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Testing the Dark Side

Cosmologists propose a new way to probe dark energy.
Supernova 1987A Rings
Three rings of glowing gas encircle the site of supernova 1987A.
Christopher Burrows, ESA / STScI / NASA
May 26, 2005
While it might sound like something out of the fantasy world of Star Wars, dark energy is a mysterious force astronomers grapple with as they try to explain the ever-increasing speed with which the universe is expanding. The concept of antigravity flinging galaxies apart at an accelerating pace is puzzling theoreticians as well. Everyone expected the universe's expansion rate to be slowing down due to the gravitational pull clusters of galaxies have on each other. Now, the race is on to explain what this dark energy is and to understand its effect on the cosmos.

Cosmologists from Princeton University think they have found a way to understand this cosmic acceleration and identify dark energy's nature. They combined observational surveys of distant supernovae and weak gravitational lensing around galaxy clusters with experimental simulations. They hope the tests will reveal whether expansion is caused by some unknown dark-energy form or if it is a signature of a new gravity theory — a discovery that would mark the breakdown of Einstein's general theory of relativity at large scales.

"The accelerating expansion of the universe constitutes one of the most intriguing and challenging problems in astrophysics. Our research work is focused on constraining different possible causes of this acceleration," says Ishak-Boushaki, principal investigator and research associate at Princeton. The work was presented last week at the annual meeting of the Canadian Astronomical Society in Montreal.

The Big Bang and the associated expansion of the universe became the prevailing theory in the late 1920s. But the fact that the universe is flying apart at faster and faster speeds is a recent discovery. Astronomers proposed dark energy as the reason in 1998, when observations of far-flung supernovae in distant galaxies showed they were dimmer than expected. This meant these exploding stars are farther away than originally thought. The best way to explain these observations is for the universe to have a form of gravitationally repulsive energy that accelerates expansion. More than 70 percent of the universe is thought to be comprised of this strange dark energy, while only 5 percent is ordinary matter like stars and planets.

Eighty years ago, Einstein included just such a repulsive force — called the cosmological constant — in his theory of general relativity, to balance the static universe. He later abandoned the idea when the universe was found to be expanding. Now, the debate over the cosmological constant has been revived with the discovery of dark energy.

As dark energy pushes the universe outward on a grand scale, gravity still dominates locally to keep clusters of galaxies together. But while dark energy affects the universe's expansion, it also appears to suppress the clumping of galaxies, countering the effects of gravity. "The way the galaxies are distorted by gravitational lensing depends on the makeup of the universe," adds Ishak-Boushaki. "So this presence of dark energy will make both the universe different and make the bending of light different."

By monitoring the growth rate of galaxy clusters and measuring their lensing effects, the Princeton team hopes to improve its tests and get a better insight on the underlying nature of the universe's expansion.

However, the question remains: Is dark energy a real phenomenon? "We don't know," comments co-investigator David Spergel. "It could be a whole new form of energy or the observational signature of the failure of Einstein's theory of general relativity. Either way, its existence will have a profound impact on our understanding of space and time. Our goal is to be able to distinguish the two cases."
Andrew Fazekas is an astronomy columnist and lecturer based in Montreal, Canada.
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