An ambitious new sky survey is set to map the universe in three dimensions, charting the distribution of tens of millions of galaxies and shining a spotlight on the evolution of the cosmos on the very largest of scales. Astronomers hope that the project, known as DESI, for Dark Energy Spectroscopic Instrument, will shed light on the puzzle of dark energy and perhaps yield insight into the nature of gravity itself.
In our local corner of the universe, galaxies and clusters of galaxies are the dominant structures. Over even larger scales, those clusters of galaxies appear to be strung together in vast filaments, stretching across many millions of light years, with great voids separating the filaments from one another. Astronomers don’t fully understand how this rich structure came into existence. At the time of the Big Bang, some 13.8 billion years ago, the universe is thought to have been nearly homogeneous. Whatever irregularities there may have been would have been amplified by gravity, which causes matter to clump together. Our best theory of gravity, Einstein’s theory of general relativity, has passed every test its been subjected to for more than 100 years; still, it has never been tested over the vast cosmological distances that DESI will be examining.
With data from DESI, “we’re testing our theory of gravity at the largest scales possible,” says Risa Wechsler, a cosmologist at Stanford University and director of the Kavli Institute for Particle Astrophysics and Cosmology.
Complicating the picture, however, is dark energy, a mysterious anti-gravity-like force that appears to be pushing galaxies apart from one another. And on top of that there’s dark matter, an unknown material—possibly exotic particles created in the very early universe—which has so far eluded direct detection, but whose gravitational effects suggest that it accounts for some 85 percent of the matter in the universe. To understand cosmic history, scientists need to untangle gravity’s pull from dark energy’s push, as well as the extra pull of dark matter. Data from DESI, which will reveal how much gravitational “clumping” occurred over time in more detail than ever before, will help researchers tease apart these competing influences.
While DESI is new, the telescope it’s piggybacking on is not: The instrument will be analyzing light collected by the four-meter Mayall telescope at Kitt Peak National Observatory near Tucson, Arizona. DESI will record not only a galaxy’s light but also its spectrum, by measuring how much light a given object emits at particular wavelengths. Because the universe is expanding, distant galaxies appear to be receding from us. This in turn causes their light to get stretched out, making the galaxy’s spectrum appear redder than if it were stationary—astronomers call this a “redshift.” And since there’s a relationship between distance and redshift—the more distant the galaxy, the greater the redshift—spectral data allows astronomers to figure out how far away each galaxy is. And with those figures in hand, they can map the universe in three dimensions.
Two key pieces of technology make DESI the ultimate galaxy-grabber. One is a special lens, about one meter across; positioned in front of Mayall’s primary mirror, it expands the telescope’s field of view to just over three degrees—some six times the apparent width of Earth’s moon. (Typical professional telescopes have a field of view of less than one degree.) The wider the field of view, the greater the number of galaxies that can be studied at once. After passing through this lens, incoming light hits an array of 5,000 optical fibers which guides the light from each individual galaxy to a set of spectrographs, which will measure how much light each galaxy is emitting at various wavelengths. Because DESI can access data from previous surveys that give the coordinates of each galaxy, each optical fiber can be positioned so that it “lines up” with the light from a particular galaxy. That’s where the second key—automation—kicks in: Each time the telescope is aimed at a new position in the sky, some 5,000 miniature robots quickly re-position the array of optical fibers so that they match up with the galaxies in the new field of view.
This level of automation is a game-changer, according to DESI project scientist according to David Schlegel of the Lawrence Berkeley National Laboratory, which manages the project. “When I was a student, I was involved in one of those redshift surveys—you’d go to the telescope night after night; we’d point the telescope at a galaxy; it took around 30 to 60 minutes to measure the redshift of a galaxy; then point to another galaxy. And over the course of five years we’d make these huge maps of maybe 3,000 galaxies; they were amazing. Now we can do that in ten minutes.”
DESI will be aimed at some particular part of the sky for about 15 to 20 minutes, before moving on to the next patch. Each time the telescope is moved, the little army of robots, as Schlegel likes to call them, takes about a minute to reposition itself. “It was probably the most fun part of the instrument. All of our engineers wanted to work on that. ‘Robot armies? We’re in!’”
The survey is expected to last five years, and will chart ten times as many galaxies as the most thorough previous survey, the Sloan Digital Sky Survey, which began collecting data in 2000.
A key question involves the role that dark energy has played over cosmic history. When the universe was very young, gravity is thought to have dominated over dark energy; but when the universe reached about half its current age, dark energy began to “take over.” Ever since, the push of dark energy has triumphed over the pull of gravity, causing the universe to not only expand but to accelerate. To understand why the switch-over happened, researchers need to have some idea of what the dark energy actually is. One guess is that it’s simply a property of space itself—what Einstein called a “cosmological constant.” If that’s the case, then, as the universe got bigger, gravity—which gets weaker as distances increase—played less and less of a role, allowing dark energy to become dominant.
But physicists are trying to keep an open mind. Dark energy may be the cosmological constant that Einstein described—or it may be something more exotic. If that’s the case, “that would be an exciting new discovery,” says Kyle Dawson, as astronomer at the University of Utah and a spokesperson for DESI. He wonders if dark energy could hint at “a new type of field, a new interaction, maybe a change to the way gravity works.”
Whatever dark energy turns out to be, data from DESI, which will provide our clearest picture yet of how structure formed in the early universe, is likely to play a key role in steering scientists toward the answer.
Because light travels at a finite speed, DESI, like all telescopic investigations, peers not only out into space but back in time. For example, because the Andromeda galaxy is two million light years away, scientists see it as it looked two million years ago. The instrument will allow astronomers to peer back through the eons, to see what the universe used to look like just a few billion years after the big bang. As homogenous as the early universe may have been, cosmologists know it wasn’t perfectly smooth; the evidence for this can be seen in the tiny “ripples” in the radiation left over from that era, known as the cosmic background radiation, which can be studied with radio telescopes. Those ripples may have originated in the universe’s earliest moments, when instead of space and time there was a kind of primordial quantum foam. Within this foam, subatomic particles may have been blinking in and out of existence, like the bubbles that pop in and out of existence in a boiling pot of water.
“We think those [ripples] were quantum fluctuations when the universe was a fraction of a second old,” says Schlegel. “By making these maps on very large scales, what we’re actually seeing are the imprints of those quantum fluctuations from the very early universe.”
Just as acorns grow to become mighty oaks, those quantum fluctuations became—over billions of years—the largest structures in the universe. Schlegel muses: “The question we’re trying to get at is, how did it all begin? What actually formed the universe?”