Detecting dark energy: The ESO's Very Large Telescope project in the Chilean Andes scanned thousands of galaxies to probe why the expansion of the universe is accelerating.
Credit: AFP
PARIS: Cosmologists believe they are closing in on one of the great mysteries of physics: why the expansion of the universe, triggered by the Big Bang, is accelerating.
The answer could be tantalisingly within reach, according to their study, detailed today in the British journal Nature.
A decade ago, astronomers were stunned to learn that the universe was expanding more quickly than in the past. It had long been assumed that the gravitational attraction of galaxies would slow the expansion of space, kicked off by the Big Bang some 14 billion years ago.
Opposing theories
Two very different theories have since emerged to explain this shock discovery.
One is that the universe is filled with so-called dark energy, a substance that has been inferred but never seen. Dark energy cannot be detected with present technology as it neither emits nor reflects light or radiation.
Dark energy, so the theory goes, counteracts the gravitational attraction that galaxies exert on each other and which would otherwise brake the cosmic expansion.
The other possible explanation is that dark energy does not exist. If this were true, current theories about gravitational force as the prime mover in the universe would be flawed; they would only make sense if there were additional dimensions to space.
Until now, observations have not strongly supported either scenario. But a new approach may unlock the enigma, say the paper's authors.
Cosmic tug-of-war
Using the European Space Agency's (ESO) Very Large Telescope (VLT) in Chile, an international team of 51 astronomers led by Luigi Guzzo of the Brera Astronomical Observatory in Milan, Italy, measured the distribution and movement of some 10,000 galaxies over a period of 30 years.
The goal of their observations was to assess the great forces involved in the cosmic tug-of-war: the overall expansion of the universe, which pushes galaxies away from each other and the force of gravity, which pulls them together.
By indirectly measuring the speed at which the galaxies travelled, the scientists were able to create a three-dimensional map of an expanding universe that also informs about movements within the galaxies themselves.
"Measuring this distortion at different epochs of the universe's history is a way to test the nature of dark energy," said Oliver Le Fevre, from the Laboratoire d'Astrophysique de Marseille.
The big tool was a spectrograph – an instrument to record X-rays – on one of the four 8.2-metre (320-inch) ESO telescopes perched on the summit of the 2,635-metre-tall Cerro Paranal in the Chilean Andeas. They scanned thousands of galaxies across a patch of sky roughly 20 full moons across.
"Consistent with dark energy"
The results, while not conclusive, are consistent with a dark energy explanation for the acceleration of an expanding universe, said the researchers.
The next step is to extend the observations tenfold. "This technique should be able to tell us whether cosmic acceleration originates from a dark energy component of exotic origin, or requires a modification of the laws of gravity," said Guzzo.
According the most recent models, he said, 73 per cent of cosmic energy seems to consist of "dark energy."
