Multiverse
A hypothetical set of multiple co-existing universes that together comprise all of physical reality.
Governing formula
Cosmic Inflation Bubble Universes & Many-Worlds Everett Branching
SI unit
Cosmological Hypothesis
In depth
Includes Max Tegmark's classification: Level I (beyond observable horizon), Level II (inflationary bubble universes with different physical constants), Level III (many-worlds quantum mechanics), and Level IV (mathematical structures).
Examples in the real world
Eternal cosmic inflation spawning distinct pocket universes with varying cosmological constants.