Multiverse

A hypothesized collection of universes beyond the observable universe or distinct from it. Proposed multiverses arise from different physical theories and need not share one structure or mechanism.

Observable universe: The region of the universe from which light or other signals have had time to reach Earth. A multiverse may include regions beyond this horizon that remain unobservable.
Eternal inflation: A class of inflationary models in which accelerated expansion continues in some regions while ending in others. Regions where inflation ends can form separate bubble universes.
Cosmic multiverse: A collection of physically distinct cosmic regions or universes, often proposed in inflationary cosmology. It denotes spatially separate universes rather than quantum branches.
Anthropic principle: A principle that links observations about the universe to the conditions required for observers to exist. It is used to interpret why observed constants permit life among many proposed universes.
Hugh Everett III: An American physicist who proposed the relative-state formulation of quantum mechanics in 1957. His formulation became the basis for the many-worlds version of a multiverse.
Cosmological horizon: A boundary beyond which events cannot send signals to an observer, given the universe’s expansion and history. It separates the observable universe from potentially existing regions outside it.
Cosmic inflation: A proposed brief period of extremely rapid expansion in the early universe. Eternal versions of inflation provide one route to a multiverse.
Quantum multiverse: A proposed collection of branches associated with quantum outcomes in the many-worlds interpretation. It treats alternative quantum outcomes as worlds, not separate regions of space.
Fine-tuning: The apparent sensitivity of physical outcomes, including life’s possibility, to the values of fundamental constants or initial conditions. A multiverse is one proposed explanation for apparently life-permitting values.
Andrei Linde: A Russian-American theoretical physicist known for developing chaotic inflation and eternal inflation. His work helped establish inflationary cosmology as a source of multiple universes.

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