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69-533: The multiverse is the hypothetical set of all universes . Together, these universes are presumed to comprise everything that exists: the entirety of space , time , matter , energy , information , and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes", "flat universes", "other universes", "alternate universes", "multiple universes", "plane universes", "parent and child universes", "many universes", or "many worlds". One common assumption
138-503: A Forbes blog post that parallel universes would have to remain a science fiction dream for the time being, based on the scientific evidence available to us. Scientific American contributor John Horgan also argues against the idea of a multiverse, claiming that they are "bad for science." Max Tegmark and Brian Greene have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise. Cosmologist Max Tegmark has provided
207-577: A taxonomy of universes beyond the familiar observable universe . The four levels of Tegmark's classification are arranged such that subsequent levels can be understood to encompass and expand upon previous levels. They are briefly described below. A prediction of cosmic inflation is the existence of an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions. Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having
276-614: A Level I or Level II multiverse. In effect, all the different worlds created by "splits" in a Level III multiverse with the same physical constants can be found in some Hubble volume in a Level I multiverse. Tegmark writes that, "The only difference between Level I and Level III is where your doppelgängers reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space ." Similarly, all Level II bubble universes with different physical constants can, in effect, be found as "worlds" created by "splits" at
345-509: A consequence of incoming light from distant galaxies , or even from clouds of dust surrounding our own galaxy. Modern proponents of one or more of the multiverse hypotheses include Lee Smolin , Don Page , Brian Greene , Max Tegmark , Alan Guth , Andrei Linde , Michio Kaku , David Deutsch , Leonard Susskind , Alexander Vilenkin , Yasunori Nomura , Raj Pathria , Laura Mersini-Houghton , Neil deGrasse Tyson , Sean Carroll and Stephen Hawking . Scientists who are generally skeptical of
414-404: A finite time, although the convergence time itself may not be predictable by a halting program, due to the undecidability of the halting problem . He also explicitly discusses the more restricted ensemble of quickly computable universes. Hypothesis A hypothesis ( pl. : hypotheses ) is a proposed explanation for a phenomenon . For a hypothesis to be a scientific hypothesis ,
483-422: A hypothesis. In common usage in the 21st century, a hypothesis refers to a provisional idea whose merit requires evaluation. For proper evaluation, the framer of a hypothesis needs to define specifics in operational terms. A hypothesis requires more work by the researcher in order to either confirm or disprove it. In due course, a confirmed hypothesis may become part of a theory or occasionally may grow to become
552-415: A loaf of rising bread). Such bubbles are embryonic level I multiverses. Different bubbles may experience different spontaneous symmetry breaking , which results in different properties, such as different physical constants . Level II also includes John Archibald Wheeler 's oscillatory universe theory and Lee Smolin 's fecund universes theory . Hugh Everett III 's many-worlds interpretation (MWI)
621-461: A method used by mathematicians, that of "investigating from a hypothesis". In this sense, 'hypothesis' refers to a clever idea or to a convenient mathematical approach that simplifies cumbersome calculations . Cardinal Bellarmine gave a famous example of this usage in the warning issued to Galileo in the early 17th century: that he must not treat the motion of the Earth as a reality, but merely as
690-557: A number of important statistical tests which are used to test the hypotheses. Mount Hypothesis in Antarctica is named in appreciation of the role of hypothesis in scientific research. Several hypotheses have been put forth, in different subject areas: hypothesis [...]— Working hypothesis , a hypothesis suggested or supported in some measure by features of observed facts, from which consequences may be deduced which can be tested by experiment and special observations, and which it
759-460: A particular characteristic. In entrepreneurial setting, a hypothesis is used to formulate provisional ideas about the attributes of products or business models. The formulated hypothesis is then evaluated, where the hypothesis is proven to be either "true" or "false" through a verifiability - or falsifiability -oriented experiment . Any useful hypothesis will enable predictions by reasoning (including deductive reasoning ). It might predict
