Special Issue "The Multiverse"

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (31 July 2019).

Special Issue Editors

Dr. Ana Alonso-Serrano
E-Mail Website
Guest Editor
Max Planck Institute for Gravitational Physics (Albert-Einstein-Institute)Potsdam, Brandenburg, Germany
Interests: High Energy Physics; General Relativity; Quantum Field Theory; Special and General Relativity; Quantum Physics; Cosmology; Gravitational Physics; Fundamental Physics; Quantum Cosmology; Black Holes; Universe; Multiverse
Special Issues and Collections in MDPI journals
Prof. Dr. Mariusz P. Dąbrowski
E-Mail Website
Guest Editor
Cosmology Group, Institute of Physics, University of Szczecin, Wielkopolska 15, 70-451 Szczecin, Poland
Tel. +48914441248
Interests: general relativity and cosmology; alternative gravities; superstring and brane cosmology; quantum cosmology; standard and non-standard cosmological singularities; varying constants (theory, observations, experiment); superstring landscape and the multiverse
Special Issues and Collections in MDPI journals
Prof. Dr. Thomas Naumann
E-Mail Website
Guest Editor
Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
Interests: Particle physics and its connections to astrophysics and cosmology, multiverse, anthropic principle

Special Issue Information

Dear Colleagues,

The idea of the Multiverse, as a collection of possible universes, has entered the area of physics and cosmology for good. Max Tegmark distinguished four levels of this very notion, starting from a collection of standard inflationary patches of space-time to finally defining it as a collection of all possible mathematical entities. The term 'Multiverse' was first suggested by a philosopher—William James—who said ''Visible nature is all plasticity and indifference, A MULTIVERSE, as one might call it, and not a universe'' (''Is life worth living?'', International Journal of Ethics 6, 10 (1895)).

The diversity of possible physical shapes of a universe within the multiverse can be interpreted in terms of diversity of possible ways to choose physical parameters and can be related to the issue of varying physical constants and varying physical laws. One approach is given by the superstring theory which led physicists to an idea of superstring landscape or many ways to choose the vacua after the symmetry breaking. An important idea related to the Multiverse is the Anthropic Principle which, despite being in some sense tautological, can give some insight and possible constraints onto the nature of the physics we experience here in 'Our Universe' whatever it is. 

Anyway, one can consider the studies related to the concept of the Multiverse as a new revolution that can change the current paradigm in cosmology. It can be understood as the next step in the Copernican transit, where our habitat has lost relevance gradually as unique or special. As it was already mentioned, the notion of the multiverse emerges naturally from some developments in cosmology and particle physics. It is not a theory by itself, so, there is not a closed scenario or definition of it. It always depends on the definition of what we mean under the notion of the universe (cf. Max's Tegmark discussion).

However, operationally it seems that the consideration of the idea of the multiverse can provide a solution to several open problems in physics. So we are interested in the different approaches and proposals regarding this issue. The most important point in order to make it a physical theory is the possibility to falsify this idea by some observational or experimental data. This seems to be the biggest challenge of the multiverse hypothesis.

Hoping it will serve as a basic and updated reference, this Special Issue will cover all current research avenues on the exciting track to the Multiverse starting from philosophy, throughout the theory, to its possible observational verification. The following topics will be the core of the volume:

general ideas, history or philosophy, Multiverse from string theory and pocket universes, cyclic universes, chaotic inflation and bubble universes, mediocrity principle and probability theory, classical and quantum multiverse, varying constants and alternative theories, observability and falsifiability of the idea of the Multiverse and related topics.

We hope you will enjoy writing and later reading the collected articles.

Yours sincerely,

Dr. Ana Alonso-Serrano
Prof. Dr. Mariusz P. Dąbrowski
Prof. Dr. Thomas Naumann
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Published Papers (10 papers)

