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Office of Communications Stanhope Hall, Princeton, New Jersey 08544-5264 Telephone 609-258-3601; Fax 609-258-1301

Contact: Justin Harmon, (609) 258-3601

Date: May 27, 1999

Scientists Conclude Anti-Gravity Force Is Accelerating Expansion of the Universe

PRINCETON, N.J. -- After reviewing recent astronomical observations, Princeton scientists have concluded that the evidence strongly supports the existence of a mysterious anti-gravity force that is causing the expansion of the universe to accelerate.

They presented their argument in a review article that will be published in the May 28 edition of the journal Science. The researchers are Neta Bahcall and Jeremiah Ostriker of the Department of Astrophysics and Paul Steinhardt of the Department of Physics, in collaboration with Saul Perlmutter of Berkeley National Laboratory.

Scientists have known since the 1920s that the universe is expanding, and they discovered in the last year that the expansion is likely to go on forever. In recent months, however, evidence has emerged to suggest that not only will the expansion continue, it will accelerate. The only way to account for such acceleration is the existence of a force to counteract the gravitational forces that would stabilize or shrink the universe.

The Princeton scientists have now bolstered that idea by reconciling three independent sets of data and showing that the data have a surprising degree of agreement. The data, some of which was generated at Princeton, have been used to answer three questions: How much matter is in the universe? Is the expansion rate slowing down or speeding up? And, is the universe flat? The Princeton scientists used a framework they call the "Cosmic Triangle," to relate the three questions and show for the first time how they merge into a unified picture of a universe that is flat, lightweight and expanding at an accelerating rate.

"It's a very exciting time because we are starting to reveal the status of the universe and it tells us something very unexpected," says Bahcall. It is the acceleration idea that is most surprising, she says. Bahcall cautions, however, that these conjectures must be confirmed by further improvements in the data, which are expected to come from a variety of sources over the next few years.

The expansion of the universe can be described in terms of a car coasting along a road as a result of a big push (the Big Bang). The mass of the universe, with the gravitational pull it exerts, is analogous to the friction and wind resistance that slow the car. In this analogy, there is so little resistance (gravitational tug) that the car never stops. The only way it could accelerate is if it were rolling downhill or if someone were depressing the gas pedal. The new force in the universe is like the downhill tug or an engine pushing the car.

"The evidence is now getting stronger that there really is a force in the universe that competes with gravity and causes repulsion instead of attraction," says Ostriker.

To account for this force, referred to as cosmic dark energy, scientists recently have revived a concept called the cosmological constant. In their paper, the Princeton scientists describe this cosmic dark energy as "a vacuum energy assigned to empty space itself, a form of energy with negative pressure." Einstein first introduced the cosmological constant in 1917, but later withdrew it, calling it the worst mistake of his life. Understanding the source and nature of this force poses deep new problems for physicists. "It's of very profound physical significance," says Ostriker.

The work to explain the source of this force already has begun. Steinhardt, a co-author, recently introduced a possible new force called quintessence, which may account for the dark energy.

Another implication of this new understanding of the universe is that it gives scientists a radically new picture of the future of the universe. It appears that the dark energy could eventually overwhelm the gravitational forces of matter. The density of matter in the universe would then become insignificant, so that the universe would approach an essentially uniform force field of dark energy. The researchers conclude that understanding dark energy, and hence the future of the universe, will be "one of the grand challenges of the millennium to come."


Lawrence Berkeley National Laboratory

Contacts: Saul Perlmutter, (510) 486-5203, Paul Preuss, (510) 486-6249,

May 25, 1999

Dark Energy Fills The Cosmos

BERKELEY, CA -- In an article titled "The Cosmic Triangle: Revealing the State of the Universe," which appears in the May 28, 1999, issue of the journal Science, a group of cosmologists and physicists from Princeton University and the Department of Energy's Lawrence Berkeley National Laboratory survey the wide range of evidence which, they write, "is forcing us to consider the possibility that some cosmic dark energy exists that opposes the self-attraction of matter and causes the expansion of the universe to accelerate."

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 mass 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. Visit our website at .