Big bang of citations

Researchers flourish by placing their scientific writing in journals. When they publish, they often build their investigations on the backs of other useful studies, citing them as references or footnotes. In some ways, having one's research mentioned often in others' published work is the highest form of professional flattery. So, when a far-reaching, decade-long study of the essential stuff of the universe took the top three spots on a list of the most frequently cited science articles of 2011—the first time in history that one research program has dominated such listings—the scientific world took notice.

The list of citations, gathered by ScienceWatch, a website run by the British media firm Thomson Reuters, centered on scores of papers created by the Wilkinson Microwave Anisotropy Probe. Better known by its acronym, WMAP is a telescope mounted on a NASA spacecraft launched in 2001, and it is designed to answer questions about the universe's beginnings. WMAP is headed by Charles L. Bennett, a professor of physics and astronomy at the Krieger School of Arts and Sciences. Now winding down as its team finishes a final data analysis, the project's publications also topped the ScienceWatch most-cited list in 2003, 2007, and 2009. "But we nabbed the first three spots this time around, which is amazing given the size of the field we work in," says Bennett. "Most research journals are in the biosciences. The medical research community is enormous in comparison with astrophysics and cosmology. We have reasons to be proud."

Simon Mitton, a Thomson Reuters consultant, concurs. "The WMAP papers are far and away the most successful series that we have ever featured in ScienceWatch," he says. "Nothing else comes close." Mitton adds that sky surveys such as WMAP maintain their high profiles by annually releasing new data that prove useful to astrophysicists. "WMAP is one of a small number of programs that are dedicated to finding the fundamental cosmological parameters. Everyone working in observational cosmology quotes the WMAP papers," Mitton says.

WMAP's investigations of cosmic microwave radiation, a lingering remnant of the universe's period of rapid expansion followed by cooling, popularly known as the Big Bang, have reaped more and more interest as new findings have been published. Since WMAP citations first were charted in 2003, they have grown sixfold, from around 1,100 at the start to 6,500 in 2011. "These are annual numbers, not cumulative ones," Bennett points out. "The level of interest goes up every year. Anyone who does research on the universe needs context, and WMAP provides that." And, he adds, interest likely will continue to climb as more researchers use WMAP's findings to investigate topics that include universal geometry and the expansion of the universe.

When the craft carrying the WMAP telescope was launched, Bennett and his crew wanted to measure the fluctuations and polarization of cosmic microwave radiation. But Bennett is astounded by how much more knowledge WMAP accumulated. For example, the project's team calculated within a 1 percent margin of error that the universe is 13.75 billion years old. It also quantified observations of other scientists that the universe is mostly composed of an antigravity called dark energy, while only 4.6 percent of it is composed of atoms that make up Earth's living things, as well as other planets and stars. "We have a strong standard of cosmology now," Bennett says. "It's not at all obvious that we can understand the entire universe, but we do understand an enormous amount because of the cosmic microwave background. We're very lucky."

Bennett now has turned his attention to CLASS, or the Cosmology Large Angular Scale Surveyor, another telescope-based project. This time, Bennett—who is building much of the apparatus along with Johns Hopkins undergraduate and graduate students —and Tobias Marriage, an assistant professor of physics and astronomy, will try to gather evidence of gravitational waves and their effect on inflation, the working theory of how the universe expanded immediately after it began.

Despite the success associated with WMAP, Bennett isn't exactly sentimental about its end. "Some people have come up and said, 'Isn't this a sad time?' Well no, actually. I'll miss working with this team. But we set out to get some information and ended up with much, much more. It's like we've won the lacrosse game and someone comes over and says, 'You just won the game. Aren't you sad it's over?'"