By Joel Achenbach
Washington Post Staff Writer
Wednesday, December 17, 2008
New observations offer evidence that astronomers are not simply imagining that there is a mysterious essence they call "dark energy" that is causing the universe to expand at an ever-accelerating pace.
In justifying the idea, astronomers came up with more evidence that Albert Einstein's theory of gravity works even at the grandest, most cosmic scale. And in confirming, with a new technique, an earlier measurement of dark energy, they found that there is little reason to fear that dark energy will eventually cause a "big rip" that will tear apart everything we see and blow up the smithereens.
In findings announced yesterday, astronomers at the Harvard-Smithsonian Center for Astrophysics said they measured the effects of dark energy using NASA's Chandra X-ray Observatory, and the results echo the earlier observations of supernovas made by the Hubble Space Telescope.
Chandra observed dozens of galactic clusters -- aggregations of galaxies that constitute the largest observed structures in the universe. Some of the clusters were about 5.5 billion light-years away, meaning their radiation (in the form of X-rays) was 5.5 billion years old by the time it reached Chandra. Other clusters were relatively nearby. This gave a past-and-present view of galactic clusters and showed that they are not forming as easily as they did 5 1/2 billion years ago.
Whatever dark energy is, it seems to be inhibiting the clusters' growth.
"What we find is that the growth of structure has slowed down during the last 5 1/2 billion years, and this is unmistakably a signature of dark energy," said Alexey Vikhlinin, lead author of one of the new reports, to be published in the Astrophysical Journal.
In addition to offering a new line of sight on this mysterious phenomenon, the research supports the interpretation of dark energy as a "cosmological constant," a force that permeates empty space and, bizarrely, has precisely the opposite effect of gravity.
At the human scale, dark energy is imperceptibly weak. But even the weakest of forces can add up to something powerful when the scale becomes cosmic, and all that vast, empty space between the galaxies is thrown into the mix.
"Even nothing, even empty space, weighs something, and because in our universe we've got a lot of nothing, it has a major effect on our evolution and causes space itself to accelerate," said David Spergel, an astrophysicist at Princeton University.
What remains unclear is what dark energy is, exactly.
"We've discovered this incredible dark energy; we don't understand what the hell it is," said Lawrence M. Krauss, a physicist at Arizona State University.
The universe as we know it is believed to have sparked into existence about 13.7 billion years ago. From that initial big bang onward, it expanded, but until about 5 billion years ago gravity dominated the show, gradually slowing the expansion. Then dark energy became the more powerful actor, and the expansion began to accelerate as surely as if someone had put pedal to the metal.
All manner of theories, some involving hidden dimensions and exotic particles, have been tossed around to account for dark energy. One possible solution would be to revise, or even toss out, Einstein's general theory of relativity. The theory holds that gravity is a function of the geometry of four-dimensional "space-time." Matter, as physicists like to say, tells space-time how to curve, and space-time tells matter how to move. The theory is so well established that GPS navigation devices have to account for the curvature of space-time to remain accurate.
But although general relativity is used all the time in modern cosmology, there have been suggestions that it breaks down at the cosmic scale.
"It's never been proved right on the scale of the observable universe," Spergel said.
The most famous confirmation of the theory came in 1919 when, during an eclipse, astronomers measured the way the sun bent the light from a star behind it. If the classical laws of physics developed by Sir Isaac Newton were correct, the sun's gravity should have deflected the starlight to a small but predictable degree. Einstein, with his tale of curved space, predicted twice as much deflection. When astronomers showed that Einstein was right, he became an instant global icon.
The new findings, by bolstering general relativity once again, suggest it will be a while before Einstein's universe is revised the way he revised the Newtonian universe.
Hovering over the new research is the minor matter of the fate of the universe. If the universe continues to expand at an accelerating rate, Krauss has estimated, in about 100 billion to 1 trillion years, almost all the galaxies we see will be so far away they will vanish from sight. It will be a much darker universe.
But what will not happen, apparently, is the cosmic apocalypse that scientists call the big rip. That would occur if dark energy was strong enough to rend asunder all the stars, planets, moons, rocks, dust and even atomic nuclei, as well as any and all innocent bystanders. The new measurement shows dark energy at a level that will permit our immediate surroundings and galactic environment to survive.
"The accelerated expansion will proceed forever," Vikhlinin said, "but will probably not result in a big rip."