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Nailing down dark energy

— Ten years after supernovae provided the first evidence that the expansion of the universe is accelerating, a survey of more than 10,000 galaxies has provided independent confirmation that the cosmic speed-up factor known as dark energy is for real.

Scientists say the resolution of the data isn't yet good enough to determine exactly what's behind dark energy, but they say a finer-scale survey could tell whether it's an exotic characteristic of the space-time continuum – or whether it's just that everything we know about gravity is wrong.

Klaus Dolag / ESO
This computer simulation shows a large-scale
intergalactic structure in our universe's cosmic
web. The color scale represents mass density,
and yellow lines describe the intensity and
direction of individual galaxies. The lines map
how gravity and dark energy are balanced.
Click on the image for a bigger view.

The survey, conducted using the European Southern Observatory's Very Large Telescope in Chile, tracked the distribution and motions of galaxies out to a distance of about 7 billion light-years - yielding a time-lapse view of the cosmos going back to when it was half its current age. Astronomers made more than 13,000 spectral observations to figure out the relative motions of galaxies based on their redshifts.

The results, published in this week's issue of the journal Nature, indicate that at the largest scales, our universe is indeed expanding at a faster rate today than it was billions of years ago. That's pretty much what the supernova readings suggested. But the technique promises to yield additional data about the local motions of galaxies - gravitationally governed motions that pull nearby galaxies into cosmic web patterns (or is that cosmic taffy?).

A close analysis of those local motions could reveal new secrets about the nature of dark energy, said Olivier LeFevre of the Laboratoire d'Astrophysique de Marseille, one of the 51 scientists behind the research. In today's news release from the ESO, he said the local motions "introduce small but significant distortions in the reconstructed maps of the universe."

"We have shown that measuring this distortion at different epochs of the universe's history is a way to test the nature of dark energy," LeFevre said.

The most straightforward explanation is that dark energy is merely an extra ingredient in the space-time continuum, a cosmic "fudge factor" that Albert Einstein at first built into the equations for relativity, then later eliminated. He called the extra factor, known as the cosmological constant, the "greatest blunder of my life" - but if it turns out the cosmological constant actually exists, then his real blunder was disavowing the idea.

Maps show the distribution of galaxies in the Very
Large Telescope's survey between 1.3 billion and 8.5
billion light-years away, with the data divided into
three "cones." Colors indicate density of galaxies,
going from green (less dense) to red and blue (more
dense). Click on the image for a larger view.

The latest results are consistent with what would be predicted by the cosmological constant, but because the margins of error are so large, they are also consistent with other possible explanations for dark energy - for example, that some quality called quintessence that varies over time, or that gravity is leaking into extra dimensions, or that some completely different theory of gravity is needed.

Today, astronomers can't determine which explanation is the right one. But astronomers should be able to nail down which scenario is at work if the measurements made by the Very Large Telescope could be extended over a volume of the cosmos about 10 times larger than the current survey, said Luigi Guzzo of Italy's Osservatorio Astronomico di Brera, the lead author of the research in Nature.

"Explaining why the expansion of the universe is currently accelerating is certainly the most fascinating question in modern cosmology," Guzzo said. "We have been able to show that large surveys that measure the positions and velocities of distant galaxies provide us with a new powerful way to solve this mystery."