The current formulation of the Big Bang theory, as well as recent observational evidence obtained by astronomers, shows that the universe has not always expanded at the same rate. Very soon after the Planck time, the universe went through a hyperinflationary period that suddenly increased the diameter of the universe by at least a factor of ten billion billion; this is called the inflationary model. Long after the hyperinflation ended, the expansion returned to an almost-constant rate, slowed down very slightly, and then billions of years ago started speeding up. Right now, the expansion rate of the universe is slowly but surely increasing. We live in an accelerating universe.