Thoughts on Defining the Multiverse
  The possibility that the multiverse corresponds to physical reality deserves serious investigation. Having three different important theories,(quantum mechanics, string theory and inflation), predict the existence of the multiverse is hardly coincidental. I argue that the existence of the multiverse must be expected from the underlying fundamental theory or else we can not meaningfully address the puzzle of the initial conditions. In this view, the extension of our current cosmology to a multiverse framework becomes an extension of the Copernican principle to nature. In order to discuss the ontology of the multiverse I propose to apply:   - the principle of 'No Perpetual Motion' as a criterion for the parameter of time; and,   - the principle of 'Domains Correlations' as a criterion for determining the background spacetime in which the multiverse is embedded. 
Laura Mersini-Houghton