'Dark energy' stunts galaxy growth, astrophysicists find

Causes universe to expand at ever-faster rate

Astrophysicists shed light yesterday on one of the deepest mysteries in science, the perplexing "dark energy" that makes up nearly three-quarters of our universe.

Harvard-Smithsonian researchers announced that they have measured the effects of dark energy, a mysterious antigravity force, documenting how it stunts the growth of galaxy clusters as it causes the universe to expand at an ever-faster rate.

"This may well be called the arrested development of the universe," said Alexey Vikhlinin, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge who led the work.

While the discovery has no effect on life on earth, at least not anytime soon, it provides a missing piece of the puzzle in our understanding of the universe, building on key discoveries from a decade ago.

"It's been 10 years since we discovered the universe is speeding up and not slowing down, and even those of us who are professional cosmologists have yet to get our heads around that," said Michael S. Turner, an astrophysicist at the University of Chicago.

"This is a big, big mystery. I call it the most profound mystery in science," said Turner, who coined the term "dark energy." The finding announced yesterday, he said, "puts a new arrow in our quiver" in trying to understand that "weird stuff out there."

Astrophysicists from the Harvard-Smithsonian Center for Astrophysics used NASA's Chandra X-ray Observatory to measure the masses of clusters of galaxies. Those clusters depend on gravity to grow, pulling in matter from millions of light-years away. But around five billion years ago, the growth of those clusters slowed.

Such observations, while they do little to illuminate the nature of dark energy itself, are critical, researchers said, because they back up the previous results.

"If they had found something different, we'd all be wringing our hands and wondering if there is a big problem," said astronomer Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics, who was a member of one of the two teams of scientists that rocked the cosmological world 10 years ago with their discovery that the university was expanding faster and faster. "This shows our first inklings were more or less right."

The significance of understanding dark energy may not be evident in a world where most people think of the universe as the things they can see. But it makes up 72 percent of the universe, whereas normal matter makes up less than 5 percent. More important, its antigravity effects have the power to eventually shape the universe's destiny.

"It's much more abundant and important to the universe's evolution than the atoms that make us up," said David Spergel , an astrophysicist at Princeton University.

Another notable benefit of this week's discovery is that it leaves Albert Einstein's theories intact.

To his theory of general relativity, Einstein added a fudge factor called the cosmological constant. While Einstein later considered this a mistake, the cosmological constant has regained popularity today as a leading explanation for dark energy.

Still, 13 billion years since the Big Bang, there are more questions than answers.

"We don't know what ideas are right," said Spergel. "This is kind of a brave new world."

Carolyn Y. Johnson can be reached at cjohnson@globe.com