Free Astronomy Magazine May-June 2026

7 MAY-JUNE 2026 ASTRO PUBLISHING mass-energy density of the Universe. Yet, we still know very little about it. In the following years, scientists began devising experiments to study dark energy, including DES. Today, DES is an international col- laboration of over 400 astrophysi- cists and scientists from 35 insti- tutions in seven countries led by DOE’s Fermi National Accelerator Laboratory. For the latest results, DES scientists greatly advanced methods using weak lensing to robustly reconstruct the distribution of matter in the Universe. Weak lensing is the distor- tion of light from distant galaxies due to the gravity of intervening matter, like galaxy clusters. They did this by measuring the probability of two galaxies being a certain dis- tance 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 meas- urements of weak lensing and gal- axy distribution tell scientists how much dark energy and dark matter there is at each moment. In this analysis, DES tested two mod- els of the Universe against their data. There is the currently accepted standard model of cosmology — Lambda cold dark matter ( Λ CDM) — in which the dark energy density is constant. There is also an extended model, in which the dark energy density evolves over time — wCDM. DES found that their data mostly aligned with the standard model of cosmology. Their data also fit the evolving dark energy model, but no better than they fit the standard model. However, one parameter is still off. Based on measurements of the early Universe, both the standard and evolving dark energy models predict how matter in the Universe clusters at later times. In previous analyses, galaxy clustering was found to be P revious page: The Víctor M. Blanco 4-meter Telescope has pristine access to wide open skies of the Chilean Andes from its perch at Cerro Tololo Inter- American Observatory (CTIO), a Program of NSF NOIRLab. To the upper left of the telescope is the ‘evening star’, actually the planet Venus. Below on the left are the SMARTS 1.5-meter Telescope and SMARTS 0.9-meter Telescope (furthest back). Housed within the silver dome of the Blanco Telescope is the Dark Energy Camera (DECam), mounted at the prime (first) focus near the top of the white Serrurier truss. The blue U-shaped structure holding the truss is the large bearing that sweeps the telescope around to a designated position for observing. DECam saw first light on 12 September 2012 and in its more than 10 years of operation it has contributed greatly to the field of astronomy. It was designed specifically for the Dark Energy Survey, operated by the Department of Energy and the Na- tional Science Foundation between 2013 and 2019. During this time, DECam cat- aloged nearly 1 billion objects, helping to construct the largest ever map of the night sky. [CTIO/NOIRLab/NSF/AURA/T. Matsopoulos] formation from 669 million galaxies that are billions of light-years from Earth, covering an eighth of the sky. Now, the DES Collaboration is re- leasing results that, for the first time, combine all six years of data from weak lensing and galaxy clus- tering probes — two techniques for measuring the Universe’s expansion history. The collaboration also pres- ents the first results found by com- bining all four methods of meas- uring the expansion history of the Universe — baryon acoustic oscilla- tions (BAO), Type-Ia supernovae, galaxy clusters, and weak gravita- tional lensing — as proposed at the inception of DES 25 years ago. The paper, submitted to Physical Review D, represents a summary of 18 sup- porting papers. “It is an incredible feeling to see these results based on all the data, and with all four probes that DES had planned. This was something I would have only dared to dream about when DES started collecting data, and now the dream has come true,” says Yuanyuan Zhang, assistant astronomer at NSF NOIRLab and member of the DES Collaboration. The analysis yields new, tighter con- straints that narrow down the pos- sible 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. “These results from the Dark Energy Survey shine new light on our un- derstanding of the Universe and its expansion,” said Regina Rameika, Associate Director for the Office of High Energy Physics in the DOE’s Office of Science (DOE/SC). “They demonstrate how long-term invest- ment in research and combining multiple types of analysis can pro- vide insight into some of the Uni- verse’s biggest mysteries.” The first clue for dark energy was uncovered about a century ago when astronomers noticed that dis- tant galaxies appeared to be mov- ing away from us. In fact, the farther away a galaxy is, the faster it recedes. This provided the first key evidence that the Universe is ex- panding. But since the Universe is permeated by gravity, a force that pulls matter together, astronomers expected the expansion would slow down over time. Then, in 1998, two independent teams of cosmologists used distant supernovae to discover that the Universe’s expansion is accelerating rather than slowing. To explain these observations, they proposed a new kind of phenomenon that is re- sponsible for driving the Universe’s accelerated expansion: dark energy. Astrophysicists now believe dark en- ergy makes up about 70% of the