Sanjaya Baral and Dhurba Chhetri St. Xavier’s College, Maitighar, Kathmandu, Nepal Abstract: Multiverse scenarios in assume that other exist "beyond" our own . They are an exciting challenge both for empirical and theoretical research as well as for of science. They could be necessary to understand why the occurred, why (some of) the laws of and the values of certain physical constants are the way they are, and why there is an arrow of . This essay clarifies competing notions of "universe" and "multiverse"; it proposes a classification of different multiverse types according to various aspects how the universes are or are not separated from each other; it reviews the main reasons for assuming the of other universes: empirical evidence, theoretical explanation, and philosophical arguments. Keywords: Big bang, universe, multiverse, cosmic , time, , , laws of nature, physical constants, fine- tuning, , , , falsifications

1. INTRODUCTION include , , Brian The multiverse (or meta-universe) is the hypothetical Greene, , , , of infinite or finite possible universes (including , , , the universe we consistently experience) that Raj Pathria, Sean Carroll, Alex Vilenkin,Laura together comprise that exists: the Mersini-Houghton, and Neil deGrasse Tyson. In entirety of , time, , and as well as contrast, critics such as Jim Baggott, , the physical laws and constants that describe them. , George Ellis and have The various universes within the multiverse are argued that the multiverse question is philosophical sometimes called parallel universe or "alternative rather than scientific, that the multiverse cannot be universes" a scientific question because it lacks , or even that the multiverse hypothesis is harmful or The structure of the multiverse, the nature of each pseudoscientific. universe within it and the relationships among the various constituent universes, depend on the specific multiverse hypothesis considered. Multiple universes have been hypothesized in cosmology, , , , philosophy, transpersonal psychology, and fiction, particularly in and . In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "", "alternative ", "alternative timelines", and "dimensional planes," 2. MULTIVERSE HYPOTHESES IN PHYSICS among others. The term 'multiverse' was coined in Swedish-American cosmologist Max Erik Tegmark, 1895 by the American philosopher and psychologist an American theoretical and many other in a different context. physicists have hypothesized various classification schemes that categorize the various theoretical types The multiverse hypothesis is a source of debate of multiverse. Some of the prominent hypothesis are within the physics community. Physicists disagree discussed below: about whether the multiverse exists, and whether the 2.1 Max Tegmark's four levels hypothsis multiverse is a proper subject of scientific inquiry. Cosmologist Max Tegmark has provided a taxonomy Supporters of one of the multiverse hypotheses of universes beyond the familiar universe.

109 Sanjaya Baral ... Multiverse ...

The levels according to Tegmark's classification is that certain observations are arranged such that subsequent levels can be cannot be predicted absolutely. Instead, there is a understood to encompass and expand upon previous range of possible observations, each with a different levels, and they are briefly described below. . According to the MWI, each of these possible observations corresponds to a different 2.1.1 Level I: Beyond our universe. Suppose a die is thrown that contains A generic prediction of chaotic inflation is an infinite six sides and that the numeric result of the throw ergodic universe, which, being infinite, must contain corresponds to a quantum mechanics observable. Hubble volumes realizing all initial conditions. All six possible ways the die can fall correspond to Accordingly, an infinite universe will contain an six different universes. Tegmark argues that a level infinite number of Hubble volumes, all having III multiverse does not contain more possibilities the same physical laws and physical constants. In in the than a level I-II multiverse. regard to configurations such as the distribution In effect, all the different "worlds" created by "splits" of matter, almost all will differ from our Hubble in a level III multiverse with the same physical volume. However, because there are infinitely many, constants can be found in some Hubble volume in far beyond the cosmological horizon, there will a level I multiverse. Tegmark writes that "The only eventually be Hubble volumes with similar, and difference between Level I and Level III is where your even identical, configurations. Tegmark estimates doppelgängers reside. In Level I they live elsewhere that an identical volume to ours should be about in good old three-dimensional space. In Level III 1010115 meters away from us. Given infinite space, they live on another quantum branch in infinite- there would, in fact, be an infinite number of Hubble dimensional ." Similarly, all level II volumes identical to ours in the universe. This bubble universes with different physical constants follows directly from the , can in effect be found as "worlds" created by "splits" wherein it is assumed our Hubble volume is not at the moment of spontaneous symmetry breaking special or unique. in a level III multiverse.

