For months cosmologists have been eagerly awaiting measurements of the edge of the universe gathered by an antarctic balloon. A map published in the 27 April Nature gives the most detailed glimpse yet of the primordial universe, revealing the shape of the cosmos and the distribution of matter shortly after the big bang. The data support the prevalent view that our universe is "flat," but they cast doubt on other key assumptions about the early universe.
The 800,000-cubic-meter balloon carried a set of sensitive microwave detectors called BOOMERANG. In late 1998, the balloon swooped around the pole in 10 days, carried by air currents back to its launch site. BOOMERANG probed a large swatch of sky for fluctuations in the cosmic microwave background (CMB), a constant electromagnetic hiss that bombards Earth from all directions. This distant hiss is radiation leftover from the big bang.
The CMB can reveal the shape of the universe. According to relativity theory, the four-dimensional "sheet" of space and time that we live on can be curved. For years, astrophysicists have been figuring out the ways in which curved space-time might distort the images of distant objects, in hopes that astronomers would be able to tell which sort of space we live in--spherelike, saddle-shaped, or neither. BOOMERANG and other CMB experiments show that the fluctuations are not distorted as they would be in curved space.
Although astronomers expected this evidence for a flat universe, another bit of BOOMERANG data took them by surprise. Theory suggests that the ripples in the microwave background ought to exist on many different scales, each contributing a "peak" to the data. BOOMERANG saw a peak corresponding to roughly 1-degree-sized fluctuations and should have spotted a half-degree peak as well. It didn't. "That is extremely interesting," says University of Pennsylvania physicist Max Tegmark. "The mischievous side of me wanted that to happen."
The missing peak means that astrophysicists must tweak their models of how the universe formed. Precisely how they do this will depend heavily upon future results. Much of the BOOMERANG data has yet to be processed. And more will come after NASA launches a microwave-sensing satellite, MAP, this fall. New CMB peaks would tell scientists just how much invisible dark matter and ordinary matter there is in the universe and would help nail down details of the universe's first 300,000 years. "To me," Tegmark says, "this experiment really signifies the beginning of a new era."