Mysterious dark energy demystified

Galactic growth
A team led by Alexey Vikhlinin of the Smithsonian Astrophysical Observatory in Cambridge, Mass., used the Chandra X-ray Observatory to measure the hot gas in dozens of galaxy clusters, which are the largest collapsed objects in the universe.

Some of these clusters are relatively nearby and others are more than halfway across the universe. Basically, the team was looking at X-rays emitted from this hot gas as it fell into areas chock-full of dark matter, or the mysterious material thought to act as scaffolding onto which galaxies mature. The X-rays can be converted into mass for a given cluster at a given point in time (depending on the age of the cluster).

"It's like there's a tug-of-war going on between the dark matter trying to slow things down and clump things and the dark energy trying to speed things up and eventually making it hard for the galaxies or the dark matter to cluster," Kirshner told Space.com.

When astronomers look farther across the cosmos, they are looking back in time. And in fact, the results show an increase in the mass of the galaxy clusters further back in time, which supports the idea that dark energy started to win out in the tug of war at some point in the universe's history. Astronomers are not certain on the timing of the change from an expanding universe to one whose expansion is speeding up.

With dark energy taking over, it would be more difficult for objects such as galaxies to get together and form clusters as space is being stretched. So astronomers would expect to see a slowdown of the growth of galaxy clusters in a dark-energy-dominated universe.

"This result could be described as 'arrested development of the universe'," Vikhlinin said. "Whatever is forcing the expansion of the universe to speed up is also forcing its development to slow down."

Future of dark energy
While the new galaxy-cluster results strengthen the case for an accelerating universe, scientists have a long trek before cracking the case of what dark energy is.

"I don't see us solving this in two or three years. We're going to have to bring to bear lots of different techniques to figure out what dark energy is," Turner said during a telephone interview. "This is a very big puzzle. This may be the most profound problem in all of science."

Scientists can continue to study the clustering of galaxies over time. Further study could show dark energy doesn't take the form of the cosmological constant. For instance, another idea is that general relativity falls apart on larger scales.

And perhaps dark energy is stirring up trouble in another way, not just speeding up the expansion of the universe. "One of the areas where we think we might get really lucky is that maybe there will be some other manifestation [of dark energy], but we don't have any yet," Turner said. "And there's so much of this stuff in the universe it's hard to believe there isn't another manifestation. But right now the only thing we know that dark energy is doing is causing the universe to speed up."

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