“I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative,” said physicist Adam Riess in an interview. Riess of Johns Hopkins University is one of three researchers just awarded the Nobel Prize for Physics for their part in the discovery that the expansion of the universe is accelerating.
This past week, the first test of the Dark Energy Spectroscopic Instrument, known as DESI, mounted atop a telescope in Arizona has aimed its robotic array of 5,000 fiber-optic “eyes” at the night sky and captured the first images showing its unique view of galaxy light. The long-awaited instrument is designed to explore the mystery of dark energy, which now makes up about 68 percent of the universe and is speeding up its expansion described by Riess as “incredibly strange, but actually it makes sense to me that it went unnoticed.”
5000 Galaxies in 20 Minutes
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.
DESI’s 5000 spectroscopic “eyes” can cover an area of sky about 38 times larger than that of the full moon, as seen in the overlay of below DESI’s focal plane on the night sky (top). Each one of these robotically controlled eyes can fix a fiber-optic cable on a single object to gather its light (the red circle marks the location of a single positioner). The gathered light collected from a small region in the Triangulum galaxy (bottom) by a single fiber-optic cable is split into a spectrum (bottom) that reveals the fingerprints of the elements present in the galaxy and aid in gauging the distance to the galaxy. The test spectrum shown here was collected by DESI on Oct. 22. (Dustin Lang, Aaron Meisner, DESI Collaboration/Imagine Sky Viewer; NASA/JPL-Caltech/UCLA; and Legacy Surveys project)
The latest milestone marks the opening of DESI’s final testing toward the formal start of observations in early 2020.
Poised to Unravel the Mystery of Dark Energy
“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.”
The DESI collaboration will study dark energy by mapping the distance to 35 million galaxies and 2.4 million quasars across one-third of the area of the sky. “Galaxies aren’t scattered randomly in space, but instead form a complex pattern from which we can learn about the composition and history of the universe,” says Eisenstein. “The unprecedented maps from DESI will allow us to measure how the universe has expanded over time, to see how gravity and dark energy compete to pull and push material apart.”
Installation of DESI began in February 2018 at the Nicholas U. Mayall Telescope at Kitt Peak National Observatory near Tucson, Arizona. Over the past 18 months, a bevy of DESI components were shipped to the site from institutions around the globe and installed on the telescope.
Among the early arrivals was a 5-ton set of lenses packaged into a large steel barrel. The barrel sits over the 4-meter primary mirror of the Mayall Telescope and provides an expansive field of view. The lenses, each measuring about a meter across, were successfully tested in April.
Behind these lenses sits DESI’s focal plane, with 5,000 robotic positioners that swivel in a choreographed “dance” to individually focus on galaxies.
These little robots — each holding a light-gathering fiber-optic cable that is about the average width of a human hair — serve as DESI’s eyes. It takes about 2 seconds for the positioners to swivel to a new sequence of targeted galaxies. The fiber-optic cables, 150 miles in total, transmit the light from each galaxy to a collection of spectrographs that split the light into a rainbow of thousands of individual colors.
By measuring these spectra, DESI can determine how fast each galaxy is moving away from Earth. Because the universe is expanding, galaxies are moving away from us, with more distant galaxies moving faster. This motion stretches the wavelength of the light we receive, much as the sound of a fire engine’s siren shifts to lower tones as it moves away from us.
Using these velocities and the distances they imply, DESI builds up a 3-dimensional map of the Universe. With its unprecedented surveying speed, DESI will map over 20 times more objects than any predecessor experiment.
In addition to the mapping of distances, the many terabytes of DESI spectroscopic information will provide a powerful data set for many astrophysical problems. Professor Douglas Finkbeiner of the Center for Astrophysics | Harvard & Smithsonian explains, “A beautiful aspect of DESI is the ability to combine precision calibration and novel statistical tools with the raw size of the data set. I look forward to the DESI opportunity not just to study dark energy, but the Milky Way, galaxies, quasars, and everything in between.”
With installation largely completed and the commissioning of the instrument now underway, DESI’s global collaboration will now start to tackle the challenges of its celestial data. “These large projects take years to construct, years to operate, and years to understand,” Eisenstein says. “DESI wouldn’t be possible without the commitment of hundreds of people, coming together collectively to pursue these goals. In many ways, that’s the joy and legacy of these surveys.”
The Daily Galaxy, Max Goldberg, via Harvard-Smithsonian Center for Astrophysics