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Research

Jump to: Review

Open AccessArticle
Conceptual Challenges on the Road to the Multiverse
Universe 2019, 5(10), 212; https://doi.org/10.3390/universe5100212 - 10 Oct 2019
Abstract
The current debate about a possible change of paradigm from a single universe to a multiverse scenario could have deep implications on our view of cosmology and of science in general. These implications therefore deserve to be analyzed from a fundamental conceptual level. [...] Read more.
The current debate about a possible change of paradigm from a single universe to a multiverse scenario could have deep implications on our view of cosmology and of science in general. These implications therefore deserve to be analyzed from a fundamental conceptual level. We briefly review the different multiverse ideas, both historically and within contemporary physics. We then discuss several positions within philosophy of science with regard to scientific progress, and apply these to the multiverse debate. Finally, we construct some key concepts for a physical multiverse scenario and discuss the challenges this scenario has to deal with in order to provide a solid, testable theory. Full article
(This article belongs to the Special Issue The Multiverse)
Open AccessArticle
Possible Origins and Properties of an Expanding, Dark Energy Providing Dark Multiverse
Universe 2019, 5(8), 178; https://doi.org/10.3390/universe5080178 - 24 Jul 2019
Abstract
The model of a multiverse is advanced, which endows subuniverses like ours with space and time and imparts to their matter all information about the physical laws. It expands driven by dark energy (DE), which is felt in our Universe (U) by mass [...] Read more.
The model of a multiverse is advanced, which endows subuniverses like ours with space and time and imparts to their matter all information about the physical laws. It expands driven by dark energy (DE), which is felt in our Universe (U) by mass input and expansion–acceleration. This dark multiverse (DM) owes its origin to a creatio ex nihilo, described in previous work by a tunneling process in quasi-classical approximation. Here, this origin is treated again in the context of quantum gravity (QG) by solving a Wheeler de Witt (WdW) equation. Different than usual, the minisuperspace employed is not spanned by the expansion parameter a but by the volume 2 π 2 a 3 . This not only modifies the WdW-equation, but also probabilities and solution properties. A “soft entry” can serve the same purpose as a tunneling process. Sections of solutions are identified, which show qualitative features of a volume-quantisation, albeit without a stringent quantitative definition. A timeless, spatially four-dimensional primordial state is also treated, modifying a state proposed by Hartle and Hawking (HH). For the later classical evolution, elaborated in earlier papers, a wave function is calculated and linked to the solutions for the quantum regime (QR). It is interpreted to mean that the expansion of the DM proceeds in submicroscopic leaps. Further results are also derived for the classical solutions. Full article
(This article belongs to the Special Issue The Multiverse)
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Open AccessArticle
Multiverse Predictions for Habitability: Fraction of Life That Develops Intelligence
Universe 2019, 5(7), 175; https://doi.org/10.3390/universe5070175 - 17 Jul 2019
Cited by 3
Abstract
Do mass extinctions affect the development of intelligence? If so, we may expect to be in a universe that is exceptionally placid. We consider the effects of impacts, supervolcanoes, global glaciations, and nearby gamma ray bursts, and how their rates depend on fundamental [...] Read more.
Do mass extinctions affect the development of intelligence? If so, we may expect to be in a universe that is exceptionally placid. We consider the effects of impacts, supervolcanoes, global glaciations, and nearby gamma ray bursts, and how their rates depend on fundamental constants. It is interesting that despite the very disparate nature of these processes, each occurs on timescales of 100 Myr-Gyr. We argue that this is due to a selection effect that favors both tranquil locales within our universe, as well as tranquil universes. Taking gamma ray bursts to be the sole driver of mass extinctions is disfavored in multiverse scenarios, as the rate is much lower for different values of the fundamental constants. In contrast, geological causes of extinction are very compatible with the multiverse. Various frameworks for the effects of extinctions are investigated, and the intermediate disturbance hypothesis is found to be most compatible with the multiverse. Full article
(This article belongs to the Special Issue The Multiverse)
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Open AccessArticle
Multiverse Predictions for Habitability: Fraction of Planets that Develop Life
Universe 2019, 5(7), 171; https://doi.org/10.3390/universe5070171 - 14 Jul 2019
Cited by 3
Abstract
In a multiverse context, determining the probability of being in our particular universe depends on estimating its overall habitability compared to other universes with different values of the fundamental constants. One of the most important factors in determining this is the fraction of [...] Read more.
