Free Astronomy Magazine September-October 2021

47 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING T he Dark Energy Survey camera (DECam) at the SiDet clean room. The Dark Energy Camera was designed specifically for the Dark Energy Survey. It was funded by the De- partment of Energy (DOE) and was built and tested at DOE’s Fermilab. [DOE/ FNAL/ DECam/R. Hahn/ CTIO/NOIRLab/NSF/AURA] with unprecedented preci- sion. DES concluded its ob- servations of the night sky in 2019. With the experi- ence gained from analyz- ing the first half of the data, the team is now pre- pared to handle the com- plete dataset. The final DES analysis is expected to paint an even more precise picture of the dark matter and dark energy in the Universe. The DES collaboration con- sists of over 400 scientists from 25 in- stitutions in seven countries. “The collaboration is remarkably young. It’s tilted strongly in the di- rection of postdocs and graduate students who are doing a huge amount of this work,” said DES Di- rector and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “That’s really grat- ifying. A new generation of cosmol- ogists are being trained using the Dark Energy Survey.” The methods developed by the team have paved the way for future sky surveys such as the Rubin Observa- tory Legacy Survey of Space and Time. “DES shows that the era of big survey data has well and truly be- gun,” notes Chris Davis, NSF’s Pro- gram Director for NOIRLab. “DES on NSF’s Blanco telescope has set the scene for the remarkable discover- ies to come with Rubin Observatory over the coming decade.” els through space, the gravity of both ordinary and dark matter in the foreground can bend its path, as if through a lens, resulting in a dis- torted image of the galaxy as seen from Earth. By studying how the ap- parent shapes of distant galaxies are aligned with each other and with the positions of nearby galaxies along the line of sight, DES scientists were able to infer the clumpiness of the dark matter in the Universe. To test cosmologists’ current model of the Universe, DES scientists com- pared their results with measure- ments from the European Space Agency’s orbiting Planck observa- tory. Planck used light known as the cosmic microwave background to peer back to the early Universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the Universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present. Combined with earlier results DES provides the most powerful test of the current best model of the Uni- verse to date, and the results are consistent with the predictions of the standard model of cosmology. However, hints remain from DES and several previous galaxy surveys that the Universe today is a few percent less clumpy than predicted. Ten regions of the sky were chosen as “deep fields” that the Dark En- ergy Camera imaged repeatedly throughout the survey. Stacking those images together allowed the scientists to glimpse more distant galaxies. The team then used the redshift information from the deep fields to calibrate the rest of the sur- vey region. This and other advancements in measurements and modeling, cou- pled with a threefold increase in data compared to the first year, en- abled the team to pin down the den- sity and clumpiness of the Universe !