5-Year Search for Dark Energy Begins

The Dark Energy Spectroscopic Instrument (DESI) has begun its five-year search for dark energy. The device, housed at Kitt Peak National Observatory near Tucson, Ariz., will aim its robotic array of 5000 fiber optic “eyes” at the night sky to gather and study the light from tens of millions of galaxies and other distant objects within the universe.

The five-year survey’s official start follows a four-month trial run of its custom instrumentation that captured spectra from four million galaxies — more than the combined output of all previous spectroscopic surveys.

Star trails take shape around the 14-story Mayall Telescope dome in this long-exposure image. The Dark Energy Spectroscopic Instrument was installed inside this dome. Courtesy of P. Marenfeld and NOAO/AURA/NSF.

“We’re not using the biggest telescopes,” said Berkeley Lab’s David Schlegel, a DESI project scientist. “It’s that the instruments are better and highly multiplexed, meaning that we can capture the light from many different objects at once.”

The telescope, Schlegel said, “is literally pointing at 5000 different galaxies simultaneously.” On any given night as the telescope moves into a target position, the optical fibers align to collect light from galaxies as it is reflected off the telescope mirror. From there, the light is fed into a bank of spectrographs and CCD cameras for further processing and study.

Gathering light from some 30 million galaxies will enable researchers to construct a 3D map of the universe with greater accuracy. The data will also help in studying the repulsive force associated with “dark energy” that drives the acceleration of the universe’s expansion across vast cosmic distances.

“With this map, we will be able to trace the impact of the mysterious dark energy through 11 billion years of cosmic time and attempt to understand its nature,” said University of Michigan professor Gregory Tarlé, who led the construction of DESI’s robotic eyes.

The components are designed to automatically orient themselves toward preselected sets of galaxies, gather their light, and then split that light into narrower bands of color to precisely map their distance from Earth and gauge how much the universe expanded as this light traveled to Earth. Under ideal conditions, the system is able to cycle through a new set of 5000 galaxies every 20 minutes.

“We will measure 10 times more galaxy spectra than ever obtained. These spectra get us a third dimension,” said project director Michael Levi of the Berkeley Lab.

As the universe expands, galaxies move away from one another and their light shifts to longer, redder wavelengths. The more distant the galaxy, the greater its “redshift.” Measuring those redshifts enables the researchers to create a 3D map of the universe. The detailed distribution of galaxies within the map is expected to yield new insights about dark energy’s influence and nature.

“Dark energy is one of the key science drivers for DESI,” said project co-spokesperson Kyle Dawson, a professor of physics and astronomy at University of Utah. “The goal is not so much to find out how much there is — we know that about 70% of the energy in the universe today is dark energy — but to study its properties.”

The universe, he explained, is expanding at a rate determined by its total energy contents. As DESI looks out in space and time, “we can literally take snapshots today, yesterday, 1 billion years ago, 2 billion years ago — as far back in time as possible. We can then figure out the energy content in these snapshots and see how it is evolving.”