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.
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 imagesabout 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 spaceand, indeed in timeto within less than a billion years after the Big Bang,” he says. “That’s something like 5 percent of the Universe’s present age.”
Scientists use the term “Big Bang” to describe the beginning of everythingtime, space, and matter13.7 billion years ago. Theorists suggest that at that first instant, all the fundamental particles of the Universe were squeezed into a space so small that it had no inherent size at all. There were no quasars yet, no galaxies, no structurejust the potential for it all, brewing a hot soup of particles. So by mapping quasars and galaxies as they appeared close to the time of the Big Bangand by close scientists mean within 900 million yearsthey get nearer to understanding how that original moment developed into the Universe as it is today.
If the Big Bang is the most important theoretical idea in cosmology, inflation is the second. “It’s a deceptively simple idea,” explains Turner, a cosmologist at the University of Chicago and the Assistant Director for Mathematical and Physical Sciences at the National Science Foundation. “Inflation says that when the Universe was very young it went through a terrific growth spurt.” At .000000000000000000000000000000000001 of a second after that original moment of no size, the Universe rapidly expanded to have breadth and depth. Scientists speculate that the structure of the current-day Universe was likely laid down at this key fraction of a second, and that it was overall smooth and uniform. Still, small fluctuations“lumps in the pudding”were present. During the growth spurt, these lumps blew up to enormous sizes. Today, the Universe continues to expand, although not at a rate that even comes close to that of inflation. All the blowup has resulted in the current far-flung arrangement of galaxies and clusters. “The distribution of galaxies today are relics of inflation,” explains Turner.
Sloan’s data has the power to address at least three of the Big Questions (and quite a few more that can’t fit into one essay): the structure of the early Universe, the structure of the current Universe, and the journey in between. When Sloan eyes the farthest quasars, it gets an idea of the Universe near the time of its origin. When it views nearer galaxies and the stars of the Milky Way, it maps the expansive, structured arrangement now. By mapping the positions of space objects in between, time-wise and space-wise, Sloan can help explain how and where objects have clustered as the Universe evolved. This directly tests the idea of inflation.
Scientists have already started wading through Sloan’s voluminous data to further tease apart the Big Questions. What about the events of that first moment, the Big Bang? Or the events that preceded it? “If you had asked that question 10 years ago, people would say, ‘Oh, that's not science, we can't address that,’” says Turner. “Now, we're starting to. It’s one of the big challenges for the next generation of scientists.” In the next 5 to 10 years, expect a battery of large-scale, large-survey telescopes, inspired by Sloan, to arrive on the scene. They’ll employ even more sensitive detectors, shoot an even wider field of view, cover even more sky. They’ll be able to find older, more distant quasars and understand Universe-wide patterns with more precision. As science progresses, they may also be able to answer more Big Questionsmaybe even ones we have not yet thought to ask.
Large Synoptic Survey Telescope
A future telescope survey project with an even more massive field of view.
AAS: An Ancient Universe
A guide for teachers, students, and the public about how astronomers understand the scale of cosmic time.
More About This Resource...
Supplement a study of astronomy with a classroom activity drawn from this Science Bulletin essay.
- Ask students what they know about the history of the universe. What is cosmic time? How is it measured?
- Have them read the essay (either online or a printed copy).
- As a class or in small groups, have students watch The Known Universe video. Discuss what clues have been found to some of the Big Questions.