http://arxiv.org/abs/astro-ph/0305292
Defining Multiverses
Authors: G.F.R. Ellis, U. Kirchner, W.R. Stoeger
Comments: 28 pages, 2 figures
The idea of a multiverse - an ensemble of universes - has received increasing attention in cosmology, both as the outcome of the originating process that generated our own universe, and as an explanation for why our universe appears to be fine-tuned for life. Here we carefully consider how multiverses should be defined, stressing the distinction between the collection of all possible universes, and ensembles of really existing universes or universe domains (essential for anthropic arguments). We show that such realised ensembles are by no means unique. A measure on the space of all universes or universe domains is needed so that probabilities can be calculated. We examine these issues in the case of the set of Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) universes, and consider scenarios like chaotic inflation which propose how ensembles of universe domains may be generated. Finally, we discuss philosophical and meta-scientific issues raised by the concept of a really existing ensemble.
Received on Mon May 19 2003 - 12:12:41 PDT