Free Astronomy Magazine March-April 2024

29 MARCH-APRIL 2024 A n example of a supernova dis- covered by the Dark Energy Sur- vey within the field covered by one of the individual detectors in the Dark Energy Camera. The supernova exploded in a spiral galaxy with red- shift = 0.04528, which corresponds to a light-travel time of about 0.6 billion years. In comparison, the quasar at the right has a redshift of 3.979 and a light-travel time of 11.5 billion years. [DES Collaboration/ NOIRLab/NSF/AURA/M. Zamani] T he history of the expansion Uni- verse can be traced by compar- ing recessional velocities (redshifts) with distances determined for each supernova. The Dark Energy Survey result shows that the expansion has been accelerating with cosmic time, the signature of dark energy. [DES Collaboration] technique to further probe the mys- teries of dark energy and place the strongest constraints on the expan- sion history of the Universe ever ob- tained. In a presentation at the 243 rd meeting of the American Astronom- ical Society on 8 January 2024, and in a paper submitted to The Astro- physical Journal , astrophysicists re- port results that are consistent with the now-standard cosmological model of a Universe with an accel- erating expansion. Yet, the findings are not definitive enough to rule out a possibly more complex model. The DES is an international collabo- ration comprising more than 400 scientists from over 25 institutions, led by the US Department of En- ergy’s Fermi National Accelerator Laboratory. The DES employs the Dark Energy Camera (DECam), a 570-megapixel digital camera built by Fermilab and funded by the DOE Office of Science, with significant contributions by the NSF and the DES partners. It is mounted on the Víctor M. Blanco Telescope at CTIO in Chile. By taking data on 758 nights across six years, DES scientists mapped an area almost one-eighth of the entire sky. Among the observations of about two million distant galaxies, the DES team found several thousand super- novae, making this the largest, deep- est supernova sample ever obtained from a single telescope. DES researchers then used advanced machine-learning techniques to aid in supernova classification and sift the sample into a uniform high- quality dataset with 1499 likely type Ia supernovae, thereby tripling the number of observed supernova Ia beyond a redshift of 0.2 and quintu- pling the number beyond a redshift of 0.5. “It’s a really massive scale-up from 25 years ago when only 52 su- pernovae were used to infer dark energy,” said Tamara Davis, a pro- fessor at the University of Queens- land in Australia and co-convener of the DES Supernova Working Group. This large sample of supernovae, spanning a wide range of distances, can be used to trace out the history of cosmic expansion. For each super- nova, DES scientists combine its dis- tance with a measurement of its ticular class of exploding stars, called type Ia (read “type one-A”) super- novae, and was recognized with the Nobel Prize in Physics in 2011. Now, 26 years after the initial dis- covery, the scientists working on the Dark Energy Survey (DES) have re- leased the results of an unprece- dented analysis using the same