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New clues for dark energy

darkness one.jpgPosted on behalf of Ashley Yeager

The same unexplained force—dark energy—that causes the universe to fly apart at an accelerating pace is also stifling the growth of galaxy clusters, new research shows. These clusters, cloudlike swarms that are the largest accumulations of matter in the universe, grew rapidly during the first 10 billion years of cosmic time but, due to the dark energy, they can no longer continue to grow.

“These are the largest clusters ever to be seen,” Alexey Vikhlinin of the Smithsonian Astrophysical Observatory, in Cambridge, Mass., said in a NASA press briefing yesterday. “It is a good thing we are observing them now because future cosmologists won’t even be able to see them because of the accelerating universe.”

Vikhlinin and his colleagues studied the clusters, the most massive collapsed objects in the universe, using NASA’s Chandra X-ray Observatory. By comparing the X-ray observations of distant and nearby swarms of galaxies, the team reported that they have found new, independent evidence for the existence of dark energy and that it has maintained the same density over time. The new research, to appear in the Astrophysical Journal, also suggests that dark energy closely resembles Einstein’s cosmological constant.

The new research shows, as past studies have, that there is energy associated with empty space and that the energy is constant with time. This is what Einstein calculated says astrophysicist David Spergel of Princeton University in New Jersey, who was not involved in the new study. The problem is astronomers don’t know at the quantum level what is causing this energy, so “we’re struggling to understand the cosmos. We don’t quite know why 72% universe comes in exotic forms”, he says.

He notes that the new research brings researchers closer to knowing whether Einstein’s theory of general relativity is enough to explain dark energy or whether the laws break down on a large scale and need to be revised.

What’s unique about Vikhlinin’s study, Spergel says, is that the data is consistent with two other methods that already detected the presence of dark energy. The X-ray method is distinct from research dependent on supernovas or the relic radiation left of over from the Big Bang and gives astronomers a method to probe the unknown force.

In essence, “geometry tells matter how move, and matter tells geometry how to curve in the cosmos”, astrophysicist William Forman also of Smithsonian Astrophysical Observatory and co-author of the new study said in the NASA briefing. Supernova and the cosmic microwave background is matter telling geometry how to curve; clusters provide an example of geometry telling matter how to move, he explained.

These new X-ray results therefore offer a crucial independent test of dark energy and help constrain what dark energy is, Spergel says. They provide a clear method for future cosmological observations to improve astronomers’ understanding of dark energy and they can finally help us to understand this ‘central question’ of the nature of the universe, he adds.

Image top: galaxy cluster Abell 85 on Chandra X-ray.


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