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.

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."