October 8, 2001

f you drop your watch at the edge of the universe, it will fall up. Or so Philip Mannheim predicts. His theory of gravity, if correct, will extend Albert Einstein's, and make cosmologists' worst headaches disappear.

A small group of physicists is taking notice of Mannheim's theory, following the recent discovery by astronomers that the universe is expanding at an accelerating rate. "It was a surprise to everyone but me," says Mannheim, a professor of physics, who has been predicting since 1987 that gravity can repel as well as attract. If gravity does behave this way, it would explain the universe's accelerating expansion.

The theory does not mean the man in the street will experience antigravity and spontaneously float off into the sky. Mannheim postulates that gravity becomes repulsive at large distances, the extent of, say, a galaxy.

This apparently counterintuitive idea is, in a broader context, intuitive, says Mannheim. Physicists have discovered that there are only four types of forces in the universe: the strong, the weak, the electromagnetic, and gravity. Except for gravity, all of them can both attract and repel. What if we're wrong, he asks, and gravity can repel as well?

Mannheim started with a discarded idea called conformal gravity, proposed by German mathematician Hermann Weyl soon after Einstein proposed general relativity. Mannheim has taken it further: "I'm pushing it to a degree that no one else was willing to push it."

Like Einstein's theory, Mannheim's theory uses the equivalence principle. This is the idea that gravity can be described as curved space-time. "Conformal gravity contains all the same geometric features as standard gravity," he says, "and differs only in the explicit amount of curvature which any given gravitational source is then to generate."

His theory's "field equations," which describe the curvature of space, are different from Einstein's. They are more complicated. One physicist approached Mannheim after a presentation he made and said, although he disagreed with the theory, he was amazed that Mannheim could navigate the mathematics.

Gravity in Mannheim's theory behaves like the familiar Newtonian gravity on a small scale, as it does in Einstein's theory. It even behaves the same as general relativity up to the scale of the solar system. But after that, he says, it works better.

Mannheim's theory offers nothing less than a way to make sense of the universe. Of established cosmological theory he says, "it works beautifully, but it makes no sense, as it implies we'd be living in a ridiculous universe."

Cosmology is the field of physics that explains structures and occurrences on the largest scales, like clusters of galaxies.

Brad Schaefer, a research fellow in astronomy at the University of Texas and supporter of Mannheim's work, calls cosmology's current solutions to its problems "magical tooth fairies."

Modern cosmology and the idea of an expanding universe began with Edwin Hubble in the 1920s. Hubble expected to see a uniformly expanding universe, where every object is moving away from its neighbor at the same speed. And, as far as Hubble could see with the telescopes of the time, uniform expansion was what he observed.

A few years ago, two international teams of physicists set out to look deeper into the universe and see how uniform its expansion was.

Unlike Hubble, they expected to see a universe whose expansion was slowing down. Instead, they saw the universe accelerating.

This puzzle was explained almost immediately with an old idea called the cosmological constant, the idea that space is filled with an energy that pushes the stars apart. Tooth fairy number one.

Physicists have also shown that galaxies are spinning faster than would be expected, given the observed amount of matter in the universe.

Astronomers conjectured that there is a large amount of dark matter in the universe, matter invisible to astronomers because it does not give off light. Tooth fairy number two.

Mannheim says the cosmological constant and dark matter "are the two central problems of modern cosmology." An accelerating universe and galaxies that spin fast are, however, natural consequences of Mannheim's theory that gravity can repel.

"If you had to make an Occam's razor argument," Schaefer says, "you would say Mannheim is right." Occam's razor is a principle that favors the simplest scientific theory.

Schaefer says Mannheim may be right, and if he is, he will alter the field of cosmology.

Mannheim's theory makes testable predictions, as any scientific theory must. One of these is that the universe's acceleration increases towards its edges, unlike the standard theory. With astronomy's increasing ability to look deep into space, Mannheim and Schaefer say, we should know in a few years if Mannheim is right.

Even though he has a promising theory, Mannheim faces some obstacles. "There's a huge inertia," says Schaefer, "more or less sociological rather than computational, behind Einstein's theory."

Even if Mannheim's theory does yield better predictions than the standard theory, it will not be accepted immediately.

Mannheim is also racing against the clock, says Schaefer, trying to predict phenomena before they occur. Astro-nomers may see telltale signs in the next few years, and his case will be stronger if his theory has already predicted them.

Like Albert Einstein before him, Mannheim is working in obscurity, trying to revolutionize the accepted view of the universe. He is eagerly awaiting advances in astronomy and a deeper look into space.

Brent C. Evans