  I make the case that the Universe according to unitary (no-collapse) quantum
theory has a branching structure, and so can literally be regarded as a
"many-worlds" theory. These worlds are not part of the fundamental ontology of
quantum theory - instead, they are to be understood as structures, or patterns,
emergent from the underlying theory, through the dynamical process of
decoherence. That they are structures in this sense does not mean that they are
in any way unreal: indeed, pretty much all higher-level ontology in science,
from tables to phonons to tigers, is likewise emergent. Unitary quantum theory
is therefore a "many-worlds" theory without any modification of the
mathematical structure of the theory: the Everett interpretation does not
consist in adding worlds to the formalism, but in realising that they are there
already. Our grounds for accepting the reality of those worlds is no more, but
no less, than our grounds for accepting any other not-directly observable
consequence of an empirically very successful theory.

History and Philosophy of Physics;Quantum Physics