UC Irvine

More than 2000 years ago, Epicurus taught that there are an infinite number of other worlds, both like and unlike ours, and Aristotle taught that there are none. Neither hypothesis can currently be falsified, and some versions of current multiverses perhaps never can be, which has contributed to occasional claims that "this isn't science!" (a common complaint about cosmology for centuries). Define "cosmos", or "world", or "universe" to mean the largest structure of which you and the majority of knowledgeable contemporaries will admit to being a part. This begins with the small, earth-centered worlds of ancient Egyptian paintings, Greek mythology, and Genesis, which a god could circumnavigate in a day and humans in a generation. These tend to expand and become helio- rather than geo-centric (not quite monotonically in time) and are succeeded by various assemblages of sun-like stars with planets of their own. Finite vs. infinite assemblages are debated and then the issue of whether the Milky Way is unique (so that "island universes" made sense, even if you were against the idea) for a couple of centuries. Today one thinks as a rule of the entire 4-dimensional space-time we might in principle communicate with and all its contents. Beyond are the modern multi-verses, sequential (cyclic or oscillating), hierarchical, or non-communicating entities in more than four dimensions. Each of these has older analogues, and, in every milieu where the ideas have been discussed, there have been firm supporters and firm opponents, some of whose ideas are explored here. Because astronomical observations have firmly settled some earlier disputes in favor of very many galaxies and very many stars with planets, "other worlds" can now refer only to other planets like Earth or to other universes. The focus is on the latter. © 2009 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim.