After just seven months, a huge team of scientists who work with the Dark Energy Spectroscopic Instrument have already mapped a larger swath of the cosmos than all other 3D surveys combined. And since they’re only 10 percent of the way through their five-year mission, there’s much more to come.
DESI, pronounced like Desi Arnaz’s name, has revealed a spectacular cosmic web of more than 7.5 million galaxies, and it will scan up to 40 million. The instrument is funded by the US Department of Energy and installed at the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory near Tucson, Arizona. It measures the precise distances of galaxies from Earth and their emitted light at a range of wavelengths, achieving quantity and quality at the same time. It will eventually cover some 14,000 square degrees, about one third of the sky. The science gained from parsing the data is yet to come, but it will especially aid astrophysicists as they investigate how the universe is expanding.
“It’s really a fantastic adventure. We have been able to go on despite the pandemic. We had to shut down for a few months and then we adapted,” says Julien Guy, DESI project scientist at the Lawrence Berkeley National Laboratory, the collaboration’s lead institution. Now the observations and data processing are mostly automated; every morning the scientists get data on as many as 100,000 galaxies collected overnight, he says.
“It's amazing how well this instrument is working and how well it’s been designed to go out and get the distances to these galaxies. It’s an incredibly efficient machine for harvesting them in a way that even two decades ago would’ve been mind-boggling,” says Jason Rhodes, a research scientist at the Jet Propulsion Laboratory in Pasadena who’s working on space telescopes to map out galaxies in the early universe.
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DESI actually consists of several apparatuses installed within the telescope’s 14-story dome. The circular focal plane is positioned near the top, and it’s made up of 10 wedge-shaped petals, each with 500 tiny robots. They’re what enable the instrument’s galactic cartography: These 5,000 pencil-sized robotic motors position optical fibers that have to be accurately placed to within 10 microns–less than the thickness of a human hair. That enables the instrument to collect precise data on 5,000 galaxies at once. Then the telescope points at another area of the night sky and starts work on the next 5,000. In contrast, at one of DESI’s predecessors, the Sloan Digital Sky Survey, based at a telescope in southern New Mexico, scientists had to manually drill holes into a circular aluminum plate at the telescope’s focus for each set of measurements, and they plugged in little fibers for every single galaxy they wanted to observe.
At the bottom of the telescope sits DESI’s spectrographs, which break down the light from each galaxy into its full spectrum of colors, like a rainbow. That allows them to measure a galaxy’s “redshift,” which tells scientists exactly how far away it is. (Light from objects that are moving away from Earth as the universe expands appears redder to us because of its longer wavelength.) With the data Guy and his team already have on hand, they can see complex, weblike structures of galaxies, which they’re probing out to more than 10 billion light-years away—meaning they’re capturing light emitted back when the universe was less than half its current age. When you look up at the sky on a clear night, you can only discern the full extent of the Milky Way by scanning the whole sky, not just spotting a couple of stars. Similarly, DESI uncovers these galactic megastructures only by systematically mapping vast numbers of galaxies across a wide area of the sky.
“This project has a specific scientific goal: to measure very precisely the accelerating expansion of the universe,” Guy says. He refers to “dark energy,” something simultaneously mysterious and ubiquitous, which is thought to make up around 70 percent of the universe and causes it to keep expanding faster and faster. Guy and his colleagues hope to shed light on what dark energy really is. Their survey could also finally resolve an emerging astrophysics problem: Scientists measuring the expansion rate in the nearby universe—when it was almost its current age—and those measuring the expansion rate of the universe when it was in its infancy get different answers. Nobody knows how to explain the persistent discrepancy, but if DESI shows that dark energy somehow evolves, that might resolve the problem.
Guy and his team plan their first DESI data release for 2023. That massive data dump will enable numerous scientists to try their hands at studying millions of galaxies, while developing new statistics and machine-learning tools. “I’m curious about what one can do with artificial intelligence or deep learning with such a large amount of data. You might be able to find something out of the ordinary that’s not expected. That's what I’m excited about,” says Shirley Ho, a cosmologist at the Flatiron Institute in New York City and a former researcher with the Sloan Digital Sky Survey.
In a few years, DESI won’t be the only sweeping galactic atlas in town. Starting next year, the National Science Foundation-funded Vera Rubin Observatory, which is being built on a dry mountain of northern Chile, will catalog billions of galaxies, but with less precision. Astrophysicists are also preparing for the European Space Agency’s Euclid spacecraft, with a planned 2023 launch date, and NASA’s Nancy Grace Roman Space Telescope, slated to blast off in 2027. They’ll both carry instruments to measure galaxies’ distances and spectra, and they’ll map them out past what DESI can see, in parts of the deeper universe, which formed when it was just a couple billion years old—a child, cosmologically speaking. “Both Euclid and Roman are counting on DESI to get the nearby universe. They will build on what DESI is doing in an interesting way,” Rhodes says.
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For Ho, DESI and other gigantic cosmic maps serve as a reminder of how small humanity is in the universe. “The fact that we are able to look at so many galaxies is an achievement. But each one of those galaxies has a billion stars, and each of those stars might have a system like ours,” she says. “I think it’s a humbling experience to realize how insignificant we are.”
Updated 01/13/2022 1:00 PM ET: This story has been updated with the correct area of the sky DESI will cover when it's complete.
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