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#1732780555466828-438: A problem is commonly referred to as a hypothesis—or, often, as an " educated guess " —because it provides a suggested outcome based on the evidence. However, some scientists reject the term "educated guess" as incorrect. Experimenters may test and reject several hypotheses before solving the problem. According to Schick and Vaughn, researchers weighing up alternative hypotheses may take into consideration: A working hypothesis
897-399: A problem of set theory , was the continuum hypothesis introduced by Cantor in 1878, and in the course of its statement Hilbert mentioned also the need to prove the well-ordering theorem . Zermelo began to work on the problems of set theory under Hilbert's influence and in 1902 published his first work concerning the addition of transfinite cardinals . By that time he had also discovered
966-424: A process beginning with an educated guess or thought. A different meaning of the term hypothesis is used in formal logic , to denote the antecedent of a proposition ; thus in the proposition "If P , then Q ", P denotes the hypothesis (or antecedent); Q can be called a consequent . P is the assumption in a (possibly counterfactual ) What If question. The adjective hypothetical , meaning "having
1035-1085: A scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues. Max Tegmark and Brian Greene have proposed different classification schemes for multiverses and universes. Tegmark's four-level classification consists of Level I: an extension of our universe, Level II: universes with different physical constants, Level III: many-worlds interpretation of quantum mechanics, and Level IV: ultimate ensemble . Brian Greene's nine types of multiverses include quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate. The ideas explore various dimensions of space, physical laws, and mathematical structures to explain
1104-516: A six-sided dice is thrown and that the result of the throw corresponds to observable quantum mechanics. All six possible ways the dice can fall correspond to six different worlds. In the case of the Schrödinger's cat thought experiment, both outcomes would be "real" in at least one "world" . Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than
1173-467: A synthesis. Concepts in Hempel's deductive-nomological model play a key role in the development and testing of hypotheses. Most formal hypotheses connect concepts by specifying the expected relationships between propositions . When a set of hypotheses are grouped together, they become a type of conceptual framework . When a conceptual framework is complex and incorporates causality or explanation, it
1242-402: A theory itself. Normally, scientific hypotheses have the form of a mathematical model . Sometimes, but not always, one can also formulate them as existential statements , stating that some particular instance of the phenomenon under examination has some characteristic and causal explanations, which have the general form of universal statements , stating that every instance of the phenomenon has
1311-464: A time immediately after the Big Bang , although it is still a matter of debate among physicists. Dr. Ranga-Ram Chary, after analyzing the cosmic radiation spectrum , found a signal 4,500 times brighter than it should have been, based on the number of protons and electrons scientists believe existed in the very early universe. This signal—an emission line that arose from the formation of atoms during
1380-450: A useful guide to address problems that are still in a formative phase. In recent years, philosophers of science have tried to integrate the various approaches to evaluating hypotheses, and the scientific method in general, to form a more complete system that integrates the individual concerns of each approach. Notably, Imre Lakatos and Paul Feyerabend , Karl Popper's colleague and student, respectively, have produced novel attempts at such
1449-566: Is a hypothesis that is provisionally accepted as a basis for further research in the hope that a tenable theory will be produced, even if the hypothesis ultimately fails. Like all hypotheses, a working hypothesis is constructed as a statement of expectations, which can be linked to the exploratory research purpose in empirical investigation. Working hypotheses are often used as a conceptual framework in qualitative research. The provisional nature of working hypotheses makes them useful as an organizing device in applied research. Here they act like
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#17327805554661518-518: Is an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification. Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence, it requires
1587-421: Is called " superposition ". In the 1990s, after recent works of fiction about the concept gained popularity, scientific discussions about the multiverse and journal articles about it gained prominence. Around 2010, scientists such as Stephen M. Feeney analyzed Wilkinson Microwave Anisotropy Probe (WMAP) data and claimed to find evidence suggesting that this universe collided with other (parallel) universes in
