The multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including the historical universe we consistently experience) that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The term was coined in 1895 by the American philosopher and psychologist William James. The various universes within the multiverse are sometimes called parallel universes. The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered. Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology and fiction, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "alternative realities", "alternative timelines", and "dimensional planes," among others.
You can think of universes as "bubbles". When two of these "bubbles" collide they may have caused the big bang. They can also separate to form two smaller universes.
- James, William 1895. The Will to Believe, and earlier in 1895, as cited in OED's new 2003 entry for "multiverse": 1895 W. JAMES in International Journal of Ethics 6 p10. "Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe".
2. James Cummins, Redland Green School