Despite the success of his model and his prediction of Hubble’s observations, Friedmann’s work remained largely unknown in the West until similar models were discovered in 1935 by the American physicist Howard Robertson and the British mathematician Arthur Walker, in response to Hubble’s discovery of the uniform expansion of the universe.

     Although Friedmann found only one, there are in fact three different kinds of models that obey Friedmann’s two fundamental assumptions.  In the first kind (which Friedmann found) the universe is expanding sufficiently slowly that the gravitational attraction between the different galaxies causes the expansion to slow down and eventually to stop.  The galaxies then start to move toward each other and the universe contracts.  Fig. 3.2 shows how the