2.1.2 Level II: Universes with different physical 2.1.4 Level IV: Ultimate ensemble constants The ultimate ensemble or mathematical universe In the chaotic inflation theory, a variant of the cosmic hypothesis is the hypothesis of Tegmark himself. inflation theory, the multiverse as a whole is stretching This level considers equally real all universes that can and will continue doing so forever, but some regions be described by different mathematical structures. of space stop stretching and form distinct bubbles, Tegmark writes that "abstract is so like gas pockets in a loaf of rising bread. Such general that any (TOE) that bubbles are embryonic level I . Linde and is definable in purely formal terms (independent of Vanchurin calculated the number of these universes vague human terminology) is also a mathematical to be on the scale of 101010,000,000. Different structure. For instance, a TOE involving a set of bubbles may experience different spontaneous different types of entities (denoted by words, say) symmetry breaking resulting in differentand relations between them (denoted by additional properties such as different physical constants. words) is nothing but what mathematicians call a set-theoretical model, and one can generally find a 2.1.3 Level III: Many-worlds interpretation of that it is a model of." He argues this quantum mechanics "implies that any conceivable parallel universe theory Hugh Everett's many-worlds interpretation (MWI) can be described at Level IV" and "subsumes all other is one of several mainstream interpretations ensembles, therefore brings closure to the hierarchy of quantum mechanics. In brief, one aspect of of multiverses, and there cannot be say a Level V."

110 The Himalayan Physics, Vol.5, No.5, Nov 2014 Jürgen Schmidhuber, however, says the "set  The simulated multiverse exists on complex of mathematical structures" is not even well- computer systems that simulate entire universes. defined, and admits only universe representations  The ultimate multiverse contains every describable by constructive mathematics, that is, mathematically possible universe under different computer programs. He explicitly includes universe laws of physics. representations describable by non-halting programs 3. Multiverse hypotheses in philosophy and logic whose output bits converge after finite time, 3.1 Model realism although the convergence time itself may not be Possible worlds are a way of explaining probability, predictable by a halting program, due to Kurt Gödel's hypothetical statements and the like, and some limitations. He also explicitly discusses the more philosophers such as David Lewis believe that all restricted ensemble of quickly computable universes. possible worlds exist, and are just as real as the actual 2.2 's hypothesis world (a position known as ). American theoretical and string theorist 3.2 Trans-world identity Brian Greene discussed nine types of parallel A metaphysical issue that crops up in multiverse universes in his book “The Hidden ” schema that posit infinite identical copies of any given published on 2011. universe is that of the notion that there can be identical  The quilted multiverse only works in an infinite objects in different possible worlds. According to universe. With an infinite amount of space, every the counterpart theory of David Lewis, the objects possible event will occur an infinite number of should be regarded as similar rather than identical. . However, the prevents people from being aware of these other identical areas. 3.3 Fictional realism  The inflationary multiverse is composed of The view that because fictions exist, fictional characters various pockets where fields collapse exist as well. There are fictional entities, in the same and form new universes. sense in which, setting aside philosophical disputes,  The multiverse follows from M-theory there are people, Mondays, numbers and planets. and states that each universe is a three- REFERENCES: dimensional brane that exists with many others. 1. http://en.wikipedia.org/wiki/Multiverse Particles are bound to their respective 2. ragh, H. (2009). "Contemporary History of except for gravity. Cosmology and the Controversy over the  The cyclic multiverse has multiple branes (each Multiverse". Annals of Science66 (4): 529. a universe) that collided, causing Big Bangs. The doi:10.1080/00033790903047725 universes bounce back and pass through time, until 3. Pathria, R. K. (1972). "The Universe as a Black they are pulled back together and collide again, Hole". Nature240 (5379): 298. Bibcode:1972Natur destroying the old contents and creating them anew. .240..298Pdoi:10.1038/ 240298a  The landscape multiverse relies on string 4. Tegmark, Max (May 2003). "Parallel Universes". theory's Calabi-Yau shapes. Quantum fluctuations drop the shapes to a lower energy 5. : Parallel Universes and the level, creating a pocket with a different set of Deep Laws of the , 2011 laws from the surrounding space. 6. Weinberg, Steven (2005). "Living in the  Thequantum multiverse creates a new universe Multiverse". arXiv:hep-th/0511037v1 when a diversion in events occurs, as in the many- 7. Richard J Szabo, An introduction to worlds interpretation of quantum mechanics. and D-brane dynamics (2004)  The holographic multiverse is derived from 8. Zyga, Lisa "Physicists Calculate Number of the theory that the surface area of a space can Parallel Universes" simulate the volume of the region.

111