If at first you don’t succeed … add a fudge factor.

That’s what Albert Einstein did.

Einstein’s general theory of relativity predicts that the universe should collapse under the attraction of gravity. Since Einstein thought the universe should be neither expanding nor contracting, he tweaked his equations to include another term. This term, called the cosmological constant, was supposed to balance the pull of gravity. Years later, when he received news that the universe was expanding, Einstein declared his constant to be “the greatest blunder of my life.”

Einstein may have dropped the cosmological constant from his equations, but scientists have resurrected the idea. Astrophysicists have recently found evidence of a repulsive force that is causing the universe to accelerate outward. The cause of this repulsive force is a mysterious form of energy dubbed “dark energy.” If the density of dark energy is not changing with time, then the cosmological constant may be the best explanation for the acceleration of the universe.

Last week, astrophysicist Steve Allen and colleagues reported compelling new evidence of dark energy, which suggests that its density does not vary with time.

“What we found is that the universe is accelerating. Here we have direct evidence of dark energy,” said Allen, an astrophysicist at the University of Cambridge. “And it behaves much like the cosmological constant.”

Instead of studying radiation from the Big Bang or the remnants of exploding stars, Allen and co-authors took a novel approach and studied galaxy clusters. Their approach allows scientists to get a better handle on the density of dark energy and whether it changes with time.

“It has allowed us a completely new line of attack on this problem,” said Allen. “It’s a very powerful new technique … [and] it agrees so well with previous methods.”

Using NASA’s Chandra X-ray Observatory, Allen and co-authors examined 26 large galaxy clusters between one and eight billion light-years away. They studied the hot gas within them to estimate the ratio of normal matter to dark matter, an invisible form of matter. Combining this information with models of the universe’s evolution, they calculated how far away the galaxy clusters should be.

The group found that the galaxy clusters were much farther away than they should have been, indicating the universe has been accelerating outward. Moreover, they found the density of the dark energy does not appear to be changing.

If these results are confirmed, could this be the cosmological constant that Einstein proposed almost a century ago?

Maybe, says Allen.

“Dark energy does exist and does [appear to] take the form of the cosmological constant,” said Allen. “It’s very intriguing that dark energy still fits the data for the simplest model” — the cosmological constant.

But Allen cautions that dark energy may not be constant. Some theories suggest that the force from dark energy is weakening, which eventually would allow gravity to trounce dark energy and suck the universe in toward a cosmic collapse known as “the big crunch.” Others suggest that it is growing stronger and will cause the universe to fly apart in a “big rip.”

Whether the universe will die in fiery inferno or peter away into desolate cosmic loneliness, no one yet knows. One thing is certain: What Einstein called his biggest blunder may turn out to be one of his greatest legacies.