Eternal inflation

In cosmology, eternal inflation is a prediction of some forms of the model of cosmic inflation, a theory developed to explain issues in the original Big Bang theory. According to eternal inflation, the decay of the meta-stable false vacuum in one region led to the inflationary phase of the universe's expansion. In surrounding regions, the false vacuum continues to expand and decay can occur in surrounding regions, resulting in other universes inaccessibly distant from ours. This process lasts forever. Eternal inflation, therefore, produces a hypothetically infinite multiverse, in which only an insignificant fractal volume ends inflation.

Paul Steinhardt, one of the original researchers 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", states that under reasonable assumptions "Although inflation is generically eternal into the future, it is not eternal into the past". Guth detailed what was known about the subject at the time, and demonstrated that eternal inflation was still considered the likely outcome of inflation, more than 20 years after eternal inflation was first introduced by Steinhardt.