A cosmological model in which the universe has repeated periods of expansion followed by contraction; i.e. a series of big bangs interspersed with big crunches. It was originally put forward in 1922 by the Russian physicist Aleksandr Friedmann (1888–1925). The theory was criticized because it led eventually to an increasing entropy with each cycle. In 1995 it was pointed out by John Barrow and Mariusz Dabrowski that, in the presence of any repulsive cosmological constant, the oscillations would eventually end and the expansion of the universe would accelerate. See also ekpyrotic universe.