multiverse

The multiverse is the hypothetical set of multiple universes that together comprise all that exists: space, time, matter, energy, and the laws and constants that describe them. The term is used in physics and philosophy to refer to realities beyond the observable universe.

A widely cited framework is Max Tegmark's four levels. Level I includes regions beyond the cosmic horizon that share the same physical laws but differ in initial conditions. Level II envisions other post-inflation bubble universes with possibly different physical constants or laws. Level III corresponds to the many-worlds interpretation of quantum mechanics, where every quantum event yields branching, non-communicating worlds. Level IV posits that all mathematical structures correspond to real universes. Philosophical modal realism is a related position that treats all possible worlds as real.

In physics, several mechanisms motivate a multiverse. Eternal inflation naturally produces Level II bubble universes. The string theory landscape suggests a vast number of metastable vacua with different constants. The Many-Worlds interpretation of quantum mechanics provides a branching structure of histories. Proponents argue that a multiverse could explain fine-tuning and the apparent arbitrariness of physical constants, while critics contend that the idea risks falsifiability.

Observationally, there is no direct evidence for other universes, and the scientific status of multiverse hypotheses is debated. Critics emphasize the difficulty of empirical tests, while supporters seek indirect signatures or explanatory power from probability and statistics in a broader framework.

See also: cosmic inflation, quantum mechanics, string theory, fine-tuning.