There may be no stopping the expanding universe. Lately, observations of everything from galaxy clusters to distant exploding stars have favored an "open" universe, containing too little mass for gravity to keep it from flying apart forever (ScienceNOW, 30 October ). Now a study to appear in Astronomy and Astrophysics adds another vote for an open universe, based on the number of light-bending "gravitational lenses" in the sky.
What creates these lenses is the gravity of massive clusters of galaxies, which bends light on its way to Earth from more distant galaxies and distorts their disclike images into arcs. "In order for this process to be effective, you need the cluster--that is, the gravitational lens--at roughly half the distance from us to the sources," says Matthias Bartelmann of the Max Planck Institute for Astrophysics near Munich, head of the team. The clusters develop through the entire lifetime of the universe, at a rate that depends on the overall density of matter. So the number of dense clusters that have ended up in the right place at the right time to act as lenses is a powerful probe of the overall mass density of the universe.
Bartelmann and Munich colleagues Andreas Huss and Jörg Colberg, along with Adrian Jenkins and Frazer Pearce of Britain's University of Durham, began with two computer models that simulate cluster evolution. The models incorporated various assumptions about the universe's mean mass density, along with a parameter known as the cosmological constant--a hypothetical energy embedded in empty space, which may also govern the overall expansion of the universe. The researchers allowed the models to evolve to the current age of the universe, then computed the lensing ability of the clusters that had developed and the number of arcs that would be seen from Earth.
The verdict: A universe with just a third of the mass density needed to stop its expansion--along with a small or zero value for the cosmological constant--would have dense clusters in the right numbers and places to produce the observed number of arcs, which is somewhere between 2300 and 2700. Bartelmann notes that the simulations suffer from uncertainty about how smoothly matter was distributed in the newborn universe. But Neil Turok of Cambridge University says, "[This] work may be the strongest single piece of evidence [for an open universe] if it holds up."