraints that narrow down the possible models for how the universe behaves. These constraints are more than twice as strong as those from past DES analyses, while remaining consistent with previous DES results.
For the latest results, DES scientists greatly advanced methods using weak lensing to robustly reconstruct the distribution of matter in the universe. They did this by measuring the probability of two galaxies being a certain distance apart and the probability that they are also distorted similarly by weak lensing.
By reconstructing the matter distribution over six billion years of cosmic history, these measurements of weak lensing and galaxy distribution tell scientists how much dark energy and dark matter there is at each moment.
In this analysis, DES tested their data against two models of the universe: the currently accepted standard model of cosmology - Lambda cold dark matter (ΛCDM) - in which the dark energy density is constant, and an extended model - wCDM, in which the equation of state parameter w is a constant but the dark energy density may vary with time. DES found that their data mostly aligned with the standard model of cosmology. Their data also fit the extended model, but no better than they fit the standard model.
Professor Ofer Lahav (UCL Physics & Astronomy), former co-chair of the DES Science Committee and Chair of DES:UK, said: "It is exciting to see results from the full DES data set, more than two decades after the project was first conceived. The sample of 140 million galaxies with shape measurements is phenomenal. While the headline results support a constant dark energy density, future analyses will test the intriguing possibility of an evolving dark energy."
DES is an international collaboration of over 400 astrophysicists and scientists from 35 institutions in seven countries. Led by the U.S. Department of Energy's Fermi National Accelerator Laboratory, the DES collaboration also includes scientists from UCL and several other UK universities.
UCL's Astrophysics Group (in the Department of Physics & Astronomy) has been involved in DES since 2004, and has contributed to both the instrumentation and the science.
The DES optical corrector, installed on the U.S. NSF Blanco 4-meter telescope at NSF Cerro Tololo Inter-American Observatory, was assembled at the UCL optical laboratory, led by Professors Peter Doel and David Brooks (UCL Physics & Astronomy), with the support of a grant from STFC. UCL staff, post-docs, and PhD students have been heavily involved in DES analyses over two decades.
Recently, the UCL team has led three other papers:
- The method of 'Simulation-Based Inference' (SBI) has been applied to data from the first three years of DES, and delivered tighter constraints on cosmological parameters (arXiv:2403.02314). Dr Niall Jeffrey (UCL Physics & Astronomy) said: "This work demonstrates how scientific AI techniques can improve our understanding of dark energy beyond what is possible with classical statistics."
- Based on the DES three-year data, another UCL-led analysis compared the spatial distribution of galaxies and mass around clusters and voids (arXiv:2509.18967). The lead author Dr Qianjun Ellen Hang (UCL Physics & Astronomy) said: "We quantified the correspondence between the distribution of galaxies and matter around troughs and peaks in the projected galaxy density. This method shows an interesting avenue for measuring field-level properties that can be applied to future lensing surveys such as Euclid and LSST-Rubin".
- Improved cosmological constraints from a re-analysis of DES five-year sample of Type Ia supernovae (arXiv:2511.07517). Dr Paul Shah (UCL Physics & Astronomy) said: "Using generally accepted thresholds for model preference, our updated data exhibits only a weak preference for evolving dark energy relative to LambdaCDM."
Other UCL researchers who contributed to recent DES papers include Dr Lorne Whiteway, PhD student Joshua Williamson and Professor Ofer Lahav.
Next, DES will combine this work with the most recent constraints from other dark energy experiments to investigate alternative gravity and dark energy models. This analysis is also important because it paves the way for the new large surveys such as LSST-Rubin and ESA's Euclid, in which UCL is involved.
