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In physics, the term “multiverse” refers to certain picture of cosmology. The intended meaning tends to differ between authors and/or remain vague (see below) but broadly what goes with it is the idea that, just as the configuration of matter and energy changes from place to place within our observable universe, clearly, it could be that beyond the observable universe (for immediate or not so immediate notions of “beyond”) eventually even the laws of physics might change from place to place, possibly altogether, but at least as concerns the values of coupling constants or even field-content.
The idea remains hypothetical, naturally, and might ultimately be empty as an idea about physics as opposed to philosophy, but it is arguably in historical continuation with a sequence of famously confirmed insights about cosmology, which led from an archaic picture of a cosmos centered all around the world directly experienced by humans (the continent a disk inside an ocean hung in the center of the heavenly spheres), via the Copernican revolution, then the confirmation of distant galaxies etc., to the realization that the observable universe is immense and varied and our physical place and whereabouts inside it just a random result of chance, subject only to the tautological condition that it allows us to be here at all – known as the “anthropic principle” in these discussions.
Accordingly, one motivation for the idea of the multiverse is as an explanation for apparent ‘fine-tuning’ in the “constants of nature”, in particular concerning the presence of complex life. The thought here is that were the constants/laws of nature to be ever so slightly different from what they are, then the universe would not have been to able to evolve to support complex, intelligent life forms (see SEP: Fine-tuning). But an account which proposes that a vast number of universes exist governed by laws spanning some full range of possibilities removes the surprise that we should live in such a special universe, since any form of life as ourselves could only occur within such a universe.
Another motivation of contemplating a multiverse, even in the face of its ontological elusiveness, comes from attempts of formulating fundamental theories of physics such as quantum gravity or theories of everything, since the motivation and success of such theories depends to some extent on what one thinks they can or should explain or predict, and how much room for chance and chaos there may remain in a truly fundamental theory of physics.
For example, if a fundamental theory of everything were to predict, say, the generations of fundamental particles and the values of the Yukawa couplings in the standard model of particle physics, then these would be fixed – by pure logic, ultimately – to be what they are, and would be the same strictly “everywhere”, thus making obsolete the idea of a multiverse.
Part of the interest in the idea of the multiverse derives from the feeling that this scenario of the standard model of particle physics, with its particular field content and coupling constants, being fixed by pure logic, is no more likely than Johannes Kepler‘s attempt to derive the distances in the solar system from the Platonic solids (the old Music of the Spheres), which today we understand is a confusion of what is fundamental law and what is random initial conditions.
In this sense, the idea of the multiverse is a form of empiricism and in contrast to theoretical aestheticism? such as universal exceptionalism.
On the very largest scales observable in astrophysical experiment, the cosmos is well described by an FRW model with cosmological constant , with plenty of dark matter and with primordial cosmic inflation (called the -CDM “concordance model of cosmology”). A horizon in there and everything on smaller scales is the observable universe.
There has never been a reason to assume that beyond this cosmic horizon visible to us today, the cosmos would not extend further. In fact in the simple FRW models with positive cosmological constant, the spacetime manifold is not a closed manifold and extends indefinitely beyond our observable horizon. This is directly analogous to the ancient mariner who would stand on the deck of his ship and see a few miles of ocean around him. That was the world within his horizon, but there was no telling what lay beyond.
Therefore it is important to distinguish the observable universe from the universe as such. Unfortunately, intellectual laziness tended to ignore this distinction and at some point some people who said “universe” to mean just the stretch of our cosmic horizon felt the need to have a new term for whatever may lie beyond. For better or worse, that new term has become wide-spread these days and is “multiverse”.
The main scientific observation that goes with this is the observation that cosmic inflation – for which there is by now excellent and ever increasing experimental evidence that indeed it happened – does, at least by the simple method which is currently used to model it, naturally predict a large ambient space in which local regions perpetually undergo inflationary expansion – eternal inflation.
Hence what is called multiverse these days might more sanely have been called universe with multiple observable regions or the like, but the term has seen an immense increase in popularity and is apparently here to stay.
Another reason why the old speculation (going back at least to Giordano Bruno) that the observable universe is but one of many variants in a larger ambient cosmos received more empathetic attention in some corners of fundamental physics at the beginning of the 21st century is that it was argued to lead to a plausible picture of the apparent lack of the fundamental constants of nature (such as notably the cosmological constant but also the Yukawa couplings etc.) to follow any discernible law (see also the “hierarchy problem”).
At the same time there are theories of fundamental physics where such apparent “constants” of nature are in fact just the values of “moduli fields”, hence of dynamical fields, whose value however is approximately constant over the observable universe (“moduli stabilization”). Generically Kaluza-Klein theories have this property, and in particular string theory does. This has the remarkable consequence that in such theories the “constants of nature” may dynamically move through their moduli space – hence through the “landscape” of possible sets of laws of nature – just like any other field evolves dynamically throughout spacetime. (See also at string theory FAQ, What does it mean that string theory has a “landscape” of solutions?.)
