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Mysterious dark energy demystified

Billions of years ago, the universe was crowded with tight-knit clusters of galaxies. Then, a party crasher got the upper hand. This mysterious force now called dark energy has since been expanding the universe at an increasing pace.
Image: Expansion of the universe
This illustration shows snapshots from a simulation by Volker Springel, representing the growth of cosmic structure when the universe was 900 million, 3.2 billion and 13.7 billion years old (now). The simulation shows how the universe has evolved from a smooth state to one containing a vast amount of structure. Gas is shown in these snapshots, where the yellow regions are stars and the brightest structures are galaxies and galaxy clusters.Volker Springel / MPE
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Billions of years ago, the universe was crowded with tight-knit clusters of galaxies. Then, a party crasher got the upper hand. This mysterious force now called dark energy has since been expanding the universe at an increasing pace.

New measurements of this accelerating expansion, which drives galaxies away from one another on large scales but so far shows no effects on small scales (such as within a galaxy), provide details about the nature of the unseen and unknown dark energy that is at work.

The results, announced today at a news conference organized by NASA, reveal a decrease in the mass of galaxy clusters in more recent times, which would be a consequence of this hastening and ripping force that some think could eventually tear apart even star systems, planets and eventually the very molecules we're made of.

"If there were any doubts 10 years after the initial discovery that the universe was speeding up, this should really dispel them," said Michael Turner of the University of Chicago's Department of Astronomy and Astrophysics, who was not involved in the current study.

In addition, the new results, which relied on the Chandra X-ray Observatory, suggest dark energy takes the form of what Einstein called the cosmological constant — a term in Einstein's general relativity that represents the possibility of empty space having a density and pressure associated with it.

If dark energy is indeed some kind of repulsive force that is linked with "nothing," and the density of dark energy stays the same over time, astrophysicists say the expansion of the universe will continue to speed up. So rather than galaxies mingling and merging, they will fly away from one another.

And billions of years from now, the scientists say, local superclusters of galaxies will also disintegrate and all other galaxies will ultimately disappear from the Milky Way's view.

"We don't really have a clue why the universe is speeding up. We have some ideas, but we really don't understand it," said Turner, who is credited with coming up with the term "dark energy." "And so having yet another method to study how that speed-up happened can only help us, can only make us more optimistic about eventually understanding what the dark energy is."

Invisible force

Image: Abell 85
This galaxy cluster, Abell 85, is located about 740 million light-years from Earth. The purple emission is multimillion-degree gas detected in X-rays by NASA's Chandra X-ray Observatory, and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. Abell 85 is one of 86 galaxy clusters observed by Chandra to trace how dark energy has affected the growth of these massive structures over the last 7 billion years.

Dark energy was discovered in 1998 by two teams of astronomers, who measured light coming from exploding stars called Type IA supernovae, known as "standard candles" for their consistent brightness. The striking result was that distant supernovae were dimmer (farther away) than they would be in a universe that was slowing down. The result suggested the expansion of the universe was accelerating. And the teams proposed something called dark energy could be driving this acceleration. This acceleration, it is thought, began about 5 billion years ago.

That was the first stand-alone evidence to support the idea of dark energy.

And whereas then the repulsive force could possibly be brushed off as a misguided product of wacky scientists' minds, more and more independent detections have solidified dark energy's existence.

Astronomers estimate now that out of the total mass-energy budget in the universe, about 74 to 76 percent is dark energy, 20 to 22 percent is dark matter and 4 percent or so is normal matter that makes stars, planets and everything we see. And they know that some "force" is causing galaxies to fly away from one another, operating like antigravity.

Rather than using Type IA supernovae, the new study is based on observations of clusters of galaxies at different time points in the history of the universe. Scientists say the new study marks the second stand-alone evidence for the existence of dark energy.

"This is surely the best job that anyone has been able to do so far in using clusters to measure how the universe has gotten clumpy over time," said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Massachusetts. Kirshner, who was not involved in the current study, was on one of the teams that first discovered dark energy.

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

When astronomers look farther across the cosmos, they are looking back in time. And in fact, the results show an increase in themass 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."