Summary and Keywords
The term multiverse is derived from multiple universes. A multiverse is a theoretical concept denoting a collection of universes that are causally disconnected and whatever may exist beyond or between the boundaries of these universes. In essence, it is the totality of physical reality, whatever form that may take. An equivalent term is megaverse. The physically distinct universes composing a multiverse are often referred to as alternative, alternate, quantum, parallel, or bubble universes.
The American philosopher William James invented the specific term multiverse in 1895, not in a cosmological context but in reference to his view of the natural world. In the 20th century the application of the term was broadened from James’s original intent to a range of areas including cosmology, religion, philosophy, and psychology. More recently, David Lewis (1941–2001) considered philosophical implications of a multiverse from his modal realism perspective. In fact, the concept of a cosmological multiverse and its philosophical and religious implications were actually considered more than a millennium prior throughout various societies and religions. The scientific implications have predominantly been analyzed since the early 20th century.
In efforts to answer fundamental questions about the origin and properties of our universe, many cosmologists have converged on a scientific concept of a multiverse of one form or another. Multiverses with vastly different properties have been developed. To organize the collection of multiverses in a consistent way, in 2003, MIT cosmologist Max Tegmark proposed a multiverse taxonomy. Tegmark argued that all multiverses can be fit into four classes, which he designated as Levels 1 through 4. A given higher level multiverse contains a set or sets of lower level multiverses. In 2007, string theorist Brian Greene of Columbia University refined Tegmark’s classification system. Each of Greene’s nine classes fit within one of Tegmark’s four Levels.
While theoretical multiverses take many forms, common to most all of them is the idea that a vast number of universes exists outside the limits of our observable region. Given that a multiverse beyond our universe is not currently (and perhaps never will be) empirically testable or detectable, the multiverse concept is very controversial. This is especially so within and among the science, philosophy, and religion communities. There is disagreement regarding the question of the existence of the multiverse and whether the multiverse is a proper subject of scientific inquiry. Some argue that a multiverse is a philosophical concept, rather than a scientific one. Alternatively, some scientists believe that most or all multiverse proposals present a deconstructionist science that avoids providing answers grounded in meaningful science. In contrast, many theoretical physicists (especially cosmologists) and some philosophers affirm that a multiverse offers a more likely and more robust resolution to fundamental cosmological issues that a sole (even infinite) universe cannot answer.
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