When the landscape of string theory vacua was (finally) realized to, apparently, have no reason to be “small”, hence when it was realized that indeed the theory alone did not constrain its moduli fields but allowed all kinds of values for them, then it was argued (first in (Schellekens 98)) that combining this apparent embarrassment of a surplus of theoretically admissible “kinds of universes” with the apparent embarrassment of a surplus of cosmically inflating observable universes together yields a plausible picture: each inflating “bubble” in eternal inflation randomly has its moduli fields determined by the vacuum fluctuation that gives rise to the inflationary process, and hence literally the moduli space of “constants/laws of nature” becomes inhabited by a kind of stochastic process. The particular value of the cosmological constant or the Yukawa couplings which we observe would be just as random (or just as “anthropically” constrained) as, say, the number, nature and distance of the planets in the solar system.
Whatever the status of this story, it is to a large part responsible for the renewed interest in the idea of the “multiverse”.
One must beware here that, while Kaluza-Klein theory/string theory constitute concrete theoretical frameworks realizing the concept of dynamically evolving “constants of nature”, the landscape of string theory vacua is poorly understood in detail, even by the standards of non-rigorous physics-style arguments; and, worse, string theory itself is fully understood only perturbatively, while its full microscopic non-perturbative completion (working title: “M-theory”, “F-theory”), despite a large and tight network of hints, remains an open problem, for the time being. But it is exactly non-perturbative effects that may be expected to be relevant for true understanding of the landscape of string theory vacua, by possibly ruling out regions which look consistent only in perturbation theory. Hence for all that is known, universal exceptionalism instead of a multiverse remains a possibility in string theory. Only improved mathematical understanding of the theory will eventually be able to tell.
From (Ellis 11):
Similar claims [about a multiverse] have been made since antiquity by many cultures. What is new is the assertion that the multiverse is a scientific theory, with all that implies about being mathematically rigorous and experimentally testable. I am skeptical about this claim. I do not believe the existence of those other universes has been proved—or ever could be. Proponents of the multiverse, as well as greatly enlarging our conception of physical reality, are implicitly redefining what is meant by ‘science.’—pg 39
For a cosmologist, the basic problem with all multiverse proposals is the presence of a cosmic visual horizon. The horizon is the limit to how far away we can see, because signals traveling toward us at the speed of light (which is finite) have not had time since the beginning of the universe to reach us from farther out. All the parallel universes lie outside our horizon and remain beyond our capacity to see, now or ever, no matter how technology evolves. In fact, they are to far away to have had any influence on our universe whatsoever. That is why none of the claims made by multiverse enthusiasts can be directly substantiated. —pg 40-41
A remarkable fact about our universe is that physical constants have just the right values needed to allow for complex structures, including living things. Steven Weinberg, Martin Rees, Leonard Susskind and others contend that an exotic multiverse provides a tidy explanation for this apparent coincidence: if all possible values occur in a large enough collection of universes, then viable ones for life will surely be found somewhere. This reasoning has been applied, in particular, to explanation the density of the dark energy that is speeding up the expansion of the universe today. I agree that the multiverse is a possible valid explanation for the value of this density; arguably, it is the only scientifically based option we have right now. But we have no hope of testing it observationally. —pg 42
All in all, the case for the multiverse is inconclusive. The basic reason is the extreme flexibility of the proposal: it is more a concept than well-defined theory. Most proposals involve a patchwork of different ideas rather than a coherent whole. The basic mechanism for eternal inflation does not itself cause physics to be different in each domain in a multiverse; for that, it needs to be coupled to another speculative theory. Although they can be fitted together, there is nothing inevitable about it. … Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is. —pg 43
As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on 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.
Vaguely related is the possible worlds semantics in modal logic for the formalization of metaphyisical necessity and possibility. For instance (Kripke 80) is much concerned with the logic of “possible worlds”.
Not really related but arguably somewhat analogous is, in the foundations of mathematics, the set-theoretic multiverse of different set theories, and similarly the 2-category Topos of toposes, hence of constructive foundations of mathematics (see at topos theory for more on this).
Apparently the first mentioning of the idea that a multiverse due to eternal inflation together with the landscape of string theory vacua provides a plausible story for the nature of “constants of nature” is due to
reproduced as
More recent account of how the relevant community pictures the situation includes
Raphael Bousso, The State of the Multiverse: The String Landscape, the Cosmological Constant, and the Arrow of Time, 2011 (pdf)
Brian Greene, Is our universe the only universe?, talk at TED2012 (web)
Andrei Linde, A brief history of the multiverse (arXiv:1512.01203)
Further and popular expositions include
For more references see at eternal cosmic inflation.
Of course a bunch of links are also here:
though the discussion there seems to show an undue preference of some points of view.
“Possible worlds” in the context of logic are discussed in modal logic, notably in
Last revised on May 19, 2019 at 10:03:00. See the history of this page for a list of all contributions to it.