Dark energy is hardly science fiction, although no less intriguing and full of mystery for being real science.

"The universe is made mostly of dark matter and dark energy," says Saul Perlmutter, leader of the Supernova Cosmology Project headquartered at Berkeley Lab, "and we don't know what either of them is." He credits University of Chicago cosmologist Michael Turner with coining the phrase "dark energy" in an article they wrote together with Martin White of the University of Illinois for Physical Review Letters.

In the May 28 Science article, Perlmutter and Neta Bahcall, Jeremiah Ostriker, and Paul Steinhardt of Princeton use the concept of dark energy in discussing their graphic approach to understanding the past, present, and future status of the universe. The Cosmic Triangle is the authors' way of presenting the major questions cosmology must answer: "How much matter is in the universe? Is the expansion rate slowing down or speeding up? And, is the universe flat?"

The possible answers are values for three terms in an equation that describes the evolution of the universe according to the general theory of relativity. By plotting the best experimental observations and estimates within the triangle, scientists can make preliminary choices among competing models.

The mass density of the universe is estimated by deriving the ratio of visible light to mass in large systems such as clusters of galaxies, and in several other ways. For several decades the evidence has been building that mass density is low and that most of the matter in the universe is dark.

Changes in expansion rate are estimated by comparing the redshifts of distant galaxies with the apparent brightness of Type 1a supernovae found in them. These measurements suggest that the expansion of the universe is accelerating.

Curvature is estimated from measurements of the anisotropy (temperature fluctuation) of the cosmic microwave background radiation (CMB), a remnant of the Big Bang. Although uncertainty is large, current results suggest a flat universe.

The Cosmic Triangle eliminates some popular models, such as a high-density universe that is slowing down and will eventually recollapse, as well as a nearly empty universe with no dark energy and low mass. While the evidence from galactic clusters shows that mass density is low, supernova evidence for acceleration shows that dark energy must be abundant.

"These two legs of the Cosmic Triangle agree with the evidence from the CMB that the universe is flat," Perlmutter says, adding that "this is a remarkable agreement for these early days of empirical cosmology."

Thus the Cosmic Triangle suggests that the standard inflationary scenario is on the right track: one of its key predictions is a flat universe.

Various types of dark energy have been proposed, including a cosmic field associated with inflation; a different, low-energy field dubbed "quintessence"; and the cosmological constant, or vacuum energy of empty space. Unlike Einstein's famous fudge factor, the cosmological constant in its present incarnation doesn't delicately (and artificially) balance gravity in order to maintain a static universe; instead, it has "negative pressure" that causes expansion to accelerate.

"The term Cosmic Triangle sounds kind of New Agey," says Perlmutter, "but it's a good way to portray the quantities in these comparisons, and it's fun for people who like to plot the possibilities" -- an evolving task that, among other choices, will require finding an answer to "the most provocative and profound" issue of all, the nature of cosmic dark energy.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.