1656-562: Is definable in purely formal terms (independent of vague human terminology) is also a mathematical structure. For instance, a TOE involving a set of different types of entities (denoted by words, say) and relations between them (denoted by additional words) is nothing but what mathematicians call a set-theoretical model, and one can generally find a formal system that it is a model of. He argues that this "implies that any conceivable parallel universe theory can be described at Level IV" and "subsumes all other ensembles, therefore brings closure to
1725-457: Is generally referred to as a theory. According to noted philosopher of science Carl Gustav Hempel , Hempel provides a useful metaphor that describes the relationship between a conceptual framework and the framework as it is observed and perhaps tested (interpreted framework). "The whole system floats, as it were, above the plane of observation and is anchored to it by rules of interpretation. These might be viewed as strings which are not part of
1794-538: Is one of several mainstream interpretations of quantum mechanics . In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different probability . According to the MWI, each of these possible observations corresponds to a different "world" within the Universal wavefunction , with each world as real as ours. Suppose
1863-440: Is proposed to subject to an extended course of such investigation, with the hope that, even should the hypothesis thus be overthrown, such research may lead to a tenable theory. Ernst Zermelo Ernst Friedrich Ferdinand Zermelo ( / z ɜːr ˈ m ɛ l oʊ / , German: [tsɛɐ̯ˈmeːlo] ; 27 July 1871 – 21 May 1953) was a German logician and mathematician , whose work has major implications for
1932-436: Is static, and time is a simple illusion. Another version of the many-worlds idea is H. Dieter Zeh 's many-minds interpretation . The ultimate mathematical universe hypothesis is Tegmark's own hypothesis. This level considers all universes to be equally real which can be described by different mathematical structures. Tegmark writes: Abstract mathematics is so general that any Theory Of Everything (TOE) which
2001-410: Is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics." The concept of multiple universes, or a multiverse, has been discussed throughout history, including Greek philosophy . It has evolved and has been debated in various fields, including cosmology, physics, and philosophy. Some physicists argue that the multiverse is a philosophical notion rather than
2070-656: The Middle Ages . The American philosopher and psychologist William James used the term "multiverse" in 1895, but in a different context. The concept first appeared in the modern scientific context in the course of the debate between Boltzmann and Zermelo in 1895. In Dublin in 1952, Erwin Schrödinger gave a lecture in which he jocularly warned his audience that what he was about to say might "seem lunatic". He said that when his equations seemed to describe several different histories, these were "not alternatives, but all really happen simultaneously". This sort of duality
2139-543: The University of Berlin , the University of Halle , and the University of Freiburg . He finished his doctorate in 1894 at the University of Berlin, awarded for a dissertation on the calculus of variations ( Untersuchungen zur Variationsrechnung ). Zermelo remained at the University of Berlin, where he was appointed assistant to Planck , under whose guidance he began to study hydrodynamics . In 1897, Zermelo went to
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2208-450: The University of Göttingen , at that time the leading centre for mathematical research in the world, where he completed his habilitation thesis in 1899. In 1910, Zermelo left Göttingen upon being appointed to the chair of mathematics at Zurich University , which he resigned in 1916. He was appointed to an honorary chair at the University of Freiburg in 1926, which he resigned in 1935 because he disapproved of Adolf Hitler 's regime. At
2277-461: The Zermelo's navigation problem is a classic optimal control problem. The problem deals with a boat navigating on a body of water, originating from a point O to a destination point D. The boat is capable of a certain maximum speed, and we want to derive the best possible control to reach D in the least possible time. Without considering external forces such as current and wind, the optimal control
2346-562: The foundations of mathematics . He is known for his role in developing Zermelo–Fraenkel axiomatic set theory and his proof of the well-ordering theorem . Furthermore, his 1929 work on ranking chess players is the first description of a model for pairwise comparison that continues to have a profound impact on various applied fields utilizing this method. Ernst Zermelo graduated from Berlin's Luisenstädtisches Gymnasium (now Heinrich-Schliemann-Oberschule [ de ] ) in 1889. He then studied mathematics , physics and philosophy at
