- Dark energy makes up 68% of the universe and is expanding
- DESI can scan 5000 galaxies every 20 minutes and measure redshift of distant galaxies
- DESI will create most detailed 3D map of the universe and shed light on universe’s early development
A new instrument mounted atop a telescope in Arizona has aimed its robotic array of 5,000 fiber-optic “eyes” at the night sky to capture the first images showing its unique view of galaxy light.
It was the first test of the Dark Energy Spectroscopic Instrument, known as DESI, with its nearly complete complement of components. The long-awaited instrument is designed to explore the mystery of dark energy, which makes up about 68 percent of the universe and is speeding up its expansion.
DESI’s components are designed to automatically point at preselected sets of galaxies, gather their light, and then split that light into narrow bands of color to precisely map their distance from Earth and gauge how much the universe expanded as this light traveled to Earth. In ideal conditions DESI can cycle through a new set of 5,000 galaxies every 20 minutes.
Like a powerful time machine, DESI will peer deeply into the universe’s infancy and early development – up to about 11 billion years ago – to create the most detailed 3D map of the universe.
“After a decade in planning and R&D, installation and assembly, we are delighted that DESI can soon begin its quest to unravel the mystery of dark energy,” said DESI Director Michael Levi of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the lead institution for DESI’s construction and operations.
“Most of the universe’s matter and energy are dark and unknown, and next-generation experiments like DESI are our best bet for unraveling these mysteries,” Levi added. “I am thrilled to see this new experiment come to life.”
Seeing farther into the universe
“With DESI we are combining a modern instrument with a venerable old telescope to make a state-of-the-art survey machine.” said Lori Allen, director of Kitt Peak National Observatory at the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory.
DESI’s focal plane, which carries 5,000 robotic positioners that swivel in a choreographed “dance” to individually focus on galaxies, is at the top of the telescope.
These little robots – which each hold a light-gathering fiber-optic cable that is about the average width of a human hair – serve as DESI’s eyes. It takes about 10 seconds for the positioners to swivel to a new sequence of targeted galaxies. With its unprecedented surveying speed, DESI will map over 20 times more objects than any predecessor experiment.
The focal plane, which is comprised of a half-million individual parts, is arranged in a series of 10 wedge-shaped petals that each contain 500 positioners and a little camera to help the telescope point and focus.
The focal plane plus other DESI components weighs 11 tons, and the Mayall Telescope’s movable arm that DESI is installed on weighs 250 tons and rises 90 feet above the floor in the Mayall’s 14-story dome.
Among the more recent arrivals at Kitt Peak are the spectrographs designed to split up the gathered light into three separate color bands to allow precise distance measurements of the observed galaxies across a broad range of colors.
These spectrographs, which allow DESI’s robotic eyes to “see” even faint, distant galaxies, are designed to measure redshift, which is a shift in the color of objects to longer, redder wavelengths due to the objects’ movement away from us. There are now eight spectrographs installed, with the final two arriving before year-end.
“This is a very exciting moment,” said Nathalie Palanque-Delabrouille, an astrophysics researcher at France’s Atomic Energy Commission (CEA) who has participated in the selection process to determine which galaxies and other objects DESI will observe.
“This is a very significant advance compared to previous experiments,” she said. “By looking at objects very far away from us, we can actually map the history of the universe and see what the universe is composed of by looking at very different objects from different eras.”
Gregory Tarlé, a physics professor at the University of Michigan (UM) who led the student teams that assembled the robotic positioners for DESI and related components, said it’s gratifying to reach a stage in the project where all of DESI’s complex components are functioning together.
UM delivered a total of 7,300 robotic positioners, including spares. During the production peak, the teams were churning out about 50 positioners a day.
Now that the hard work of building DESI is largely done, Tarlé said he looks forward to DESI discoveries.
“I want to find out what the nature of dark energy is,” he said. “We finally have a shot at really trying to understand the nature of this stuff that dominates the universe.”