On a mountaintop near Tucson, Arizona, a telescope is changing our view of the cosmos. The Dark Energy Spectroscopic Instrument (DESI) – a high-tech, robot-operated device – is in the process of creating the largest 3D map of the universe to date, spanning 11 billion years.
And it’s only 10% through its five-year mission.
Once the detailed map is finished, it will give us a better idea of how the universe evolved – and clue us into its eventual fate.
“There is a lot of beauty to it,” says physicist Julien Guy, from the US’s Department of Energy’s Lawrence Berkeley National Laboratory.
“In the distribution of the galaxies in the 3D map, there are huge clusters, filaments, and voids. They’re the biggest structures in the universe. But within them, you find an imprint of the very early universe, and the history of its expansion since then.”
DESI, which is attached to the Mayall 4-meter Telescope at Kitt Peak National Observatory, was designed to collect detailed spectra from millions of galaxies, spanning a third of the entire sky.
Essentially, the instrument can break the light from each galaxy down to its constituent colours in order to find out how much the light has been redshifted. This occurs as the universe expands: as galaxies get further away from us, their light “stretches” on its way to Earth, shifting towards the red end of the electromagnetic spectrum.
Measuring this redshift tells us how far away a galaxy is: the greater the redshift, the further the light had to travel to get to us.
By collecting data from tens of millions of galaxies and seeing how far away each one is, DESI can see the depth of the sky and create a 3D map of the distribution of matter across the cosmos. This will show clusters and superclusters of galaxies, which contain “echoes” of their formation.
“Our science goal is to measure the imprint of waves in the primordial plasma,” says Guy. “It’s astounding that we can actually detect the effect of these waves billions of years later, and so soon in our survey.”
The map will also give us more information about dark energy.
About 70% of the universe is made up of dark energy, an enigmatic substance that is driving the accelerating expansion of the universe. It will ultimately decide the universe’s fate: will it expand forever, rip itself apart, or collapse?
To understand the fate of our cosmos, researchers must look to the past to see how dark energy has behaved through time. DESI’s 3D map will be instrumental in this.
So far, the survey has taken data from 7.5 million galaxies, smashing the previous speed record from the Sloan Digital Sky Survey. By the end of its observing run in 2026, DESI will have catalogued around 35 million – so we’ll have to wait a while before the map is completed.
And the survey has already been delayed – though first installed on the telescope in 2019, DESI experienced several setbacks due to the pandemic and so only started its survey in May 2021.
The way this instrument works is fascinating: it has 5000 robots that position the instrument’s optical fibres to ensure they are accurate within 10 microns.
“Ten microns is tiny,” says physicist Klaus Honscheid of Ohio State University, an instrument scientist on the project. “It’s less than the thickness of a human hair. And you have to position each robot to collect the light from galaxies billions of light-years away. Every time I think about this system, I wonder how could we possibly pull that off?”
While DESI is busily working on finishing its 3D map of the universe, it will also provide astrophysicists more and more information about the physics of galaxy formation and evolution, as well as data on extreme objects like black holes and quasars.
“We’re finding quite a lot of exotic systems, including large samples of rare objects that we just haven’t been able to study in detail before,” says Victoria Fawcet, a graduate student at Durham University in the UK who is working on understanding quasars.
Stay tuned for 2026.