2415-459: The scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous observations that cannot satisfactorily be explained with the available scientific theories. Even though the words "hypothesis" and " theory " are often used interchangeably, a scientific hypothesis is not the same as a scientific theory . A working hypothesis is a provisionally accepted hypothesis proposed for further research in
2484-488: The Ancient Greek Atomists , beginning with Leucippus and Democritus in the 5th century BCE, followed by Epicurus (341–270 BCE) and Lucretius (1st century BCE). In the third century BCE, the philosopher Chrysippus suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time. The concept of multiple universes became more defined in
2553-464: The Multiverse", author and cosmologist Paul Davies offered a variety of arguments that multiverse hypotheses are non-scientific: For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there
2622-406: The alternative hypothesis. The alternative hypothesis, as the name suggests, is the alternative to the null hypothesis: it states that there is some kind of relation. The alternative hypothesis may take several forms, depending on the nature of the hypothesized relation; in particular, it can be two-sided (for example: there is some effect, in a yet unknown direction) or one-sided (the direction of
2691-420: The axiom of choice was a paradigm of non-constructive mathematics. In 1908, Zermelo succeeded in producing an improved proof making use of Dedekind's notion of the "chain" of a set, which became more widely accepted; this was mainly because that same year he also offered an axiomatization of set theory. Zermelo began to axiomatize set theory in 1905; in 1908, he published his results despite his failure to prove
2760-424: The concept of a multiverse or popular multiverse hypotheses include Sabine Hossenfelder , David Gross , Paul Steinhardt , Anna Ijjas, Abraham Loeb , David Spergel , Neil Turok , Viatcheslav Mukhanov , Michael S. Turner , Roger Penrose , George Ellis , Joe Silk , Carlo Rovelli , Adam Frank , Marcelo Gleiser , Jim Baggott and Paul Davies . In his 2003 New York Times opinion piece, "A Brief History of
2829-422: The consistency of his axiomatic system. See the article on Zermelo set theory for an outline of this paper, together with the original axioms, with the original numbering. In 1922, Abraham Fraenkel and Thoralf Skolem independently improved Zermelo's axiom system. The resulting system, now called Zermelo–Fraenkel axioms (ZF), is now the most commonly used system for axiomatic set theory . Proposed in 1931,
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2898-413: The context of the anthropic principle. According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher Anaximander in the sixth century BCE. However, there is debate as to whether he believed in multiple worlds, and if he did, whether those worlds were co-existent or successive. The first to whom we can definitively attribute the concept of innumerable worlds are
2967-426: The criterion of falsifiability or supplemented it with other criteria, such as verifiability (e.g., verificationism ) or coherence (e.g., confirmation holism ). The scientific method involves experimentation to test the ability of some hypothesis to adequately answer the question under investigation. In contrast, unfettered observation is not as likely to raise unexplained issues or open questions in science, as would
3036-441: The data to be tested are already known, the test is invalid. The above procedure is actually dependent on the number of the participants (units or sample size ) that are included in the study. For instance, to avoid having the sample size be too small to reject a null hypothesis, it is recommended that one specify a sufficient sample size from the beginning. It is advisable to define a small, medium and large effect size for each of
3105-560: The distant past. However, a more thorough analysis of data from the WMAP and from the Planck satellite , which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a bubble universe collision. In addition, there was no evidence of any gravitational pull of other universes on ours. In 2015, an astrophysicist may have found evidence of alternate or parallel universes by looking back in time to
3174-592: The end of World War II and at his request, Zermelo was reinstated to his honorary position in Freiburg. In 1900, in the Paris conference of the International Congress of Mathematicians , David Hilbert challenged the mathematical community with his famous Hilbert's problems , a list of 23 unsolved fundamental questions which mathematicians should attack during the coming century. The first of these,
