DESI, the Dark Energy Spectroscopic Instrument, is an exceptional apparatus designed to create a high-definition, 3-D map of a swath of the universe.
The U.S. Department of Energy has announced its approval of Critical Decision 2 (CD–2), authorizing the project’s scientific scope, schedule, and funding profile.
University of Michigan researchers will build critical components of DESI. Its mission is to shed light on the role of dark energy in the history of the universe’s expansion. Dark energy is believed to have competed with gravity over time to shape the structure of the universe.
“Seventy-five percent of our universe is made of this stuff and ultimately we don’t have any understanding of its nature. DESI will help us understand the fabric of empty space.”
DESI is a state-of-the-art spectrometer that will be mounted on top of the National Optical Astronomy Observatory-operated Mayall Telescope located near Tucson, Ariz.. To accommodate DESI, the telescope has been retrofitted with new glass lenses that will focus light onto DESI’s focal plane.
The DESI spectrometer will upgrade this old battleship to world-leading capability. The telescope’s top end will be replaced with DESI’s optical corrector and focal-plane system. The corrector’s six glass lenses, each a meter across, will focus the light from the existing 4-meter mirror onto the new focal plane.
The focal plane will have 5,000 small robotic arms, each of which holds an optical fiber. Once the telescope and spectrometer are operational, the robotic arms will position the fibers to capture light from a single galaxy or quasar at a time.
After a 15 to 20-minute exposure, the telescope aims at a new patch of sky; in less than a minute the robots rotate and reposition thousands of fibers.
DESI will measure the redshifts of more than 30 million galaxies and quasars with unprecedented precision. Redshift, the shift of a distant astronomical object’s spectrum to longer (redder) wavelengths, is a direct measure of how much space stretched while the light was on its way from the object to the observer. The bigger the redshift, the older the object.
At the end of the project, researchers will be able to measure the spectra of 10 times as many galaxies as DESI’s predecessor, the Baryon Oscillation Spectroscopic Survey of the Sloan Digital Sky Survey.
From the data, they will be able to create the largest three-dimensional map of the universe yet — one that reaches earlier in the universe’s history than ever before. The map will show how galaxies and other matter cluster together. The map also can be used to visualize fluctuations of density in the early universe called baryon acoustic oscillations (BAOs), which act as a standard ruler to measure the effect dark energy has on the universe’s expansion.
Beyond BAOs, DESI will be used to weigh the total mass of neutrinos in the universe, providing insight into the standard model of particle physics. It also provide data that can be used to test the theory of General Relativity, Einstein’s theory of gravity and models for the expansion of the universe.
“DESI aims to use the fossil imprint of sound waves from the first 400,000 years of the universe (still detectable as a pattern of temperature variations in the cosmic microwave background radiation) to study the mysterious composition of today’s universe.”
Those temperature differences map early variations in density (sound waves) that subsequently evolved into the clustering of galaxies, intergalactic gas and dark matter at recurrent intervals throughout space. Called “baryon acoustic oscillations,” these regularly spaced clusterings are consistent over time—like a ruler to gauge the universe, with the cosmic microwave background at one end—allowing direct measures of dark energy’s effect on expansion.
“The Mayall telescope is built like a battleship,” says Natalie Roe, Director of Berkeley Lab. The telescope’s moving weight is 375 tons, and it is “so well engineered it can support this very heavy new instrument”—which weighs five tons—“suspended way up there in the air.”
Two hundred physicists and astronomers make up the international DESI Collaboration, which is based at DOE’s Lawrence Berkeley National Laboratory. Seven U-M faculty members will be involved with the project in areas such as software development, survey planning, data distribution and simulation work. They will also be on the science team when DESI sees first light in 2019.
The camera will contain 5,000 optical fibers, each of which can be pointed at an individual galaxy. Over the next few years, U-M researchers will build its unique, robotic positioner system, which they call as “the heart of DESI.”
Key to DESI’s present and future success is its robust scientific collaboration, supported by many organizations, among them 31 universities and 18 government and private institutions, both U.S. and foreign, including five DOE national labs.
DOE and NSF will shortly begin joint support for Mayall telescope operations, preparatory work, and installation of the DESI instrument. Beginning in fiscal year 2019, DOE will support the full operations of the telescope throughout the five-year DESI survey.
