Image: Graviton
Fermilab
In this artist's conception, the blue ball represents a graviton escaping from the three spatial dimensions that define our universe, into a hard-to-visualize extradimensional space.
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Scientists may not have to go over to the dark side to explain the fate of the universe.

The theory that the accelerated expansion of the universe is caused by mysterious "dark energy" is being challenged by New York University physicist Georgi Dvali. He thinks there's just a gravity leak.

Scientists have known since the 1920s that the universe is expanding. In the late 1990s, they realized that it is expanding at an ever-increasing pace. At a loss to explain the stunning discovery, cosmologists blamed it on dark energy, a newly coined term to describe the mysterious antigravity force apparently pushing galaxies outward.

This repulsive, unknown force is believed to make up more than 70 percent of the mass-energy budget of the universe.

But the existence of dark energy is far from proven, and some researchers believe they and their colleagues simply don't understand gravity at larger scales. The gravitational pull between any two objects becomes less with distance. But in Dvali's view, it weakens more than standard theory predicts.

Dvali would modify the theory of gravity so that the universe becomes self-accelerating, eliminating the need for dark energy. He presented his work here earlier this month at the annual meeting of the American Association for the Advancement of Science.

Dvali borrows from string theory, which states that there are extra, hidden dimensions beyond the four we are familiar with: three directions and time. String theory suggests that gravitons  — hypothetical elementary particles transmitting gravitational forces — can escape to other dimensions. Dvali says this would cause "leaks" in gravity over cosmic proportions, reducing gravitational pull at larger distances more than expected.

"The gravitons behave like sound in a metal sheet," says Dvali. "Hitting the sheet with a hammer creates a sound wave that travels along its surface. But the sound propagation is not exactly two-dimensional as part of the energy is lost into the surrounding air. Near the hammer, the loss of energy is small, but further away, it's more significant."

The effect is to alter the space-time continuum, speeding up universal expansion.

"Virtual gravitons exploit every possible route between the objects," Dvali said, "and the leakage opens up a huge number of multidimensional detours, which brings about a change in the law of gravity."

The speeding up of the universe suggest that Einstein’s general theory of relativity, describing the interaction of space and matter, must be modified at large cosmic distances.

"It is this modification, and not dark energy, that is responsible for the accelerated expansion of the universe," Dvali concludes.

The idea might be testable.

Gravity leakage should create minor deviations in the motion of planets and moons. Astronauts on the Apollo 11 mission installed mirrors on the lunar surface. By shooting lasers at the mirrors, a reflected beam can be monitored from Earth to measure tiny orbital fluctuations. Dvali said deviations in the moon's path around Earth might reveal whether gravity is really leaking away.

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