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A five-year quest to map the universe and unravel the mysteries of “dark energy” is beginning officially today, May 17, at Kitt Peak National Observatory near Tucson, Arizona, USA. To complete its quest, the Dark Energy Spectroscopic Instrument (DESI) will capture and study the light from tens of millions of galaxies and other distant objects in the universe.

By gathering light from some 30 million galaxies, project scientists say DESI will help them construct a 3D map of the universe with unprecedented detail. The data will help them better understand the repulsive force associated with “dark energy” that drives the acceleration of the expansion of the universe across vast cosmic distances.

What sets DESI apart from previous sky surveys? “DESI will allow us to see an order of magnitude more galaxies than ever before, and study the evolution of the Universe from 11 billion years ago to the present day”, explained Héctor Gil-Marín, a scientist at Institut de Ciències del Cosmos at the University of Barcelona (ICCUB) and at the Institute of Space Studies of Catalonia (IEEC), that will co-lead the first analysis of the galaxy maps. The DESI telescope collects light, or spectra, from galaxies and quasars, which get us their recession velocity. “We know that the farther the object is from us, the higher its recession velocity is, which allows us to build a 3D map of the universe”, Gil-Marín explained.

“DESI is the front runner of a new generation of instruments around the globe that will study dark energy from different angles,” said Andreu Font-Ribera, a cosmologist at Institut de Física d’Altes Energies (IFAE) that will co-lead the first analysis of the most distant quasars. He said the scientific program will allow researchers to address with precision two primary questions: what is dark energy; and the degree to which gravity follows the laws of general relativity, which form the basis of our understanding of the cosmos. “It has taken 10 years of effort, from the instrument design to this moment, when DESI starts collecting the data that is going to revolutionize our understanding of the Universe” says Violeta Gonzalez-Perez, a scientist at Universidad Autónoma de Madrid (UAM) that is one of the coordinators for developing computer simulations of what DESI will observe.

The disk of the Andromeda Galaxy (M31), which spans more than 3 degrees, is targeted by a single DESI pointing, represented by the large, pale green, circular overlay. The smaller circles within this overlay represent the regions accessible to each of the 5000 DESI robotic fiber positioners. In this sample, the 5000 spectra that were simultaneously collected by DESI include not only stars within the Andromeda Galaxy, but also distant galaxies and quasars. The example DESI spectrum that overlays this image is of a distant quasar (QSO) 11 billion years old. Credit: DESI collaboration and DESI Legacy Imaging Surveys

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

The DESI instrument resides at the retrofitted Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of the National Science Foundation’s (NSF) NOIRLab. The instrument includes new optics that increase the field of view of the telescope and includes 5,000 robotically controlled optical fibers to gather spectroscopic data from an equal number of objects in the telescope’s field of view.

“What is special about DESI is not so much the telescope, but the instrument” says Otger Ballester, an engineer at IFAE who has been part of the team developing the guiding, focusing and alignment cameras for DESI, one of the Spanish contributions to the project. In fact, the instrument “can simultaneously gather light from 5,000 different objects and obtain their spectra in just 20 minutes” Ballester said. As the telescope is moved 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. On a good night, DESI collects spectra from some 150,000 objects.

Photo of a small section of the DESI focal plane, showing the one-of-a-kind robotic positioners. The optical fibers, which are installed in the robotic positioners, are backlit with blue light in this image. Credit: DESI collaboration

“The outstanding capability of DESI to collect spectra is also thanks to the instrument software,” said Santiago Serrano, an engineer at Institute of Space Sciences (ICE, CSIC) and IEEC, who has developed part of the algorithms needed to guide the telescope. He recognizes the invaluable effort of scores of scientists in Spain and around the world which have made the instrument and the experiment possible.

Spectra collected by DESI are the components of light corresponding to the colors of the rainbow. Their characteristics, including wavelength, reveal information such as the chemical composition of objects being observed as well as information about their relative distance and velocity.

As the universe expands, galaxies move away from each other, and their light is shifted to longer, redder wavelengths. The more distant the galaxy, the greater its “redshift.” By measuring galaxy redshifts, DESI researchers will create a 3D map of the universe. The detailed distribution of galaxies in the map is expected to yield new insights on the influence and nature of dark energy.

“Unraveling the properties of the mysterious Dark Energy is the main goal of DESI” said Licia Verde , ICREA professor at ICCUB. “We know that at present 70% of the energy content of the Universe is made of Dark Energy, but we know very little about its properties”.

Dark Energy determines the expansion rate of the universe , Verde explains. As the DESI instrument looks out in space and time, she says, “we can simultaneously observe the universe at different epochs, and by comparing them, figure out how the energy content evolves as the universe gets older.”

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the U.S. National Science Foundation, the Science and Technologies Facilities Council of the United Kingdom, the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, the French Alternative Energies and Atomic Energy Commission (CEA), the National Council of Science and Technology of Mexico, the Ministry of Economy of Spain, and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.