Through a glass, darkly. Microwave data observed by the WMAP satellite (upper left), supernovae (lower left), and galaxy clusters (above) all reveal a universe dominated by dark energy.

Breakthrough of the Year: Microwave Mapping

A lonely, distant satellite has captured the very essence of the universe. In February, the Wilkinson Microwave Anisotropy Probe (WMAP) produced an image of the infant cosmos, of all of creation when it was less than 400,000 years old. The brightly colored picture marks a turning point in the field of cosmology: Along with a handful of other observations revealed this year, it ends a decades-long argument about the nature of the universe and confirms that our cosmos is much, much stranger than we ever imagined.

Half a decade ago, astronomers studying supernovae were surprised to find that the universe was expanding ever faster rather than decelerating, as general relativity--and common sense--led astrophysicists to believe. This was the first sign of a mysterious "dark energy," an unknown force that counteracts the effects of gravity and flings galaxies away from each other. Teams of astronomers across the world rushed to test the existence of this irresistible force in independent ways. That quest ended this year.

Important confirmation came from the WMAP satellite, which took the most detailed picture ever of the cosmic microwave background (CMB; ScienceNOW, 11 February). The CMB is the most ancient light in the universe, the radiation that streamed from the newborn universe when it was still a glowing ball of plasma. This faint microwave glow surrounds us like a distant wall of fire. The writing on the wall--tiny fluctuations in the temperature (and other properties) of the ancient light--reveals what the universe is made of.

The answer was disturbing and comforting at the same time. WMAP confirmed an incredibly strange picture of the universe. The universe is only 4% ordinary matter, the stuff of stars and trees and people. Another 23% is exotic matter: dark mass that astrophysicists believe is made up of an as-yet-undetected particle. And the remainder, 73%, is dark energy. Another project, called the Sloan Digital Sky Survey (SDSS), came to the same conclusion (ScienceNOW, 28 October).

WMAP data also answered a lot of long-standing questions about the basic properties of the universe. A year ago, cosmologists would likely have said that the universe is between 12 billion and 15 billion years old. Now they say it's 13.7 billion years, plus or minus a few hundred thousand. It's expanding at 71 kilometers per second per megaparsec. And its "shape" is slate flat.

This year, thanks to WMAP, the SDSS data, and new supernova observations, cosmologists finally know for sure that the universe is made of dark matter and was blown apart by dark energy. They're starting to ask new questions, too. It is, perhaps, a sign that scientists will finally begin to understand the beginning.

Related sites
Extended coverage of Science's Breakthrough of the Year
Wilkinson Microwave Anisotropy Probe
Sloan Digital Sky Survey
More about the cosmic microwave background
Dark Energy in the Expanding Universe--a tutorial