In 1998, astronomers discovered an astonishing fact: Some antigravitational force was speeding up the expansion of the universe. Two decades later, this "dark energy" is still a mystery. But next month, a veteran telescope in Arizona will begin to hunt for clues, after being retrofitted with a robotic system to map an unprecedented 35 million galaxies and how they clump across space and time.
That huge sample, spanning most of the visible universe, will allow the Dark Energy Spectroscopic Instrument (DESI) to look for fluctuations in the clumping patterns that might betray the nature of dark energy. So far, dark energy appears to have the same accelerating effect everywhere, across most of the history of the universe. That suggests it is a constant pressure associated with the fabric of space itself—a so-called cosmological constant. But if dark energy turns out to vary over time, it could require a more exotic explanation, such as an additional force field, sometimes dubbed quintessence, that might change through cosmic history. Or it could point to the need for a more fundamental rewrite of general relativity, Albert Einstein's theory of gravity.
"What's the story?" asks Ofer Lahav, an astronomer at University College London and head of a U.K. consortium contributing to the $75 million project. "I'd like to know before I die."
Astronomers originally discovered cosmic acceleration by measuring the distance to supernovae in remote galaxies. They found that these distant beacons had been swept farther away than they would be if the expansion were constant. But some have questioned whether supernovae mark distance reliably enough to probe the behavior of dark energy, and astronomers have adopted new tactics with increasing precision. For example, astronomers with the Dark Energy Survey, recently completed using a telescope in Chile, looked for tiny distortions in the shapes of galaxies caused by intervening matter, the distribution of which holds clues to the stretching of space.
DESI astronomers will use what has turned out to be the most powerful dark energy probe so far. They will look for ripple patterns, called baryon acoustic oscillations, in the clumping of galaxies. These ripples began as sound waves that washed through the universe in the 400,000 years after the big bang, when it was a swirling mass of particles and energy. Once that primordial soup cooled and became transparent, the ripples—which drew matter to them and formed the foundations for galaxy clusters—became locked in, separated by a characteristic distance. By measuring how that cosmic yardstick has grown over time, astronomers can gauge the competing effects of dark energy and gravity and look for deviations. "It's a miracle nature built such a useful ruler," Lahav says.
A project called the Baryon Oscillation Spectroscopic Survey (BOSS) pioneered the technique, as a part of the Sloan Digital Sky Survey (SDSS), which relied on a 2.5-meter telescope in New Mexico. It mapped about 2 million galaxies—but it was slow and laborious. To get a distance to a galaxy, astronomers split its light into a spectrum and measure its redshift, or the stretching of its light by the expansion of the universe. BOSS astronomers did this by fitting the telescope's focal plane with a mask: an aluminum plate into which they had drilled hundreds of holes, each one at a spot where light from a known galaxy would fall. Optical fibers manually attached to each hole caught each galaxy's light and fed it to a spectrograph. But the researchers had to make a new plate and reattach the fibers every time the telescope swiveled to a new part of the sky. "It was horrible but effective," says DESI team member John Peacock of the University of Edinburgh.
By using the 4-meter Nicholas U. Mayall Telescope on Kitt Peak in Arizona, DESI will capture galaxies from deeper in space and time than the SDSS, and automation will boost its per-night productivity. The instrument has 5000 fibers attached to its 0.8-meter-wide focal plane. Each fiber tip can be repositioned in a matter of seconds by a tiny robotic actuator. The 5000 fibers snake down the back of the telescope to a temperature-controlled room containing 10 spectrographs, each analyzing the light of 500 galaxies simultaneously. The entire instrument can be reconfigured for a new patch of sky in a few minutes. "DESI is 10 times more effective than the last survey from SDSS," says project scientist David Schlegel of Lawrence Berkeley National Laboratory in California. "It's the next generation."
Next month, for the first time, the DESI team will point the telescope at the sky to test the whole system. "A little bit at a time, we'll show that the robots are working and gathering light," says team member Nathalie Palanque-Delabrouille of the University of Paris-Saclay. By 2025, astronomers hope to have their full cosmic galaxy map in hand. It will hold clues not only to dark energy, but also to other exotic questions in cosmology, such as whether neutrinos—tiny fleeting particles that are almost impossible to detect—could account for the universe's hidden dark matter and whether, as theorists believe, the universe was born in an exponential burst of expansion lasting just a fraction of a second, known as inflation.
"We're aimed at all the weirdest things we see in the universe," Schlegel says.