A model of the universe that postulates that the big bang occurred because of the collision of two parallel branes of a type predicted to occur in string theory (see brane world). The term ‘ekpyrotic’ means ‘coming out of fire’. It has been developed into a cyclic model of the universe, which avoids the problems of entropy associated with the oscillatory universe. There are some differences between the predictions of the ekpyrotic theory and those of the more established inflationary universe but neither possibility has been definitively ruled out yet.