May the Dark Energy Be With You

Going, going ... The universe as we see it—that starry expanse in the night sky—may never get better than it is now, at least in a visual sense: dark energy, the mysterious springy stuff that is causing the cosmic expansion to accelerate, is also squelching the growth of the largest entities in the universe, clusters of galaxies. From here on out, those clusters will grow no more than a ballerina on a diet.

The discovery, to be reported in two separate papers in the Feb. 10 issue of The Astrophysical Journal, also gives astronomers the best clues yet about the nature of dark energy, which was discovered in 1998, and what it means for the fate of the universe.

Using the orbiting Chandra X-Ray Observatory, astronomers led by Alexey Vikhlinin of the Smithsonian Astrophysical Observatory in Cambridge, Mass., measured how dark energy has affected the growth of galaxy clusters—the largest entities in the universe—over billions of years, which required spying some that lie halfway across the cosmos.

What they found is that the mass of the clusters increases over time, with the oldest clusters also being the most massive. Galactic clusters grew like Topsy for the first 10 billion or so years after the big bang, the scientists infer, but for the last 5 billion have been bulking up no more than an anorectic ballerina. In other words, dark energy was a wimp for the first two thirds of the universe’s existence, but strengthened into a bully, keeping galactic clusters in check, in the most recent one third.

The anemic growth of galactic clusters caused by dark energy makes sense: just as it would be harder for any two partygoers to find themselves up close and personal if the gathering were held in some sci-fi expanding room, so it is harder for cosmic objects to come together gravitationally if the universe is being stretched, as it is under the influence of dark energy. It’s sort of like “arrested development of the universe,” said Vikhlinin. “Whatever is forcing the expansion of the universe to speed up is also forcing its development to slow down.”

That suggests that dark energy, a sort of repulsive or anti-gravity, is the energy of empty space rather than the result of a more general energy field or the result of something weird happening with the shape of spacetime (which would mean Einstein’s theory of general relativity breaks down at very large scales). Einstein first gave this repulsive force the name cosmological constant, though he later said it was the worst mistake of his scientific life. “Putting all of this data together gives us the strongest evidence yet that dark energy is the cosmological constant, or in other words, that ‘nothing weighs something’,” said Vikhlinin. “A lot more testing is needed, but so far Einstein’s theory is looking as good as ever.”

If dark energy is indeed the manifestation of the energy of empty space, then the expansion of the universe will continue to accelerate. "The expansion of the universe will continue forever," said Vikhlinin (sorry, Woody Allen), but slow down enough "that it will not result in a big rip, tearing the structure apart." Phew. Still, in about 100 billion years, we won’t be able to see any other galaxies from our own Milky Way. Get out there with a telescope while there’s still lots to see.