Multiverse cosmological models
  Recent advances in string theory and inflationary cosmology have led to a surge of interest in the possible existence of an ensemble of cosmic regions, or universes, among the members of which key physical parameters, such as the masses of elementary particles and the coupling constants, might assume different values. The observed values in our cosmic region are then attributed to an observer selection effect (the so-called anthropic principle). The assemblage of universes has been dubbed the multiverse. In this paper I review the multiverse concept and the criticisms that have been advanced against it on both scientific and philosophical grounds. 
P.C.W. Davies