Maps sharpen view of cosmic radiation.

Two new sky maps are giving astronomers their first direct glimpse of individual structures -- gravitational ripples -- in the primordial universe.

One of the new maps is a more accurate version of an earlier one that drew worldwide accolades when researchers presented it nearly two years ago. Based on the first year of data collected by NASA's Cosmic Background Explorer (COBE) satellite, this map showed tiny temperature fluctuations in the microwave background radiation believed left over from the explosive birth of the universe (SN: 5/2/92, p.292).

The pattern of hot and cold spots represents gravitational ripples in an otherwise smooth distribution of matter and energy in the infant universe. The ripples may have caused lumps to form there, ultimately creating clusters of galaxies. Though the overall pattern of hot and cold spots was statistically significant, the data contained about as much noise as signal. Thus, scientists could not identify any single feature as real.

Last week, at a meeting of the American Astronomical Society in Arlington, Va., the COBE team unveiled a new sky map based on an additional year of data. This distribution of hot and cold spots in the relic radiation has at least 1.4 times the amount of signal as noise, and some regions have about twice as much signal as noise, notes COBE researcher Charles L. Bennett of NASA's Goddard Space Flight Center in Greenbelt, Md. The added sensitivity means that some parts of the map probably show true structures in the microwave background, he says.

The new COBE-derived picture confirms that the tiny temperature variations were "not some fluke in the first year of data," Bennett says. Both maps show hot and cold spots that differ by 30 millionths of a kelvin from the 2.73-kelvin microwave background, but the new map halves the margin of error in this measurement. The team has two more years of COBE data to analyze; NASA ended the craft's mission last December.

Astronomers using radiotelescopes at Tenerife in the Canary Islands to study the microwave background over a small strip of sky also report actual structures in the relic radiation. Rodney D. Davies of the University of Manchester in England says the coldest and hottest spots in his team's map, which covers 10 percent of the sky, have about seven times as much signal as noise.

The group finds variations of about 42 millionths of a kelvin over patches of sky about half the angular size of that scanned by COBE. This agrees with published, first-year COBE maps, the team reports in the Jan. 27 NATURE.

Both the COBE and Tenerife maps reveal fluctuations that occurred a tiny fraction of a second after the birth of the universe. Davies says that as his group enlarges its sky coverage, it can begin to search for the particular features in these primordial fluctuations that are predicted by particular cosmological models. He cites, for example, a theory known as cosmic strings; it envisions sharp, filamentary structures in the background radiation.

In a commentary accompanying the NATURE article, cosmologist Joseph Silk of the University of California, Berkeley, notes that both maps weakly suggest that the curvature of the universe is such that the cosmos may expand forever rather than collapse in upon itself.

"Perhaps the curvature of the universe is imprinted on the sky," he writes. However, COBE researcher Edward L. Wright of the University of California, Los Angeles, says that without the ability to survey the microwave sky from another, distant galaxy, the hint of an open universe in the current maps may remain just that.