In a multiverse context, determining the probability of being in our particular universe depends on estimating its overall habitability compared to other universes with different values of the fundamental constants. One of the most important factors in determining this is the fraction of planets that actually develop life, and how this depends on planetary conditions. Many proposed possibilities for this are incompatible with the multiverse: if the emergence of life depends on the lifetime of its host star, the size of the habitable planet, or the amount of material processed, the chances of being in our universe would be very low. If the emergence of life depends on the entropy absorbed by the planet, however, our position in this universe is very natural. Several proposed models for the subsequent development of life, including the hard step model and several planetary oxygenation models, are also shown to be incompatible with the multiverse. If any of these are observed to play a large role in determining the distribution of life throughout our universe, the multiverse hypothesis will be ruled out to high significance. Full article
(This article belongs to the Special Issue The Multiverse)
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Open AccessArticle
Multiverse Predictions for Habitability: Number of Potentially Habitable Planets
Universe 2019, 5(6), 157; https://doi.org/10.3390/universe5060157 - 25 Jun 2019
Cited by 3
Abstract
How good is our universe at making habitable planets? The answer to this depends on which factors are important for life: Does a planet need to be Earth mass? Does it need to be inside the temperate zone? are systems with hot Jupiters [...] Read more.
How good is our universe at making habitable planets? The answer to this depends on which factors are important for life: Does a planet need to be Earth mass? Does it need to be inside the temperate zone? are systems with hot Jupiters habitable? Here, we adopt different stances on the importance of each of these criteria to determine their effects on the probabilities of measuring the observed values of several physical constants. We find that the presence of planets is a generic feature throughout the multiverse, and for the most part conditioning on their particular properties does not alter our conclusions much. We find conflict with multiverse expectations if planetary size is important and it is found to be uncorrelated with stellar mass, or the mass distribution is too steep. The existence of a temperate circumstellar zone places tight lower bounds on the fine structure constant and electron to proton mass ratio. Full article
(This article belongs to the Special Issue The Multiverse)
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Open AccessArticle
Multiverse Predictions for Habitability: The Number of Stars and Their Properties
Universe 2019, 5(6), 149; https://doi.org/10.3390/universe5060149 - 13 Jun 2019
Cited by 5
Abstract
In a multiverse setting, we expect to be situated in a universe that is exceptionally good at producing life. Though the conditions for what life needs to arise and thrive are currently unknown, many will be tested in the coming decades. Here we [...] Read more.
In a multiverse setting, we expect to be situated in a universe that is exceptionally good at producing life. Though the conditions for what life needs to arise and thrive are currently unknown, many will be tested in the coming decades. Here we investigate several different habitability criteria, and their influence on multiverse expectations: Does complex life need photosynthesis? Is there a minimum timescale necessary for development? Can life arise on tidally locked planets? Are convective stars habitable? Variously adopting different stances on each of these criteria can alter whether our observed values of the fine structure constant, the electron to proton mass ratio, and the strength of gravity are typical to high significance. This serves as a way of generating predictions for the requirements of life that can be tested with future observations, any of which could falsify the multiverse scenario. Full article
(This article belongs to the Special Issue The Multiverse)
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Open AccessArticle
Multiverse—Too Much or Not Enough?
Universe 2019, 5(5), 113; https://doi.org/10.3390/universe5050113 - 11 May 2019
Cited by 2
Abstract
The aim of this essay is to look at the idea of the multiverse—not so much from the standpoint of physics or cosmology, but rather from a philosophical perspective. The modern story of the multiverse began with Leibniz. Although he treated “other worlds” [...] Read more.
The aim of this essay is to look at the idea of the multiverse—not so much from the standpoint of physics or cosmology, but rather from a philosophical perspective. The modern story of the multiverse began with Leibniz. Although he treated “other worlds” as mere possibilities, they played an important role in his logic. In a somewhat similar manner, the practice of cosmology presupposes a consideration of an infinite number of universes, each being represented by a solution to Einstein’s equations. This approach prepared the way to the consideration of “other universes” which actually exist, first as an auxiliary concept in discussing the so-called anthropic principle, and then as real universes, the existence of which were supposed to solve some cosmological conundrums. From the point of view of the philosophy of science, the question is: Could the explanatory power of a multiverse ideology compensate for the relaxation of empirical control over so many directly unobservable entities? It is no surprise that appealing to a possibly infinite number of “other universes” in order to explain some regularities in our world would seem “too much” for a self-disciplined philosopher. With no strict empirical control at our disposal, it is logic that must be our guide. Also, what if logic changes from one world to another in the multiverse? Such a possibility is suggested by the category theory. From this point of view, our present concepts of the multiverse are certainly “not enough”. Should this be read as a warning that the learned imagination can lead us too far into the realms of mere possibilities? Full article
(This article belongs to the Special Issue The Multiverse)

