Dinoj Surendran, Microsoft Research, and Mark Subbarao, Adler
Planetarium/KICP/University of Chicago
Located 9,200 feet above sea level, atop the Apache Point Observatory in Sunspot, New Mexico, the Sloan Digital Sky Survey telescope cannot match the incredibly sharp vision of the Hubble Space Telescope, which orbits above Earth’s blurring atmosphere. And, at a modest 2.5 meters (8 feet) across, the Sloan telescope’s main mirror cannot see the incredibly dim objects that the 10-meter (33-foot) Keck telescopes in Hawaii can. What the Sloan telescope does have in spades is a voracious appetite for sky—an appetite that is producing some of the most amazing discoveries in astronomy.
With its giant set of light-sensitive imaging sensors, the Sloan telescope has a field of view so wide it can image 36 full moons’ worth of sky at once (Hubble, in contrast, is limited to a view less than one-tenth of a moon across). Night after night it scans vast swaths of the heavens and downloads its observations into a 73-terabyte (and growing) digital database that covers almost half the night sky as seen from Apache Point. Swept up in the Sloan’s relentless gaze are stars, galaxies, supernovas, nebulas, and more—over 350 million celestial objects in total—adding up to the most complete census of the universe ever conducted.
The result of all this activity is the Sloan Digital Sky Survey (SDSS), originally established “to determine the large-scale structure of the universe,” says Richard Kron, a University of Chicago astrophysicist and a Sloan survey director. “We wanted to map out the galaxies that form clusters and the clusters that form superclusters.” Achieving this goal required a huge step up from the 1950s-era Palomar Sky Survey, whose photographic plates have guided astronomers to celestial curiosities for decades. “We knew that to make real progress, we needed a hundred times more data,” Kron says. The Sloan survey captures the sky in full color rather than just through red and blue filters, produces images twice as sharp as Palomar’s, and detects objects one-tenth the brightness of those detectable by its predecessor. The Sloan also introduced two huge innovations. First, it delivers all the data in digital form, so the images are easy to categorize and study electronically, even from halfway around the world. Second, it does not just capture sky images; it also gauges the distance to many of the objects—a million galaxies and 100,000 quasars so far—that pass through its field of view, providing a unique three-dimensional perspective on deep space.
The Sloan telescope went into operation in 2000 and has since yielded two landmark surveys, known as SDSS-I and SDSS-II. Last August astronomers affiliated with the project gathered in Chicago to review results from SDSS-II and to prepare for a third survey—SDSS-III, of course—which recently began and will continue until 2014.
Taken together, the Sloan results lay out one of the most astonishing stories in science: The visible universe is merely the foam atop a much grander cosmic sea. The vast majority of what is out there is more dynamic and complicated and just plain weirder than the tiny fraction we knew. Only now are we starting to see the universe as it truly is.
The Universe Is Doubly Dark
By plotting the precise positions of more than 46,000 galaxies in a volume of space approximately 5 billion light-years in diameter, the Sloan Digital Sky Survey has cast some light on the biggest mystery faced by cosmologists today: the nature of dark energy.
The Sloan telescope is housed within a box-shaped wind baffle.
Image courtesy of Fermilab Visual Media Services
Astronomers figured out some years ago that most of the matter of the universe is not in the form of the stuff that makes up stars and planets, you and me. Most of it (83 percent, by the latest estimates) is so-called dark matter, an unknown something—perhaps an invisible, as yet undetected elementary particle. Then the story got even more confusing. In 1998, while plans for the SDSS were being finalized, two teams of observers reported that the cosmos is pervaded by another unseen entity, a force dubbed dark energy. This energy acts like an antigravity force that pushes galaxies apart, making the universe expand faster and faster over time.
The evidence for dark energy came from studies of a kind of exploding star known as a Type 1a supernova. The wonderful characteristic of these stars is that they all seem to explode exactly the same way, producing flare-ups with a predictable luminosity. Knowing the true brightness of a Type 1a supernova allows astronomers to measure its distance by noting how dim it appears in our sky. Those measurements, in turn, can be interpreted to show how the expansion of the universe has changed over time. Examining a number of extremely distant supernovas, researchers inferred the presence of dark energy and managed to measure its abundance. It appears to make up a staggering 70 percent of the universe’s content. (All matter, light and dark, adds up to just 30 percent.)
The findings left researchers with all kinds of new questions. What is dark energy, and how does it act? Was it stronger or weaker in the early years of the universe? Does it vary in intensity from place to place across the cosmos? Scientists also wanted an independent way to confirm the dark energy story told by the Type 1a supernovas. It seemed possible, for example, that these supernovas were not truly uniform, which would invalidate the underlying assumption of the original studies.
