Chubby Galaxy Cluster Suggests Dark Energy Stronger Long Ago

This map of the dark matter around the galaxy cluster Abell 1689 shows structures of mass, outlined with colored contour lines within the central area. Galaxies are marked with pink squares.

Astronomers have created a new, incredibly detailed map of dark matter by using the Hubble Space Telescope to peer through a huge cluster of galaxies as if it were a cosmic magnifying glass.

Though invisible, dark matter makes its presence known through its gravitational tug on normal stuff. Scientists now calculate that dark matter could make up 80 percent of all the matter in the universe.

The new dark matter map could reveal secrets not just about dark matter, but about its equally enigmatic sibling, dark energy. This is the name given to the perplexing force that is pulling against gravity, causing the universe to balloon in size ever more rapidly. [New map of dark matter]

A curved path

The dark matter map was created with observations from the Hubble  telescope of a large galaxy cluster called Abell 1689, located 2.2 billion light-years from Earth. This cluster is famous as a stunning example of gravitational lensing – a phenomenon predicted by Einstein that happens when massive objects warp the space-time around them, causing even light to travel on a bent path when it passes by.

When astronomers look at Abell 1689, they can see distorted pictures of the galaxies that lie beyond it in our line of sight: As those galaxies' light travels from them to us, it passes through Abell 1689 and is bent and magnified.

By studying this so-called lensing effect, scientists can deduce the mass that is causing the warping.

Astronomer Dan Coe of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Edward Fuselier of the United States Military Academy at West Point teamed up to apply a new mathematical formulation to Hubble observations of Abell 1689. The result is the most accurate, detailed calculation so far of the cluster's mass distribution, including the mass that can't be accounted for by the visible matter – meaning, the dark matter.

"The lensed images are like a big puzzle," Coe said. "Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions."

Dark energy

The new dark matter map reveals that Abell 1689 is denser at its center than physical models would predict.

"Abell 1689 appears to have been well fed at birth from the high density of dark matter surrounding it," Coe told  "This has given it a chubby belly which it has carried through its adult life to appear as we observe it today."

Hints of similar heavy centers have been found in other large galaxy clusters recently, he added.

"What it might be saying is that clusters may have formed earlier than simulations show," Coe said. "My work lends more support to this idea we've had from these other analyses."

Because the universe has been continually expanding since its birth, it was once much denser than it is now. The extra heavy cores of galaxy clusters suggest they were born during these early stages when such dense conglomerations of matter were around.

But if galaxy clusters did get an early start in forming, that presents a quandary, because scientists would expect to see a lot more of them around today.

That indicates that perhaps the force of dark energy was stronger in the young universe than scientists have thought. Because dark energy works against gravity, pulling matter apart, its force would have suppressed the formation of galaxy clusters, and could have counteracted the head start clusters had in forming, scientists think.

Wider sample

To test this hypothesis, the researchers want to investigate even more galaxy clusters. They plan to begin a project called CLASH (the Cluster Lensing and Supernova survey with Hubble) to look at 25 other clusters over the next three years.

"We're going to see whether your regular-Joe clusters also have these chubby bellies," Coe said.

If the dense centers found in Abell 1689 and other large clusters hold for the larger sample, it will lend support to the idea that dark energy was stronger earlier in the universe. If not, then it could mean that these few mega clusters just happen to have been special.

All in all, scientists may be getting closer to solving the riddles of dark matter and dark energy than ever before. For example, detectors on Earth and in space are currently searching for elusive signals from dark matter that could reveal the nature of this befuddling matter.

"I think we have a little bit better idea of what dark matter might be and we have a lot of different ways we're looking for it," Coe said. "Within the next five years or so we hope to maybe have a clear signal from one or more of these experiments."

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Clara Moskowitz
Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She has written for both and Live Science.