Over the course of the LSST's 10-year exploration of the cosmos, the Rubin Observatory will take 5.5 million data-rich images of the sky. Wider and deeper in volume than all previous surveys combined, the LSST will provide an unprecedented amount of information to astronomers and cosmologists working to answer some of the most fundamental questions in science.
Heavily involved in the LSST Dark Energy Science Collaboration (DESC), scientists at DOE's Argonne National Laboratory are working to uncover the true nature of dark energy and dark matter. In preparation for the LSST, they're performing advanced cosmological simulations and working with the Rubin Observatory to shape and process its data to maximize the potential for discovery.
Simulating the dark side
Together, dark energy and dark matter make up a staggering 95% of the energy and matter in the universe, but scientists understand very little about them. They see dark matter's effects in the formation and movement of galaxies, but when they look for it, it seems like it's not there. Meanwhile, space itself is expanding faster and faster over time, and scientists don't know why. They refer to this unknown influence as dark energy.
"Right now, we have no clue what their physical origins are, but we have theories," said Katrin Heitmann, deputy director of Argonne's High Energy Physics (HEP) division. ?"With the LSST and the Rubin Observatory, we really think we can get good constraints on what dark matter and dark energy could be, which will help the community to pursue the most promising directions."
In preparation for the LSST, Argonne scientists are taking theories about particular attributes of dark matter and dark energy and simulating the evolution of the universe under those assumptions.
It's important that the scientists find ways to map their theories to signatures the survey can actually detect. For example, how would the universe look today if dark matter had a slight temperature, or if dark energy was super strong right after the universe began? Maybe some structures would end up fuzzier, or maybe galaxies would clump in a certain way. Simulations can help researchers predict what features will actually appear in real-world data from the LSST that would indicate a certain theory is true.
Simulations also allow the collaboration to validate the code they will use to process and analyze the data. For example, together with LSST DESC and the collaboration behind NASA's Nancy Grace Roman Space Telescope, Argonne scientists recently simulated images of the night sky as each telescope will actually see it. To ensure their software performs as intended, scientists can test it on this clean, simulated image data before they begin processing the real thing.
To perform their simulations, Argonne scientists leverage the computational resources of the Argonne Leadership Computing Facility (ALCF), a DOE Office of Science user facility. Among its suite of supercomputers, the ALCF houses Aurora, one of the world's first exascale machines, which can perform over one quintillion - or one billion billion - calculations per second.
"Aurora's impressive memory and speed will allow us to simulate larger volumes of the universe and account for more physics in the simulations than ever before, while maintaining high enough resolution to get important details right," said Heitmann, who formerly served as spokesperson for the LSST DESC.
w.ezWhat to expect when you're expecting an astronomical amount of data During the LSST, light emitted a long time ago from galaxies far away will reach the observatory. Sensors on the observatory's camera will convert the light into data, which will travel from the mountain to several Rubin Project data facilities around the world. These facilities will then prepare the data to be sent to the larger community for analysis.
As part of the LSST DESC, Argonne scientists are currently working with the Rubin Observatory to ensure the data is processed in ways that are most conducive to their scientific goals. For example, Argonne physicist Matthew Becker works closely with the Rubin Project to develop algorithms for data processing that will enable investigation of dark matter and dark energy through a phenomenon called weak gravitational lensing.
"As light from distant galaxies travels to the observatory, its path is influenced by the gravitational pull of the mass in between, including dark matter," said Becker. ?"This means that, as the observatory will see them, the shapes and orientations of the galaxies are slightly correlated in the sky. If we can measure this correlation, we can learn about the distribution of matter - including dark matter - in the universe."
Weak gravitational lensing can also reveal how the structure of the universe has changed over time, which could shed light on the nature of dark energy. The challenge is that the signals that indicate weak gravitational lensing in the LSST data will be, well, weak. The strength of the signal the scientists are looking for will be roughly 30 times smaller than the expected level of noise, or unwanted signal disturbance, in the data.
This means the scientists need a whole lot of data to make sure their measurements are accurate, and they're about to get it. Once complete, the LSST will have generated 60 petabytes of image data, or 60 million gigabytes. It would take over 11,000 years of watching Netflix to use that amount of data.
Becker and his colleagues are developing methods to compress the data to make analysis both manageable and fruitful. For example, by combining images of the same parts of the sky taken at different times, the scientists can corroborate features in the images to uncover correlations in the shapes of galaxies that might have otherwise been too faint to detect. Becker is also focused on determining the level of confidence the community can expect to have in conclusions drawn from the compressed data.
"If we know how certain we can be in our analysis, it enables us to compare our results with other experiments to understand the current state of knowledge across all of cosmology," said Becker. ?"With the data from the LSST, things are about to get much more interesting."
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