Dec. 16, 2008 -- The next time you find yourself beneath a dark night sky, look up and consider: it might've been a much more crowded place.
Were it not for dark energy, a mysterious repulsive force in the universe, our Milky Way galaxy would've long since crashed into its only neighbor, Andromeda. And instead of being lonely travelers in space, the galaxies would be nestled in the Virgo cluster, with hundreds more neighboring galaxies so close they'd be visible to the naked eye.
Now a team of researchers have analyzed 85 galactic clusters, measuring how dark energy has influenced their evolution. Over the past five billion years, they found, it has had a profound effect on the clusters' structure, significantly slowing down the rate at which they collapse.
"What we see can only be explained with a substantial amount of dark energy," said Alexey Vikhlinin of Harvard University, the team leader. The observations confirm scientists' long-held notion that the strange energy makes up two-thirds of the entire universe.
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The result is also a profound validation for the theory of general relativity, which predicts a consistent force throughout the universe -- known to Einstein as the "cosmological constant" -- that dominates gravity at scales of tens of millions of light-years or more. All 85 clusters appear to show the same effects from dark energy, so relativity emerges unscathed.
"The equations of general relativity have never been tested on scales of galaxy clusters," Alexey Vikhlinin said. "Gravity works as expected; that leaves little room for modification."
The team's finding appears to rule out a competing theory that dark energy is composed of subatomic particles that apart the fabric of space. If that were true, you'd expect to see some change in the particles' behavior over the last five billion years, said Robert Kirshner of Harvard University, who was not involved in the study.
Instead, dark energy remains constant. In areas with high densities of normal and dark matter (which together make up only about 30 percent of all the stuff in the universe), gravity dominates -- holding planets, stars and galaxies together.
But across huge distances, the slight effects of dark energy add up enough to push the universe apart faster and faster every day. The team's findings cement what cosmologists have long suspected about the fate of the universe.
"Expansion of the universe will keep speeding up," Kirshner said. As the Milky Way continues on its solitary trip, flying away from its nearest neighbors in Virgo, "it will eventually be kind of dark and cold and lonely," he said.
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