German-American physicist Albert Einstein proposed a theory of the universe, based on general relativity, in which gravitation is equivalent to a curvature of four-dimensional space. His solution indicated that the universe was not static but must be expanding or contracting. Of various non static theories, that proposed by Russian mathematician Alexander Friedmann is the currently accepted one. The fate of the Friedmann universe depends on the average density of matter in the universe. If there is relatively little matter in the universe, the gravitational attraction among the galaxies will be slight, and the universe will expand forever to an infinite extent. If the density of matter is above a critical value, the expansion will slow to a halt and reverse to contraction, ending in the total collapse of the entire universe. The fate of this collapsed universe in uncertain. One theory is that it would explode, producing a new expanding universe, which would again collapse, and so on forever.