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Multiverse Scenarios in Cosmology: Classification, Cause, Challenge, Controversy, and Criticism

Multiverse scenarios in cosmology assume that other universes exist "beyond" our own universe. They are an exciting challenge both for empirical and theoretical research as well as for philosophy of science. They could be necessary to understand why the big bang occurred, why (some of) the laws of nature and the values of certain physical constants are the way they are, and why there is an arrow of time. This essay clarifies competing notions of "universe" and "multiverse"; it proposes a classification of different multiverse types according to various aspects how the universes are or are not separated from each other; it reviews the main reasons for assuming the existence of other universes: empirical evidence, theoretical explanation, and philosophical arguments; and, finally, it argues that some attempts to criticize multiverse scenarios as "unscientific", insisting on a narrow understanding of falsification, is neither appropriate nor convincing from a philosophy of science point of view. -- Keywords: big bang, universe, multiverse, cosmic inflation, time, quantum gravity, string theory, laws of nature, physical constants, fine-tuning, anthropic principle, philosophy of science, metaphysics, falsificationism