The Universe is expanding, and it's doing so ever faster, although astronomers don't quite know why. To find out, a team of cosmologists from the University of Toulouse III - Paul Sabatier and the University of Geneva have put the current model of the Universe predicted by Einstein's own theories to the test. The results, published in the magazine Nature Communications, indicate that the theory of general relativity, which the German physicist developed more than a hundred years ago, may be insufficient to accurately describe the evolution of the universe. "We have enough evidence to believe that Einstein's theory is not definitive," says Isaac Tutusaus, a Catalan cosmologist at the University of Toulouse III - Paul Sabatier and co-author of the study.