GREENBELT, MD. — In their quest to answer the biggest questions about the universe, scientists are turning to a 1,700-pound satellite taking shape here at NASA's Goddard Space Flight Center.
The $88-million spacecraft is designed to make the most precise all-sky maps yet of tiny changes in the afterglow from the fireball that, scientists say, formed the universe. The changes, researchers say, constitute a cosmic code that will help them answer the ultimate question: Will the universe expand forever or slow, contract, and collapse in the "big crunch"?
Known as cosmic microwave background radiation, the afterglow first picked up as a "hiss" in 1964 carries details on key features of the cosmos, such as the nature and density of its matter, its evolution, and its expansion rate.
Competing notions about how the universe evolved and what its future holds depend on those details, explains Charles Bennett, the lead scientist on the satellite project, called the Microwave Anisotropy Probe (MAP).
"Basically, Joe Blow comes up with a model of cosmology. He says: I like this kind of matter and this much of it. He can tell you exactly what fluctuations you should see in the cosmic background radiation," Dr. Bennett says. "This means that the cosmic background radiation becomes an incredibly powerful tool" for testing theories. "That's why people are so excited" about the mission, adds George Smoot III, an astrophysicist at the Lawrence Berkeley National Laboratory in Berkeley, Calif. "Either we will understand the universe, or we'll have to come up with new ideas" if the satellite's data fail to match theories' predictions.
The cosmic microwave background presents a picture of the universe when it was only 300,000 years old. It has provided some of the most convincing evidence yet that the universe exploded outward from an extremely dense, hot spot in space some 15 billion years ago. In 1964, Bell Laboratory researchers Arno Penzias and Robert Wilson discovered the radiation while testing microwave relay antennas designed for long-distance telephone service. They found the radiation to be uniform in all directions.
"At first it was great," Bennett recalls. "But then it started to be a bit of a problem," because the uniform temperature implied a uniform distribution of matter. To anyone looking through a mountaintop telescope, matter appeared to be anything but uniformly spread. Galaxies gathered in groups, then formed clusters, which themselves were clumped into superclusters.
"At some level, there had to be some temperature differences in the early universe that were the seeds of those structures," Bennett continues, with slightly cooler spots representing regions of greater density. In 1989, NASA launched the Cosmic Background Explorer (COBE) spacecraft, which spent four years mapping the radiation more closely. It found the predicted temperature differences, but not in enough detail to allow cosmologists to test their various notions about the universe. COBE fueled expectations that the answers were there to be found.
"We got to be confident about that because of COBE," says Dr. Smoot, the lead scientist for one of COBE's three detectors. MAP, he adds, "represents a big step forward." The craft, a collaboration between researchers at Goddard, Princeton University, the University of Chicago, and the University of California at Los Angeles, is designed to measure the temperature difference between two tightly spaced regions of sky.
Scheduled for launch in 2000, MAP will be looking for temperature variations as small as one-millionth of a degree in the background radiation. And it will be able to spot features half the diameter of the moon, instead of 15 times the size of the moon, as was the case with COBE.
MAP's prime value, the researchers say, is in verifying models and deriving most of the universe's key attributes directly from the background radiation. But as keenly sensitive as they are, MAP's instruments won't be able to nail all of the 15 parameters astrophysicists would like to measure. That effort, Smoot says, will await the European Space Agency's Planck mission, scheduled for launch in 2005.