Princeton Weekly Bulletin June 21, 1999
Antigravity force, methane dwarfs
By Steven Schultz
Princeton astrophysicists have been involved in recent advances in the study of the universe, including assessement of evidence of acclerating expansion and discovery of a new type of celestial body.
Antigravity force
After reviewing recent astronomical observations, Princeton researchers have concluded that there is strong evidence for the existence of a mysterious antigravity force that is causing the expansion of the universe to accelerate.
Astrophysics professors Neta Bahcall and Jeremiah Ostriker, and physics professor Paul Steinhardt, in collaboration with Saul Perlmutter of Berkeley National Lab, presented their argument in a review article in the May 28 edition of Science.
Since the 1920s scientists have known that the universe is expanding. In the last year they discovered that the expansion is likely to go on forever, and in recent months they have begun to suggest that it is accelerating.
The Princeton scientists have bolstered that idea by reconciling three independent sets of data and showing that the data have a surprising degree of agreement. The data have been used to answer three questions: How much matter is in the universe? Is the expansion rate slowing down or speeding up? Is the universe flat? Using a framework they call the "Cosmic Triangle," the researchers 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're starting to reveal the status of the universe, and it tells us something very unexpected," says Bahcall. She cautions 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.
Nevertheless, "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.
This force, called cosmic dark energy, is described in the Science paper as "a vacuum energy assigned to empty space itself, a form of energy with negative pressure." To account for it, cosmologists have revived a concept called the cosmological constant, introduced by Einstein in 1917 but later rejected. (Einstein called it the worst mistake of his life). Steinhardt has introduced a possible new force called quintessence, which may be the source of the dark energy.
One implication of this new understanding of the universe is 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."
New kind of celestial body
Princeton astronomers have contributed to the discovery of an entirely new kind of celestial body called methane dwarfs, which are part way between planets and stars.
This past spring graduate student Xiaohui Fan and Assistant Professor of Astrophysical Sciences Michael Strauss identified one of these objects as part of the Sloan Digital Sky Survey. Using the Sky Survey's telescope in Apache Point, NM, they discovered a faint object that was unusually red. Analysis showed that it had much the same makeup as an object called Gliese 229B, the only previously known example of such a cool substellar body. The Princeton discovery was followed by a similar find by astronomers at Johns Hopkins University.
One characteristic of the newly discovered objects is that they contain a lot of methane, which appears in planets like Jupiter but not in stars. Astronomers have dubbed the new objects "methane dwarfs" and estimate that they are 10 to 70 times the mass of Jupiter.
The new objects are the second major discovery to come out of the Sky Survey in its first year, even though the project is still in a trial run. In December Fan and Strauss announced they had discovered the oldest and most distant quasars in the universe.
The Sloan Digital Sky Survey is a joint project of Princeton, the University of Chicago, Fermilab, the Institute for Advanced Study, Japan Participation Group, Johns Hopkins, Max Plank Institute for Astronomy, US Naval Observatory and University of Washington. Funding has been provided by the Alfred P. Sloan Foundation, Sky Survey member institutions, National Science Foundation, NASA and US Department of Energy.