The New Riddle of the Universe

Astronomers have long known that the universe is expanding, and, until now, they assumed that gravity was slowing the enlargement down. But recent observations of distant exploding stars have shown that instead the expansion may be accelerating— and this has cosmologists scratching their heads in wonder. Writing in Scientific American (Jan. 1999), a half-dozen astronomers and cosmologists ponder the astonishing development. Craig J. Hogan, Robert P. Kirshner, and Nicholas B. Suntzeff, astronomers at the University of Washington, Harvard University, and Cerro Tololo Inter-American Observatory in La Serena, Chile, respectively, belong to one of the two teams that have tracked a few score of the supernovae (exploding stars), in galaxies hundreds of millions of light-years away. Such blasts occur when a dead star becomes a natural thermonuclear bomb; these took place four to seven billion years ago. The big surprise was that the supernovae were "fainter than expected," and therefore farther away, the astronomers say. Though the difference in brightness was slight—only 25 percent less than forecast—it was "enough to call longstanding cosmological theories into question."

"Taken at face value," the three astronomers write, "our observations appear to require that expansion [of the universe] is actually accelerating with time." But that cannot happen if the cosmos is made up exclusively of normal matter, because "gravity must steadily slow the expansion." It could happen, however, if all the empty space in the universe were filled with an unknown form of matter or energy whose gravity repelled rather than attracted.

That weird idea runs counter to the big bang theory, as well as the inflation theory that shores it up. The big bang theory, which holds that the universe has been expanding for about 12 billion years, assumes that matter is spread out evenly and is governed by only one force, gravity. To correct for certain shortcomings in the theory, cosmologists in the early 1980s adopted inflation theory, which, borrowing ideas from particle physics, holds that there was an early period of very rapid expansion after the big bang.

But a decade ago, notes physicist Lawrence M. Krauss, of Case Western Reserve University, Cleveland, it became clear that when the visible contents of the universe were added up, the collective gravitational force was not enough to bring the outward impulse from the big bang into eventual balance. That balance would be necessary if the universe were to avoid expanding forever or, alternatively, collapse in a fiery "big crunch." So cosmologists concluded that invisible matter ("dark matter") must exist in space, exerting sufficient gravitational force to make up the deficit.

But if, as astronomers’ recent observations of exploding stars suggest, the expansion of the universe is speeding up, then even the unseen matter is not enough. A kooky form of antigravity matter or energy apparently must exist, or else the universe will keep expanding forever. Physicists Martin A. Bucher, of the University of Cambridge, and David N. Spergel, of Princeton University, do not rule out the latter possibility, and contend that inflation theory can be modified to take an eternally expanding universe into account. Krauss, however, believes that the other alternative—that the universe is "filled with an energy of unknown origin"—is more likely. In either case, he observes, "a dramatic new understanding of physics" is now required.