based on the standard model of cosmology, the universe has been and is expanding. the classical big bang theory assumes that the universe expanded at the same rate throughout universal history. inflation says that the universe "has not" always expanded at the same rate, and that the universe has experienced many periods of brief but immense expansions in which the size of the universe nearly doubled. these periods, as said, were extremely short, lasting less than a second. after a single period, the expansion slowed down until the next rapid inflation came and, again, massively increased the size of the universe.