Free Astronomy Magazine March-April 2024

30 MARCH-APRIL 2024 ASTRO PUBLISHING this is an intriguing new piece of the puzzle. A more complex explana- tion might be needed.” The innovative tech- niques that the DES has pioneered will shape and further drive future astrophysical analyses. Projects like the upcom- ing Legacy Survey of Space and Time, to be conducted by Vera C. Rubin Observatory, which is operated jointly by NSF’s NOIRLab and DOE’s SLAC National Ac- celerator Laboratory, as well as NASA’s Nancy Grace Roman Space Tel- escope, will pick up where the DES left off. “We’re pioneering tech- niques that will be di- rectly beneficial for the next generation of supernova sur- veys,” said Kron. “This result clearly shows the value of astronomical survey projects that continue to yield excellent science well after data collection has ended,” says Nigel Sharp, a pro- gram director in NSF’s Astronomical Sciences Division. “We need as many diverse approaches as we can get in order to understand what dark en- ergy is, and what it isn’t. This is an important route to that under- standing.” Alistair Walker, DECam Instrument Scientist at NOIRLab, adds, “Multi- ple elements came together to per- mit this important advance in our understanding of dark energy — the pristine skies of Chile, the large Blanco Telescope equipped with the superbly-made DECam, inten- sive data calibration efforts that achieved unprecedented levels of measurement accuracy and a decade of analysis effort by a very talented group of scientists.” R edshift is the term used to describe the stretching of wavelengths of the light from an ob- ject as a result of the expansion of the Universe; the greater the object’s distance, the greater the redshift. The detailed history of the expansion of the Universe is determined with a precise relation between the distances to galaxies — or supernovae — and their redshifts. [DES Collaboration] redshift — how quickly it is moving away from Earth as a result of the expansion of the Universe. To- gether, these two factors can lend insight into whether the Universe’s dark energy density has remained constant or changed over time. “As the Universe expands, the mat- ter density goes down,” said DES di- rector and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “But if the dark energy density is a constant, that means the total proportion of dark energy must be increasing as the volume increases.” The standard cosmological model is known as Λ CDM, or ‘Lambda cold dark matter’. This mathematical model describes how the Universe evolves using just a few features such as the density of matter, the type of matter and the behavior of dark energy. While Λ CDM assumes the density of dark energy in the Universe is constant over cosmic time and doesn’t dilute as the Uni- verse expands, the DES Supernova Survey results hint that this may not be true. Results were obtained by combining the DES data with complementary data from the European Space Agency’s Planck telescope. An in- triguing outcome of this survey is that it is the first time that enough distant supernovae have been meas- ured to make a highly detailed measurement of the decelerating phase of the Universe, and to see where the Universe transitions from decelerating to accelerating. And while the results are consistent with a constant density of dark energy in the Universe, they also hint that dark energy might possibly be vary- ing. “There are tantalizing hints that dark energy changes with time,” said Davis, “We find that the sim- plest model of dark energy — Λ CDM — is not the best fit. It’s not so far off that we’ve ruled it out, but in the quest to understand what is acceler- ating the expansion of the Universe !