The University of Michigan has signed on to be a major player in a new instrument that aims to help answer a burning scientific question: Why is the expansion of the universe accelerating? Cosmologists suspect a mysterious property called dark energy. Although it is thought to comprise 75 percent of the universe, its nature and the physics behind it are still a mystery.      

The Dark Energy Spectroscopic Instrument (DESI) will address this mystery by creating a high-definition, 3-D map of a swath of the universe going back 10 billion light-years. By exploring how structure in the universe has evolved through time, scientists hope to uncover the tug-of-war between the forces of gravity and dark energy (see Testing for Dark Energy).

U-M was chosen to build a major system for this Department of Energy (DoE) project, and seven faculty members from the Departments of Physics and Astronomy have committed to support the project in related areas such as software development, survey planning, data distribution, and simulation work. They also plan to do ground-breaking science when DESI sees first light in 2018.

The instrument itself is a giant prism-like camera that will sit within the Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona (see image above). It will contain 5,000 optical fibers, each of which can be pointed at an individual galaxy, thanks to the unique positioning system U-M was chosen to develop.

“Our system features an array of 5,000 little robots that will simultaneously position each optical fiber on a galaxy,” says Gregory Tarlé, physics professor and member of the DESI Executive Committee, who is heading this effort. “The light will be routed to spectrographs, which will measure each galaxy’s ‘redshift’ and precisely determine its distance from us."

Over five years of operation, DESI will determine the 3-D position of 30 million galaxies, providing scientists an unprecedentedly large and high-quality data set to work with.

"This is what makes the project such a big leap,” says Chris Miller, assistant professor of astronomy and DESI Data Distribution Committee co-chair. “In just a couple of decades, we’ve gone from collecting spectra from one object to 500 at a time. Having a dedicated 4-meter class telescope collecting spectra for 5,000 galaxies every hour, every night will really drive the science forward."

U-M’s robotic positioner is key to making this happen, and the university’s culture of student engagement is in part what made U-M’s system a winner. “We were chosen for this effort because our design was small, durable, and simple – but also because we had a track record of engaging U-M students to build sophisticated systems like this,” Tarlé says.

The contribution to DESI’s instrumentation has allowed U-M to become a full institutional member in the project. This means that U-M faculty interested in contributing to DESI and using it for science can join without the standard cost of individual membership.

This is not just a boon for scientists interested in dark energy. Each night, there will be about 10 percent of the optical fibers without an assigned galaxy in the telescope’s field of view. In addition, on nights where the moon makes the sky too bright to observe dim targets like distant galaxies, the whole instrument will be available for other science.

“While many of us will use DESI to study dark energy, a number of U-M faculty plan to use it in other ways,” says Miller. “Spectroscopic data from DESI is an ideal complement to imaging surveys like Gaia and the Large Synoptic Survey Telescope, allowing us to measure the motion and chemistry of local and distant galaxies as well as stars in the Milky Way. But no matter how we use it, because we’re helping to build the system, we have a head start in understanding the data and will be much more efficient in using it for our science.”

U-M faculty currently committed to DESI include Chris Miller, Eric Bell and Monica Valluri from astronomy and Gregory Tarlé, David Gerdes, Dragan Huterer and Michael Schubnell from physics.

The University of Michigan’s involvement in the DESI project was made possible by the University of Michigan Departments of Astronomy and Physics; the U-M College of Literature, Science, and Arts; the U-M Office of Research; and the University administration. The research is funded by DESI and DOE, grant number DE-SC0007859. DESI is a $55 million international project that is managed by the Lawrence Berkeley National Laboratory. This research is supported by the Director, Office of Science, Office of High Energy Physics of the U.S. Department of Energy under Contract No. DE–AC02–05CH11231; additional support is provided by the U.S. National Science Foundation, Division of Astronomical Sciences under Contract No. AST-0950945 to the National Optical Astronomy Observatory; the Science and Technologies Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; and by the DESI Member Institutions.

Image credit: Credit: NOAO/AURA/NSF. 

Testing for Dark Energy

DESI aims to precisely measure the evolution of structure in the universe. Because galaxy clustering represents the result of a competition between gravity working to pull galaxies together and dark energy working to stretch space-time apart, the measurement of structure over time helps reveal how dark energy operates.

As galaxies move away from Earth, the light they emit is stretched into longer wavelengths by the Doppler Effect, making them appear redder. The amount of this “redshift” can tell scientists how far away a galaxy is from us. Scientists currently have a good 2-D map of the universe but need precision depth measurements for a large number of galaxies to map the universe’s structure in three dimensions.

To see how this structure has evolved over time, scientists use a naturally occurring standard ruler of the distance between clusters of matter called “baryon acoustic oscillations.” At a unique point in cosmological history – about 380,000 years after the Big Bang, when the hot plasma of the early universe cooled and atoms formed – small fluctuations in matter density that had been carried along by sound waves became frozen into a pattern on the sky. These ripples of more concentrated matter formed the seeds from which galaxies grew, and the space between galaxies still reflects this pattern today. Measuring how this pattern has stretched through time provides insight into the nature of dark energy.