A Cosmic Archipelago: Multiverse Scenarios in the History of Modern   Cosmology
  Multiverse scenarios are common place in contemporary high energy physics and cosmology, although many consider them simply as bold speculations. In any case there is nothing like a single theory or a unified model of the multiverse. Instead, there are innumerable theoretical proposals often reciprocally incompatible. What is common to all these scenarios is the postulated existence of many causally disjointed regions of space/time (if not completely separated space-times) and the consideration of the observable universe as atypical in a global perspective. This paper examines the history of modern cosmology focusing on the forerunners of current ideas, and shows how some fundamental issues tend to present themselves on increasingly deeper levels of physical knowledge. 
Stephano Bettini