3243-534: The era of recombination—is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. There is a 30% chance that this signal is noise, and not really a signal at all; however, it is also possible that it exists because a parallel universe dumped some of its matter particles into our universe. If additional protons and electrons had been added to our universe during recombination, more atoms would have formed, more photons would have been emitted during their formation, and
3312-441: The existence and interactions of multiple universes. Some other multiverse concepts include twin-world models, cyclic theories, M-theory, and black-hole cosmology . The anthropic principle suggests that the existence of a multitude of universes, each with different physical laws, could explain the asserted appearance of fine-tuning of our own universe for conscious life. The weak anthropic principle posits that we exist in one of
3381-414: The existence of a relation may be assumed. Otherwise, any observed effect may be due to pure chance. In statistical hypothesis testing, two hypotheses are compared. These are called the null hypothesis and the alternative hypothesis . The null hypothesis is the hypothesis that states that there is no relation between the phenomena whose relation is under investigation, or at least not of the form given by
3450-404: The few universes that support life. Debates around Occam's razor and the simplicity of the multiverse versus a single universe arise, with proponents like Max Tegmark arguing that the multiverse is simpler and more elegant. The many-worlds interpretation of quantum mechanics and modal realism , the belief that all possible worlds exist and are as real as our world, are also subjects of debate in
3519-417: The formulation of a crucial experiment to test the hypothesis. A thought experiment might also be used to test the hypothesis. In framing a hypothesis, the investigator must not currently know the outcome of a test or that it remains reasonably under continuing investigation. Only in such cases does the experiment, test or study potentially increase the probability of showing the truth of a hypothesis. If
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#17327805554663588-421: The hierarchy of multiverses, and there cannot be, say, a Level V." Jürgen Schmidhuber , however, says that the set of mathematical structures is not even well-defined and that it admits only universe representations describable by constructive mathematics —that is, computer programs . Schmidhuber explicitly includes universe representations describable by non-halting programs whose output bits converge after
3657-470: The hypothesized relation, positive or negative, is fixed in advance). Conventional significance levels for testing hypotheses (acceptable probabilities of wrongly rejecting a true null hypothesis) are .10, .05, and .01. The significance level for deciding whether the null hypothesis is rejected and the alternative hypothesis is accepted must be determined in advance, before the observations are collected or inspected. If these criteria are determined later, when
3726-431: The idea of the multiverse as a way of explaining the nature of existence . He points out that it ultimately leaves those questions unresolved because it is a metaphysical issue that cannot be resolved by empirical science. He argues that observational testing is at the core of science and should not be abandoned: As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on
3795-509: The inference of a multiverse to explain the apparent fine-tuning of the universe is an example of Inverse Gambler's Fallacy . Stoeger, Ellis, and Kircher note that in a true multiverse theory, "the universes are then completely disjoint and nothing that happens in any one of them is causally linked to what happens in any other one. This lack of any causal connection in such multiverses really places them beyond any scientific support". In May 2020, astrophysicist Ethan Siegel expressed criticism in
3864-431: The intended interpretation usually guides the construction of the theoretician". It is, however, "possible and indeed desirable, for the purposes of logical clarification, to separate the two steps conceptually". When a possible correlation or similar relation between phenomena is investigated, such as whether a proposed remedy is effective in treating a disease, the hypothesis that a relation exists cannot be examined
3933-488: The lack of empirical testability and falsifiability is a major concern, but he is opposed to that line of thinking: Many physicists who talk about the multiverse, especially advocates of the string landscape , do not care much about parallel universes per se . For them, objections to the multiverse as a concept are unimportant. Their theories live or die based on internal consistency and, one hopes, eventual laboratory testing. Ellis says that scientists have proposed
4002-466: The moment of spontaneous symmetry breaking in a Level III multiverse. According to Yasunori Nomura , Raphael Bousso , and Leonard Susskind , this is because global spacetime appearing in the (eternally) inflating multiverse is a redundant concept. This implies that the multiverses of Levels I, II, and III are, in fact, the same thing. This hypothesis is referred to as "Multiverse = Quantum Many Worlds". According to Yasunori Nomura , this quantum multiverse
