Galaxies today are struggling to clump together against the incredible repulsive power of dark energy, hints a new survey of thousands of galaxies. Measuring this anti-clumping effect puts a new arrow in the quiver of cosmologists seeking to uncover the nature of the mysterious force.
Scientists proposed the existence of a mysterious repulsive force called dark energy in 1998 to explain supernova observations showing the universe is expanding at ever faster rates.
Since then, researchers have been trying to measure the properties of dark energy more precisely, in the hope of discovering what it is. Possible explanations include fluctuating energy fields from quantum physics and the effects of unseen extra spatial dimensions. In some scenarios, the strength of dark energy changes with time in characteristic ways.
Now, a study led by Luigi Guzzo of Brera Astronomical Observatory in Merate, Italy, may pave the way for researchers to decide between the different theories. They wanted to see if dark energy had any effect on the motion of galaxies at different times since the big bang.
On the whole, galaxies are moving away from each other because of the expansion of the universe. But they also have small extra motions, called peculiar velocities, caused by their gravitational attraction to each other – individual galaxies fall into massive clusters of galaxies, for example.
Previous studies have measured the rate at which galaxies clump together in the modern universe using data from the Two Degree Field Galaxy Redshift Survey (2dFGRS).
Guzzo’s team compared that rate with observations by the VIMOS-VLT Deep Survey (VVDS), which measured the motions of about 6000 galaxies in the distant universe. Because of the time it takes their light to reach us, the galaxies appear as they were about 7 billion years ago, when the universe was half its present age.
While the data cannot conclusively prove that dark energy is affecting the galaxies’ motions, it suggests that dark energy is slowing down the rate of clumping more and more over time. Galaxies apparently have to fight harder to clump together in the modern universe due to the faster expansion of space.
Unfortunately, the uncertainties in the measurements are too large to distinguish between different kinds of dark energy. But improved surveys may be able to do it, says team member Enzo Branchini of the University of Rome III in Italy.
“We think that in future surveys covering much larger areas and using a larger number of objects, you will be able to use this very method to constrain very precisely the amount of dark energy and the nature of this dark energy,” he told New Scientist.
The study demonstrates a new tool that scientists can use to try to unlock the secret of dark energy in an independent way, says Michael Strauss of Princeton University in New Jersey, US, who is not a member of Guzzo’s team.
Other techniques being pursued include observing large numbers of distant supernovae and searching for dark energy’s effects on repeating patterns in the distribution of galaxies that were caused by booming sound waves in the early universe.
Strauss says it is hard to know what technique has the best chance of pinning down the nature of dark energy. “Given that we don’t know what dark energy is, it’s a little hard to come up with the experiment that will crack the nut,” he told New Scientist. “It’s going to be a hard game – there’s no question about it.”
In the next four to five years, the VVDS study should have measured 50,000 to 100,000 galaxies, which may be enough to start distinguishing between different explanations, Branchini says.
And 2 million galaxies could be studied by a space-based observatory called SPACE (Spectroscopic All-sky Cosmic Explorer) that the European Space Agency is considering for launch in 2017. That would provide an even better ability to distinguish between the different explanations, he says.
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Journal reference: Nature (DOI: 10.1038/nature06555)