Review

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Open AccessReview
The String Theory Landscape
Universe 2019, 5(7), 176; https://doi.org/10.3390/universe5070176 - 20 Jul 2019
Cited by 2
Abstract
String/M theory is formulated in 10 and 11 space-time dimensions; in order to describe our universe, we must postulate that six or seven of the spatial dimensions form a small compact manifold. In 1985, Candelas et al. showed that by taking the extra [...] Read more.
String/M theory is formulated in 10 and 11 space-time dimensions; in order to describe our universe, we must postulate that six or seven of the spatial dimensions form a small compact manifold. In 1985, Candelas et al. showed that by taking the extra dimensions to be a Calabi–Yau manifold, one could obtain the grand unified theories which had previously been postulated as extensions of the Standard Model of particle physics. Over the years since, many more such compactifications were found. In the early 2000s, progress in nonperturbative string theory enabled computing the approximate effective potential for many compactifications, and it was found that they have metastable local minima with small cosmological constant. Thus, string/M theory appears to have many vacuum configurations which could describe our universe. By combining results on these vacua with a measure factor derived using the theory of eternal inflation, one gets a theoretical framework which realizes earlier ideas about the multiverse, including the anthropic solution to the cosmological constant problem. We review these arguments and some of the criticisms, with their implications for the prediction of low energy supersymmetry and hidden matter sectors, as well as recent work on a variation on eternal inflation theory motivated by computational complexity considerations. Full article
(This article belongs to the Special Issue The Multiverse)
Open AccessReview
Anthropic Selection of Physical Constants, Quantum Entanglement, and the Multiverse Falsifiability
Universe 2019, 5(7), 172; https://doi.org/10.3390/universe5070172 - 14 Jul 2019
Abstract
This paper evaluates some important aspects of the multiverse concept. Firstly, the most realistic opportunity for it which is the spacetime variability of the physical constants and may deliver worlds with different physics, hopefully fulfilling the conditions of the anthropic principles. Then, more [...] Read more.
This paper evaluates some important aspects of the multiverse concept. Firstly, the most realistic opportunity for it which is the spacetime variability of the physical constants and may deliver worlds with different physics, hopefully fulfilling the conditions of the anthropic principles. Then, more esoteric versions of the multiverse being the realisation of some abstract mathematics or even logic (cf. paper by M. Heller in this volume). Finally, it evaluates the big challenge of getting any signal from “other universes” using recent achievements of the quantum theory. Full article
(This article belongs to the Special Issue The Multiverse)
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Open AccessReview
Time Reversal Symmetry in Cosmology and the Creation of a Universe–Antiuniverse Pair
Universe 2019, 5(6), 150; https://doi.org/10.3390/universe5060150 - 13 Jun 2019
Cited by 1
Abstract
The classical evolution of the universe can be seen as a parametrised worldline of the minisuperspace, with the time variable t being the parameter that parametrises the worldline. The time reversal symmetry of the field equations implies that for any positive oriented solution [...] Read more.
The classical evolution of the universe can be seen as a parametrised worldline of the minisuperspace, with the time variable t being the parameter that parametrises the worldline. The time reversal symmetry of the field equations implies that for any positive oriented solution there can be a symmetric negative oriented one that, in terms of the same time variable, respectively represent an expanding and a contracting universe. However, the choice of the time variable induced by the correct value of the Schrödinger equation in the two universes makes it so that their physical time variables can be reversely related. In that case, the two universes would both be expanding universes from the perspective of their internal inhabitants, who identify matter with the particles that move in their spacetimes and antimatter with the particles that move in the time reversely symmetric universe. If the assumptions considered are consistent with a realistic scenario of our universe, the creation of a universe–antiuniverse pair might explain two main and related problems in cosmology: the time asymmetry and the primordial matter–antimatter asymmetry of our universe. Full article
(This article belongs to the Special Issue The Multiverse)
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