News from
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."