"A range of theoretical developments has gradually qualified the interpretation of 'universe.'  [...]  Sometimes it's applied to separate realms, ones that are partly or fully, temporarily or permanently, inaccessible to us; in this sense, the word relegates our universe to membership in a large, perhaps infinitely large, collection.” —Thank you, Brian Greene, for The Hidden Reality:  Parallel Universes and the Deep Laws of the Cosmos, 2011, pp.12-13.