Ebene Magazine – The Dark Energy Spectroscopic Instrument (DESI) starts a 5-year survey

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May 17, 2021

by Tracy Marc, Fermi National Accelerator Laboratory

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

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DESI is an international scientific collaboration administered by the Lawrence Berkeley National Laboratory or Berkeley Lab of the Department of Energy with primary funding from the DOE Office of Science.

By collecting light from approximately 30 million galaxies, project scientists can loudly DESI create a 3D map of the universe with unprecedented detail. The data will help them better understand the repulsive force associated with « dark energy » which is driving the acceleration of the universe’s expansion over vast cosmic distances.

Jim Siegrist, associate director of high energy physics at DOE, says : « We are very excited about the launch of DESI, the first next-generation Dark Energy project, which is commencing its scientific investigation. In addition to its primary purpose of Dark Energy studies, the data is before set by the broader scientific community for a multitude of purposes. » of astrophysical studies. « 

What is the difference between DESI and previous sky surveys? The project leader, Michael Levi of Berkeley Lab, said: « We will measure ten times more galaxy spectra than ever before. These spectra bring us a third dimension. » Instead of two-dimensional images of galaxies, quasars and other distant objects, he explained, the instrument collects light or spectra from the cosmos so that it « becomes a time machine in which we place these objects on a timeline that goes back as far as 11 billion years ago. « 

 » DESI is the most ambitious instrument of a new generation that aims to better understand the cosmos – especially its dark energy component, « said project spokeswoman Nathalie Palanque-Delabrouille, cosmologist at the French commission for alternative energies Nuclear energy CEA. She said the scientific program – including her own interest in quasars – will allow researchers to precisely answer two main questions: What is dark energy? and how gravity follows the laws of general relativity that form the basis of our understanding of the cosmos.

« It was a long way from the first steps we took almost a decade ago to design the survey , then decide what goals to watch and now have the tools to achieve those scientific goals, « said Palanque-Delabrouille. said. « It’s very exciting to see where we are today. »

The formal start of the five-year DESI survey follows a four-month test run of its custom instrumentation that captured 4 million galaxy spectra – more than the combined output of any previous spectroscopic Investigations.

The DESI instrument is located in the upgraded 4-meter Nicholas U. Mayall telescope at Kitt Peak National Observatory, a program of the National Science Foundation’s NOIRLab. The instrument includes new optics that increase the telescope’s field of view and 5,000 robotic optical fibers to collect spectroscopic data from an equal number of objects in the telescope’s field of view.

« We don’t use the largest telescopes, » said David Schlegel from Berkeley Lab, DESI project scientist. « The fact is that the instruments are better and very much multiplexed, which means that we can capture light from many different objects at the same time. »

In fact, the telescope « literally points to 5,000 different galaxies at the same time, » Schlegel said . Every night, he explains, as the telescope moves into a target position, the optical fibers align to collect light from galaxies as it reflects off the telescope mirror. From there, the light is fed into a number of spectrographs and CCD cameras for further processing and examination.

« It really is a factory that we have – a spectra factory, » said Christophe Yeche, head of survey validation, also a cosmologist at CEA. « We can collect 5,000 spectra every 20 minutes. On a good night we collect spectra from around 150,000 objects. »

« But it’s not just the instrument hardware that got us here – it’s the instrument software too, the central nervous system of DESI, « said Klaus Honscheid, professor of physics at Ohio State University who led the design of the DESI instrument control and monitoring systems. He credits numerous people in his group and around the world who have built and tested thousands of DESI components, most of which are unique to the instrument.

Spectra collected by DESI are the components of light that make up colors correspond to the rainbow. Their properties, including wavelength, reveal information such as the chemical composition of the objects being observed, as well as information about their relative distance and speed.

As the universe expands, galaxies move away from each other and their light is shifted to longer, reddish wavelengths . The further away the galaxy is, the greater its « redshift » is. By measuring the redshifts of galaxies, DESI researchers create a 3D map of the universe. The detailed distribution of the galaxies on the map is expected to provide new insights into the influence and nature of dark energy. « Dark energy is one of the most important scientific drivers for DESI, » said Kyle Dawson, co-spokesman for the project, Professor of Physics and Astronomy at the 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 is expanding at a rate that is determined by its total energy content, explains Dawson. While the DESI instrument looks into space and time, he says: « We can literally take snapshots today, yesterday, 1 billion years ago, 2 billion years ago – as far back as possible in time. We can then find out. » Energy content in these snapshots and see how it evolves.  »

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