Eternal Inflation

Eternal inflation is the cosmological scenario in which cosmic inflation never ends globally. While local regions ("pocket universes") thermalize and exit the inflationary phase, the larger spacetime continues expanding exponentially, continually spawning new pocket universes with potentially different physical properties. This mechanism is the dominant theoretical basis for the level II multiverse, though it relies on the persistence of a false vacuum energy density that has not been directly observed.

The eternal inflation framework connects to backreaction and inhomogeneous cosmology in a critical way: if local structures significantly affect global expansion, the assumption of a globally persistent inflating background may itself be an artifact of averaging. The multiverse that eternal inflation produces may be less a prediction than a mathematical artifact of extrapolating a smoothed model beyond its domain of validity. The bubble nucleation mechanism by which pocket universes form remains theoretically conjectural, with no observational signature.