Parallel universe
A parallel universe, also sometimes called an alternate universe, is a hypothetical universe which exists separately from our own. Some theories of physics postulate the existence of many parallel universes, possibly even an infinite number. Depending on the details of the theory, these universes may or may not interact with each other. The word "multiverse" has been proposed to refer to the collection of all universes.
Of the theories of parallel universes, that taken most seriously by modern physicists is the many worlds interpretation of quantum mechanics, proposed by Hugh Everett III in 1956, or some minor modification of it. A champion of this theory is the Oxford-based physicist David Deutsch.
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Everett's "many-worlds" theory
Main article: Many-worlds interpretationParallel universes as an explanation of 'fine-tuning'
The idea of multiple universes has been used together with the anthropic principle to explain why our universe seems fine-tuned for intelligent life.