Sloan Digital Sky Survey: Mapping the Universe
The Big Questions
In science, there are questions, and there are Questions. Astronomers, in particular, want to know the structure of the Universe in detail. What is its arrangement today, and what did it look like at its birth? And how did we get from then to now? Theoreticians are particularly eager to know what exactly happened at the Universe’s first moment … and, of course, what came before that.
Galaxy M100 and surroundings, as viewed by the Sloan Digital Sky Survey telescope.
Even the sharpest astrophysicists and cosmologists don’t know all the answers to the Big Questions, but they’re making headway. Veteran theoretician Michael Turner describes how 20 years ago, he and his peers could only speculate about such bold inquiries, since the technology didn’t exist to test them. Today, scientists have instruments like the Sloan telescope, which, with its unprecedentedly wide field of view, is taking the largest-ever survey of the Universe. This cosmological catalog, called the Sloan Digital Sky Survey, is one significant tool to test scientists’ educated guesses at the Big Answers. As Turner reasons, “If there weren’t big questions to answer, who would want to spend years of their life mapping the sky?”
The Here and Now
The question that’s nearest to being answered is: What is the structure of the Universe today? We know that matter is not cast evenly across the Universe. Billions of stars congregate in groups called galaxies, which themselves congregate in sheets and clusters. Every congregation is separated from the others by voids filled with sparse atoms of gas as well as a mysterious, invisible “dark matter.”
“Sloan set out to map the Universe’s structure,” says Michael Strauss, a Princeton University astronomer and the deputy project scientist for the survey. “Before Sloan began, we had a pretty good idea of the distribution of galaxies in an anecdotal way. But we set out to measure things very precisely.” Since 1998, the Sloan telescope has been pointed skyward on a mountaintop at Apache Point, New Mexico. When it completes operations in 2008, it will have identified the positions of hundreds of millions of objects in one contiguous quarter of the night sky.
Most of the sharp-edged points of light that Sloan records are local stars: those less than 100,000 light-years away in our own galaxy. Bright but fuzzy objects are galaxies, which are found billions of light-years away. Some objects Sloan detects are both sharp-edged and distant:. They’re quasars, high-energy galactic cores so bright that they outshine their resident stars and gas. Quasars are enormous distances from Earth, on the order of 11, 12, even 13 billion light-years away. Yet quasars are so luminous that they’re plainly visible on Sloan’s digital images—about 50 per square degree of space, the equivalent area of four full moons. Indeed, quasars are among the most luminous and most distant objects known in the Universe, and Sloan has spotted the most distant ones yet.
The There and Then
Because of their incredible distances, quasars are portals into the past. The light from the quasars traveled across a space so vast that it took 11, 12, 13 billion light-years to arrive at the Sloan telescope. The quasars themselves may be long gone, perhaps “turned off” or developed into other types of objects, but their light is still in transit. So when Strauss and other researchers map the distribution of the farthest quasars visible, they get a snapshot of the Universe as it looked 13 billion years ago. “We're getting a baby picture of quasars 13 billion years after they actually were babies. Sloan is probing back in space—and, indeed in time—to within less than a billion years after the Big Bang,” he says. “That’s something like 5 percent of the Universe’s present age.”
