American physicist Alan Guth proposed a model of the universe based on the big bang theory. He incorporated a short early period of exponential cosmic inflation in order to solve the uncertainties of the standard big bang model, such as horizon and flatness problems. This became known as the inflationary model. Another variation of the inflationary model was the cyclic model developed by Pul Steinhardt and Neil Turok in 2002, which incorporated the ideas based on the superstring theory.