WASHINGTON – Mysterious dark energy, which likely causes the universe to keep expanding, seems to have another effect: It prevents the biggest clusters of galaxies from getting too fat.
Astronomers used X-rays to study the formation of galactic clusters billions of years ago. Their research supports the hard-to-fathom concept of dark energy as a potent force that governs the growth of the universe.
It also means Albert Einstein's century-old theory of general relativity passes another crucial, but not conclusive, real-world test.
Scientist Alexey Vikhlinin used NASA's Earth-orbiting Chandra X-ray Observatory to find that dark energy acts as a force that keeps clusters of galaxies — around 1,000 bright galaxies or more — from essentially overeating and getting too big.
Without dark energy, these giant clusters would keep forming, getting denser and bigger because of gravity. But in the last few billion years that hasn't happened, said Vikhlinin, a scientist at the Harvard Smithsonian Astrophysical Observatory in Cambridge, Mass.
He looked at when 86 such clusters formed and saw the slowdown in girth, starting about 5.5 billion years ago. The logical cause: dark energy.
"They're put on a diet, a permanent diet," Vikhlinin said in a telephone interview.
Three outside experts praised the new study, which will be in February's Astrophysical Journal, as important to understanding a concept that is counterintuitive but crucial to figuring out the evolution of the universe.
The history of the universe has been a battle between "the two dark titans, dark matter and dark energy," said University of Chicago astrophysicist Michael Turner.
"This is the first time you've seen the effect of dark energy taking over," he said.
Dark energy is still a relatively new idea that came out a decade ago when measurements of star supernovae showed the universe was surprisingly expanding. One way to explain the expansion — building on an idea Einstein first broached and called his biggest mistake — is that another force in the universe is essentially fighting gravity. That force is dark energy.
"It's much more important and abundant in the evolution of the universe than the atoms that make us up," said Princeton theoretical astrophysicist David Spergal.
Clusters of galaxies are a good way to look at the effects of dark energy because they are "cosmic frogs" that are sensitive to small changes, Turner said. Our galaxy, the Milky Way, is not in a cluster but we are not too far from the Virgo cluster.
As the universe expands thanks to dark energy, clusters such as Virgo and other galaxies will get farther and farther way.
And in tens of billions of years, they will eventually disappear from sight, Vikhlinin said.
"Sometime down the road, we'll have nothing to observe," he said.