Free Astronomy Magazine May-June 2021

MAY-JUNE 2021 dicted that this warping of space- time might be observable, as a stretching and distortion of the light from a background galaxy by a fore- ground cluster of galaxies. The lenses typically appear in images as arcs and streaks around fore- ground galaxies and galaxy clusters. Only 1 in 10,000 massive galaxies are expected to show evidence of strong gravitational lensing, and locating them is not easy. Gravitational lenses allow astronomers to explore the most profound questions of our Uni- verse, including the nature of dark matter and the value of the Hubble constant, which defines the expan- sion of the Universe. A major limita- tion of the use of gravitational lenses until now has been the small number of them known. “A massive galaxy warps the space- time around it, but usually you don’t notice this effect. Only when a gal- axy is hidden directly behind a giant galaxy is a lens possible to see,” notes the lead author of the study, Xiaosheng Huang from the Univer- sity of San Francisco. “When we started this project in 2018, there were only about 300 confirmed strong lenses.” “As a co-leader in the DESI Legacy Surveys, I realized this would be the perfect dataset to search for gravi- tational lenses,” explains study co- author David Schlegel of Lawrence Berkeley National Laboratory (LBNL). “My colleague Huang had just fin- ished teaching an undergraduate class on machine learning at the University of San Francisco, and to- gether we realized this was a per- fect opportunity to apply those techniques to a search for gravita- tional lenses.” Doubling the number of known gravitational lenses A stronomers hunting for gravitational lenses utilized machine learning to inspect the vast dataset known as the DESI Legacy Imaging Surveys, uncovering 1,210 new lenses. The data were col- lected at Cerro Tololo Inter-Ameri- can Observatory (CTIO) and Kitt Peak National Observatory (KPNO), both Programs of the National Sci- ence Foundation’s NOIRLab. The ambitious DESI Legacy Imaging Sur- veys just had its ninth and final data release. Discussed in scientific journals since the 1930s, gravitational lenses are products of Einstein’s General The- ory of Relativity. The theory says that a massive object, such as a cluster of galaxies, can warp spacetime. Some scientists, including Einstein, pre- by NOIRLab - Amanda Kocz