Free Astronomy Magazine July-August 2025

45 JULY-AUGUST 2025 ASTRO PUBLISHING ergy-fabricated Dark Energy Camera (DECam), mounted on the U.S. Na- tional Science Foundation (NSF) Víc- tor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Obser- vatory (CTIO) in Chile, a Program of NSF NOIRLab. By taking data on 758 nights across six years, DES scientists mapped an area almost one- eighth of the entire sky. The project employs multiple ob- servational techniques, in- cluding supernova measure- ments, galaxy clustering analysis, and weak gravita- tional lensing, to study dark energy. Two key DES measurements — Baryon Acoustic Oscilla- tions (BAO) and distance measurements of exploding stars (Type Ia supernovae) — track the Universe’s expan- sion history. BAO refers to a standard cosmic ruler form- ed by sound waves in the early Universe, with peaks spanning approximately 500 million light-years. Astronomers can measure these peaks across several periods of cosmic history to see how dark energy has stretched the scale over time. Santiago Avila from the Cen- tre for Energy, Environmen- tal and Technological Research (CIEMAT) in Spain, who was respon- sible for the BAO analysis in DES, says, “By analyzing 16 million galax- ies, DES found that the measured BAO scale is actually 4% smaller than predicted by Λ CDM.” Type Ia supernovae serve as ‘stan- dard candles’, meaning they have a known intrinsic brightness. There- fore, their apparent brightness, combined with information about their host galaxies, allows scientists to make precise distance calcula- tions. In 2024 DES published the most extensive and detailed su- pernova dataset to date, providing highly accurate measurements of cosmic distances. These new findings from the combined supernovae and BAO data independently confirm the anomalies seen in the 2024 su- pernova data. By integrating DES measurements with cosmic microwave background data, researchers inferred the prop- erties of dark energy — and the re- sults suggest a time-evolving nature. If validated, this would imply that dark energy, the cosmological con- stant, is not constant after all, but a dynamic phenomenon requiring a new theoretical framework. “This result is intriguing because it hints at physics beyond the standard model of cosmology,” says Juan Mena-Fernández of the Subatomic Physics and Cosmology Laboratory in Grenoble, France. “If further data support these findings, we may be on the brink of a scientific revolution.” Although the current re- sults are not yet defini- tive, upcoming analyses incorporating additional DES probes — such as galaxy clustering and weak lensing — could strengthen the evidence. Similar trends have emerged from other major cosmological proj- ects, including the Dark Energy Spectroscopic In- strument (DESI), raising anticipation within the scientific community. “These results represent years of collaborative ef- fort to extract cosmologi- cal insights from DES data,” says Jessie Muir of the University of Cincin- nati. “There is still much to learn, and it will be ex- citing to see how our un- derstanding evolves as new measurements be- come available.” The final DES analysis, expected later this year, will incorporate addi- tional cosmological probes to cross- check findings and refine constraints on dark energy. The scientific com- munity eagerly awaits these results, as they could pave the way for a par- adigm shift in cosmology. O n the previous page: The U.S. National Science Foundation Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, a Program of NSF NOIRLab. The Blanco is home to the 570-megapixel De- partment of Energy-fabricated Dark Energy Camera (DECam) conducting DES. [CTIO/NOIRLab/NSF/AURA/P. Horálek (Institute of Physics in Opava)] T he Dark Energy Camera (DECam) is mounted on the Víctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter- American Observatory (CTIO) in north-central Chile. Telescope construction started in 1969 with the casting of the primary mirror. The assembly at the Cerro Tololo mountaintop was finished in 1974. Upon completion of construction, it was the 3 rd largest telescope in the world, behind the 200-inch Hale telescope at Palomar Observatory in California and the BTA-6 in southern Russia, and was the largest in the Southern Hemi- sphere (a title that it held for 22 years). It was later named in 1995 in honor of Víctor M. Blanco, Puerto Rican astronomer and former director of CTIO. [CTIO/NOIRLab/DOE/NSF/AURA/ R. Hahn (Fermi National Accelerator Laboratory)] !