Astronomers have been puzzling over the expansion rate of the Universe and its mass for decades. If the mass of the Universe is large enough, the expansion will eventually decrease and the Universe will then collapse in on itself. However, if the density of matter in the Universe is less than a certain critical density, it will continue to expand for ever. The ratio of the actual density to this critical density is called Omega.

Many astrophysicists believe that the Universe underwent a period of incredibly rapid expansion shortly after the big bang. One consequence of this "inflationary" model is that the Universe is "flat" with a value of Omega equal to one. However, when all the visible mass in the Universe is added up, it is much less than that needed to give a flat Universe. This is one of the main motivations of the search for invisible or "dark" matter in the Universe.

Supernovae allow astronomers to measure the expansion rate of the Universe and hence determine how much mass there is in the Universe. A distant supernovae observed by Perlmutter and colleagues last year gave a result that contradicted the inflationary model. It suggested that the expansion rate was actually increasing rather than decreasing (Nature 391 51). Further research by other groups confirmed their results.

Some astronomers believe that dust clouds could have altered the luminosity of the supernovae and affected Perlmutter's calculations. Dust absorbs blue light more readily than light from any other part of spectrum, making objects appear redder than they actually are. However, Perlmutter and Filippenko have now excluded those supernovae that they believe are affected by dust, and they still conclude that the expansion rate is still increasing. Other astronomers are slowly coming round to agree with them. "I'm reaching the point that I'm beginning to believe the two teams, " says Jeremiah Ostriker of Princeton University.

Some cosmologists believe that the acceleration is caused by quantum effects, which result in a non-zero cosmological constant, Lambda. If the sum of Lambda and Omega equals 1, then the Universe will remain "flat", as predicted by inflation theory.

Some groups have tried to measure Lambda by studying gravitational lensing. If Lambda is non-zero, then astronomers should see more lensing events than if Lambda were zero. According to Matthias Bartelmann of the Max Planck Institute for Astrophysics in Garching, Germany, computer simulations can predict the number of lensing events you should see for different values of Lambda. His results approximately match observations of the gravitational lensing of radio galaxies carried out by Chris Kochanek and colleagues from the Harvard-Smithsonian Center for Astrophysics in the US (Astrophys J. 495 157). Their results place an upper limit of 0.7 on Lambda and a lower limit of 0.3 on Omega. Both these figures match the supernova data.

Astronomers hope that the European Space Agency's PLANCK mission and NASA's Microwave Anisotropy Probe (MAP) will make more accurate measurements of both Lambda and Omega within the next decade.