The distribution of pairs of galaxies that orbit one another has verified key elements of the standard model of cosmology — suggesting the presence of dark energy. Credit: NASA/ESA

The claim that mysterious dark energy is accelerating the Universe's expansion has been placed on firmer ground, with the successful application of a quirky geometric test proposed more than 30 years ago.

The accelerating expansion was first detected in 1998. Astronomers studying Type 1a supernovae, stellar explosions called "standard candles" because of their predictable luminosity, made the incredible discovery that the most distant of these supernovae appear dimmer than would be expected if the Universe were expanding at a constant rate.1 This suggested that some unknown force - subsequently dubbed dark energy - must be working against gravity to blow the universe apart.

Since that time, studies comparing variations in the cosmic microwave background radiation — an echo from the Big Bang — with the distribution of galaxies today have allowed cosmologists to trace how the Universe has expanded, supporting the idea of dark energy. They have also suggested that the Universe is 'flat' — that is, it contains just enough matter to keep it delicately poised between collapsing in on itself and expanding forever2.

It's a very clever idea, it's unexpected, and it's going to take a while to determine whether it's accepted or not. Charles Alcock , Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts

These two assumptions have become a fundamental part of cosmologists' understanding of the Universe. Now Christian Marinoni and Adeline Buzzi of the Centre for Theoretical Physics at the University of Provence in Marseilles, France, have independently checked these ideas by analysing the geometry of orbiting pairs of galaxies. Their study is published this week in Nature3.

The researchers used a version of the Alcock–Paczynski test, which relies on identifying symmetrical objects in space and using them as 'standard spheres'. Any distortions in space caused by the expansion of the cosmos would cause the most distant standard spheres to appear asymmetrical. "This provides a similar level of accuracy to supernovae," says Marinoni. "It's a direct proof of dark energy."

Object lessons

For example, if the universe is expanding outwards due to dark energy, distant objects will appear elongated along the line of sight from Earth, because Earth and the objects are being propelled away from one another along that direction.

Several groups have tried to apply the test, for example by considering clusters of galaxies as the standard spheres, but largely failed because they could not measure distant objects with sufficient accuracy.

To get round this, Marinoni and Buzzi instead studied the distribution in orientations of pairs of galaxies that orbit each other. In a Universe without dark energy, that distribution is expected to be spherically symmetrical — in other words, the number of galaxy pairs oriented in any particular direction should be the same.

The researchers found that the farther away the galaxy pairs were, the more asymmetrical the distribution was, with more galaxy pairs oriented along the line of sight from Earth. The pattern matched what would be expected in a flat Universe expanding due to dark energy.

The reliability of the test depends on the assumption that the distribution in orientations of galaxy pairs doesn't change depending on their distance from Earth — an idea that is largely untested. But researchers are still excited by the result.

"It's a very clever idea, it's unexpected, and it's going to take a while to determine whether it's accepted or not," says Charles Alcock, director of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and co-proposer, in 1979, of the test Marinoni and Buzzi used4.

"This is a cool new tool. It's another indication that everything is consistent," says Anthony Tyson, who studies cosmology at the University of California, Davis. Tyson adds that he is sceptical about some of the assumptions underlying theories of dark energy, so he finds independent efforts to confirm its presence extremely valuable. "For those of us who are doubters, it's making us think twice," he says.

Future surveys of distant objects are planned, for example using the European Space Agency's planned Euclid space telescope, and the Wide Field Infrared Survey Telescope, a proposed mission that was the top recommendation of the US National Academy of Sciences Decadal Survey of Astronomy and Astrophysics. Marinoni says he hopes these will find further remote pairs of galaxies that can be analysed to pin down the nature of dark energy, perhaps even enough to overtake the accuracy offered by studies of type Ia supernovae.