A multi-national group of scientists, including Harvard Professor of Astronomy Robert P. Kirshner, made a startling discovery this past year.

They found that the expansion of the universe, once thought to be slowing down, is actually accelerating.

"Our observations show that the universe is expanding faster today than yesterday," said Adam Reiss, an astronomer at the University of California at Berkeley, in The New York Times in March.

Reiss, who conducted graduate research at Harvard, is a member of the High-Z Supernova Team, a group that includes Kirshner and scientists from Chile, Australia and Washington.

These astronomers and astrophysicists have been measuring the velocities and positions of 16 supernovas to study the rate at which the universe is expanding.

"The idea is to look at the relationship between velocity and distance," Kirshner says. "If the universe was expanding faster in the past, then you'd measure high velocities when you looked at distant objects."

But when the scientists looked at the data, they instead found evidence that the stars were moving away from the earth at an even faster rate than they had in the past.

"The supernova are dimmer than you would expect if the universe was expanding at a constant rate," Kirshner says. "They are about 25 percent dimmer than you would expect."

When Einstein developed his theory of general relativity in 1916, Kirshner says his initial equations showed that the universe was either expanding or contracting. But like most scientists of his time, Einstein believed the size of the universe was constant.

To correct this mistake, Einstein added a term to his equations that he called the "cosmological constant." This constant altered his equations to show that the universe would expand for a while and then stay relatively the same size.

But when Edwin P. Hubble discovered in 1929 that the universe was expanding, Einstein was deeply embarrassed. For the remainder of his life, he referred to the cosmological constant as his "greatest blunder."

"We've known since 1929 that the universe is expanding," Kirshner says. "But we've also known there's gravity. And the question is, `How does gravity affect the cosmic expansion?'"

Until now, scientists believed that the force of gravity would eventually slow this expansion. But the recent evidence suggests that Einstein's original cosmological constant--although inserted into his equations for the wrong reasons--might represent a real force present in the universe.

"What [the cosmological constant] means is something like negative pressure," Kirshner says. "For an ordinary gas, the thing expands as the pressure goes down. But in this, the pressure would stay constant as it expands."

As the force represented by the cosmological constant pushes the edges of the universe farther apart, the gravitational attraction with the potential to bring the expansion to a standstill grows weaker, Kirshner says.

"As time passes, the cosmological constant gets more important," he says. "The universe will expand faster and faster."

However, the cosmological constant is only one way in which scientists are explaining the accelerating expansion of the universe.

"It could be the cosmological constant, or it could be something more complex but more related to particle physics," Kirshner says.

At a conference held two weeks ago at the Fermi Lab, Kirshner says theoretical physicists presented their theory of "quintessence," and expansionary force that would change over time.

"What was intriguing to me was how ready to accept this idea the theoretical community was," Kirshner says. "These people have very nimble minds, very flexible minds."