4071-568: The nature of a hypothesis", or "being assumed to exist as an immediate consequence of a hypothesis", can refer to any of these meanings of the term "hypothesis". In its ancient usage, hypothesis referred to a summary of the plot of a classical drama . The English word hypothesis comes from the ancient Greek word ὑπόθεσις hypothesis whose literal or etymological sense is "putting or placing under" and hence in extended use has many other meanings including "supposition". In Plato 's Meno (86e–87b), Socrates dissects virtue with
4140-492: The nature of science and on the ultimate nature of existence: why we are here. ... In looking at this concept, we need an open mind, though not too open. It is a delicate path to tread. Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is. Philosopher Philip Goff argues that
4209-425: The network but link certain points of the latter with specific places in the plane of observation. By virtue of those interpretative connections, the network can function as a scientific theory." Hypotheses with concepts anchored in the plane of observation are ready to be tested. In "actual scientific practice the process of framing a theoretical structure and of interpreting it are not always sharply separated, since
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#17327805554664278-440: The outcome of an experiment in a laboratory setting or the observation of a phenomenon in nature . The prediction may also invoke statistics and only talk about probabilities. Karl Popper , following others, has argued that a hypothesis must be falsifiable , and that one cannot regard a proposition or theory as scientific if it does not admit the possibility of being shown to be false. Other philosophers of science have rejected
4347-410: The researcher already knows the outcome, it counts as a "consequence" — and the researcher should have already considered this while formulating the hypothesis. If one cannot assess the predictions by observation or by experience , the hypothesis needs to be tested by others providing observations. For example, a new technology or theory might make the necessary experiments feasible. A trial solution to
4416-526: The same physical laws and physical constants . In regard to configurations such as the distribution of matter , almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon , there will eventually be Hubble volumes with similar, and even identical, configurations. Tegmark estimates that an identical volume to ours should be about 10 meters away from us. Given infinite space, there would be an infinite number of Hubble volumes identical to ours in
4485-473: The same leap of faith. George Ellis , writing in August 2011, provided a criticism of the multiverse, and pointed out that it is not a traditional scientific theory. He accepts that the multiverse is thought to exist far beyond the cosmological horizon . He emphasized that it is theorized to be so far away that it is unlikely any evidence will ever be found. Ellis also explained that some theorists do not believe
4554-406: The same way one might examine a proposed new law of nature. In such an investigation, if the tested remedy shows no effect in a few cases, these do not necessarily falsify the hypothesis. Instead, statistical tests are used to determine how likely it is that the overall effect would be observed if the hypothesized relation does not exist. If that likelihood is sufficiently small (e.g., less than 1%),
4623-458: The signature line that arose from all of these emissions would be greatly enhanced. Chary himself is skeptical: Many other regions beyond our observable universe would exist with each such region governed by a different set of physical parameters than the ones we have measured for our universe. Chary also noted: Unusual claims like evidence for alternate universes require a very high burden of proof. The signature that Chary has isolated may be
4692-508: The so-called Russell paradox . In 1904, he succeeded in taking the first step suggested by Hilbert towards the continuum hypothesis when he proved the well-ordering theorem ( every set can be well ordered ). This result brought fame to Zermelo, who was appointed Professor in Göttingen, in 1905. His proof of the well-ordering theorem , based on the powerset axiom and the axiom of choice , was not accepted by all mathematicians, mostly because
4761-405: The universe. This follows directly from the cosmological principle , wherein it is assumed that our Hubble volume is not special or unique. In the eternal inflation theory, which is a variant of the cosmic inflation theory, the multiverse or space as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles (like gas pockets in
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