The DESI instrument, installed in 2019 at the four-meter diameter Mayall Telescope at Kitt Peak National Observatory in Arizona (USA), began its observation program in May 2021 after a four-month validation period.

While it took the SDSS (Sloan Digital Sky Survey) collaboration fifteen years to observe five million galaxies and quasars, DESI only needed a few months to break this record. Indeed, every twenty minutes, five thousand optical fibers are each aimed at a previously designated and selected target, and the collected signal is transmitted to one of the ten DESI spectrographs. All this at a breathtaking pace!

DESI's smooth, automated operation relies in particular on the individual positioning (to within 10 microns) of the optical fibers by five thousand robots. Quite a technological challenge! This achievement is also attributed to DESI's spectrographs, built by the French company Winlight Optics, and to the cryogenic enclosures protecting their CCD sensors, designed and built at the CEA-Irfu. "Our cryo-mechanical system maintains the CCDs at the required temperature to within 0.1 degrees, without incident so far," says Pierre-Henri Carton, a researcher at the Irfu.

For each galaxy or quasar, the spectrographs provide the "redshift" of their light emission, which the researchers link to the distance of the object in question, in order to reconstruct the large structures of the Universe over a third of the sky.

The choice of targets was thoroughly considered before the start of the observations. "To ensure the quality of our data, we conducted an intense testing campaign to validate the selection algorithms for the different classes of objects to be observed," explains Christophe Yèche, cosmologist at the Irfu. "Thanks to these tests, the observation strategy has been refined and optimized for each class of objects."

Indeed, DESI can observe the following different types of galaxies, all of which are witnesses to the last twelve billion years of the Universe (which itself is 13.8 billion years old):

• bright galaxies located up to 4 billion light-years from Earth
• "red" galaxies up to 8 billion light-years away
• "blue" galaxies up to 10 billion light-years away
• quasars up to 12 billion light-years away

"Artificial intelligence techniques make it possible to efficiently correct the systematic errors that affect each class of objects," notes Edmond Chaussidon, a doctoral student in cosmology at the Irfu. "They are also used to select quasars and determine their redshift from their spectrum."
 
The large structures that emerge from these observations still bear the imprint left by the acoustic waves that traversed the primordial plasma at the time of their formation in the early Universe, billions of years ago. By extracting this fine information, the physicists can reconstruct a history of the expansion of the Universe.
 
This "history" is crucial because it determines the fate of the Universe. Will dark energy, which is estimated to make up almost 70% of the energy content of the Universe, continue to expand it, make it collapse back on itself, or tear it apart? The knowledge of the past could enlighten the future.
 
Moreover, by comparing the history of the expansion with that of the growth of the great structures of the Universe, the cosmologists will be able to verify if Einstein's theory of general relativity applies to these immense expanses of space and time.
 
The DESI international collaboration is directed by the Lawrence Berkeley National Laboratory of the US Department of Energy. It brings together almost 500 researchers from 75 institutions in 13 countries. The design of the spectrographs is the result of a collaboration between the Berkeley Lab, the CEA, and the CNRS, along with Winlight Optics, which specializes in precision optics.