Which is itself an outgrowth or extension of the Big Bang theory.According to eternal inflation, the inflationary phase of the universe's expansion lasts forever throughout most of the universe. Because the regions expand exponentially rapidly,most of the volume of the universe at any given time is inflating. Eternal inflation, therefore, produces a hypothetically infinite multiverse, in which only an insignificant fractal volume ends inflation.Paul Steinhardt, one of the original architects of the inflationary model, introduced the first example of eternal inflation in 1983,and Alexander Vilenkin showed that it is generic.Alan Guth's 2007 paper, "Eternal inflation and its implications"

⇌Eternal inflation is a hypothetical inflationary universe model,

                                            

A schematic of the scalar-field potential. Region 1 controls the tunneling behavior. The field, for example, might tunnel from one red dot to another. Region 2 controls the early-time behavior inside each bubble, and therefore also controls the collision behavior between two bubbles. Region 3 controls the slow-roll inflation inside the bubble. Inflation inside the bubble needs to last long enough to solve the horizon and flatness problems in cosmology.