Anthony Aguirre: His theory is kicking up dust. Photo by Rose Lincoln.

A student at Harvard University has thrown cold dust on the hottest science story of 1998.

Last year, two teams of scientists announced that the universe is expanding at a faster rate than anyone had believed.

"In a triumph for astronomers' ability to look deep into the past, the independent teams came to their conclusions by observing far-off exploding stars called supernovae that turn out to be surprisingly dim, revealing an acceleration that has swept them to unexpectedly large distances from Earth," wrote Science, the august journal of the American Association for the Advancement of Science. "We name their findings, which transform our view of the universe and pose fundamental new questions for physics, as 'Breakthrough of the Year for 1998.' "

But Anthony Aguirre, a 25-year-old graduate student, thinks those teams may have had dust in their eyes. On Feb. 10, The Astrophysical Journal, published by the American Astronomical Society, will publish Aguirre's idea that the exploding stars look so dim simply because space contains a kind of celestial pollution. Specifically, he postulates the presence of minute needles of carbon dust floating around between the galaxies.

"I argue that bright, dusty galaxies, where many new stars are being made, eject needle-like dust that can account for the dimming of these supernovae," Aguirre says.

The favored explanation for the acceleration of the universe is an antigravity force that pushes matter apart faster that gravity pulls it together. "If my theory is right," Aguirre needles the pros, "such a force is not necessary."

Aguirre came up with the idea of a dust screen on his own, according to David Layzer, the Donald H. Menzel Research Professor of Astrophysics who advises him. "The whole astronomical community has bought into the acceleration idea, so I think it will make a stir," he adds. "I hope it does."

The student works in the same building as Robert Kirshner, a Harvard professor of astronomy and a member of one of the teams credited with the acceleration discovery. Aguirre discussed with Kirshner the possibility that dust accounted for the dimness, and Kirshner encouraged him to pursue it.

"It's good for science to have people on both sides of all issues," Kirshner comments.

Peter Garnavich, a researcher at the Harvard College Observatory who made the supernovae sightings with Kirshner and others, says that Aguirre's theory "is an interesting idea. We have been concerned that some kind of strange dust might make the supernovae look dimmer. Another explanation is that these supernovae may have been intrinsically dimmer when they first exploded billions of years ago." (The supernovae are so far away, it has taken billions of years for their light to reach Earth.)

Garnavich admits that Aguirre's careful work puts the dust theory on a sounder scientific basis than before, and, if true, eliminates the need for postulating an antigravity force.

Einstein's Blunder

To explain universal acceleration, astronomers dusted off an old idea proposed by Albert Einstein. In 1917, he introduced reverse gravity as a fudge factor to hold the universe in place. Einstein did not believe that it was expanding or collapsing. The pull of gravity that every star and planet exerts on every other celestial body would tend to make the universe collapse on itself, so a countering repulsive force was needed for balance.

By 1929, however, astronomer Edwin Hubble conclusively proved that every star and galaxy is moving away from every other star and galaxy. Most scientists accept the theory that this expansion results from the origin of the universe in a tremendous explosion called "The Big Bang," which began space and time some 15 billion years ago.

Certain types of stars, called white dwarfs, give off a fixed and predictable amount of light when they become unstable and explode. Such supernovae serve as so-called "standard candles." When looking out at an expanding universe, you can't tell if a brilliant light comes from a large star far away or a small, bright one that is closer. Standard candles provide a reference; since their inherent brightness is known, the dimmer they appear the farther away they are.

Last year, two teams of astronomers studied supernovae estimated to be between 5 billion and 8 billion years old, that is, light has been traveling from them toward Earth since the universe was about a third or half of its present age. Even at that distance, their dimness startled the researchers. They believe it means that these exploding stars lie 10-to-15 percent farther away than expected. The quickly accepted reason: the expansion of the universe has been accelerating.

To account for this speed-up in expansion, most astronomers and astrophysicists favor bringing back Einstein's fudge factor. But keep in mind that no real evidence exists for reverse gravity; Einstein just made it up so that his equations would support the idea of a static universe. In a twist he could never have imagined, antigravity has returned to support the idea of a universe expanding forever at an increasing rate.

Aguirre thinks the idea of dust is much simpler, but he too is short on evidence. Some interstellar dust in needle-like form has been collected from meteorites, although it is not the predominant type of dust in these celestial rocks. Also, when carbon, which is abundant in the universe, solidifies from gas in laboratory experiments, it takes the form of needles.

This interstellar debris would not be like ordinary house dust that you find under a bed or sofa. The little gray whiskers would be too small to see without a magnifier. "If you took lead from a mechanical pencil, shattered it, then shrunk it 10,000 times, that's what the dust would look like," Aguirre says.

"You would expect such dust to be patchy," Kirshner points out. "If so, supernovae seen through the patches would look dimmer; those seen through gaps in the dust would be brighter. We don't see that range in brightness."

"The dust would be more or less uniformly distributed," Aguirre replies. "There is bound to be some patchiness, but the clumps would not be big enough to block some supernovae and not others."

There are other objections to the needles idea, but there also is a way to settle the controversy. Any acceleration will decrease the farther you go back in time (or out in space), because the effect would be smallest at the beginning of the universe.

"As part of our continuing research, we are now trying to measure supernovae that are farther away to see if we can detect a decrease in the acceleration," Kirshner says. "That should settle the question."

"It's in everyone's interest to come up with the correct explanation," Aguirre says. "If dust is responsible for the dimming effect, it's going to come out sooner or later; you can't keep it a secret."