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What is the multiverse, and what is its significance?
David H. Bailey
Updated 27 November 2022 (c) 2022
The "multiverse" is a term for the totality of physical reality, extending beyond the observable universe to a larger realm that encompasses other regions. It is curious that the term "multiverse" was not actually invented by modern physicists, astronomers or cosmologists, but instead was coined by American philosopher William James in 1895 [James1895, pg. 43], although his usage of the term was quite different than that of modern scientists.
Proposed levels of the multiverse
Various contemporary authors use different terms and distinguish different realms. Physicist-cosmologist Paul Davies frames the discussion with these terms [Davies2007, pg. 31-32]:
Max Tegmark, in his 2014 book Our Mathematical Universe: My Quest for the Ultimate Nature of Reality, classifies the multiverse as follows [Tegmark2014]:
- Observed universe: This is all of space and its contents out as far as our instruments can currently probe.
- Observable universe: Everything within the "horizon" of what is visible from our vantage point looking around 13.7 billion light years (since the big bang was 13.7 billion years ago). This nearly coincides with the observed universe.
- Entire universe: This includes all space within and beyond our horizon and its contents.
- Pocket universe: This is the region of space out as far as it resembles the observable universe today, recognizing that there may be regions (other pocket universes) with different laws than those that prevail in our universe.
- Multiverse: This is the collection of all pocket universes (possibly infinite in number) plus "gaps" between them. Some authors, such as Susskind, prefer the term "megaverse."
Brian Greene, in his 2011 book The Hidden Reality, sums up the numerous proposals for a multiverse as follows (including some that are not mentioned above) [Greene2011, pg. 309]:
- Level I: The universe external to ours, as a result of the inflation at the big bang.
- Level II: Pocket universes, the result of "eternal inflation."
- Level III: The bifurcating ensemble of universes, as a consequence of the "many worlds" formulation of quantum mechanics.
- Level IV: Tegmark's own notion of all mathematical structures, which he hypothesizes actually exist and constitute the irreducible "stuff" of existence.
Greene concludes his analysis not by endorsing any (much less all) of these lines of thought, but by simply urging his readers to recognize that ultimate reality might be much broader and more exotic than we have heretofore imagined: "It's only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality." [Greene2011, pg. 322].
- The "quilted multiverse" -- Greene's name for Davies' collection of "pocket universes".
- The "inflationary multiverse" -- Greene's name for the Guth-Linde collection of universes spawned in the inflation era of the very early universe.
- The "brane multiverse" -- a higher-dimensional expanse populated by other "branes" as defined in string theory.
- The "cyclic multiverse" -- a theorized collection of universes, parallel in time, resulting from collisions between branes.
- The "landscape multiverse" -- the collection of universes resulting from the huge number of distinct possible shapes (topology) of the universe's fundamental structure -- see above.
- The "quantum multiverse" -- the vast ensemble of branching parallel universes suggested by the "many worlds" interpretation of quantum theory.
- The "holographic multiverse" -- the observation, stemming from the "holographic principle" (see [Greene2011, pg. 238-273]), that our universe is mirrored by phenomena taking place on a distant bounding surface.
- The "simulated multiverse" -- a collection of universes that potentially are created as simulations running inside futuristic super-powerful computer systems.
- The "ultimate multiverse" -- the suggestion by Tegmark and others that every set of mathematical equations describing a possible universe is actually realized.
Origins of the multiverse hypothesis
The multiverse has its roots in two central paradoxes of cosmology that had been noted in the 1960s:
In the early 1980s by Alan Guth hypothesized that a phenomenon known as "false vacuum" resulted in an "inflationary" phase of the very early universe, wherein the fabric of space exploded by the stupendous factor of roughly 1030 [Guth1997, pg. 193]. This theory, in a single stroke, explained the two paradoxes above, and much more. After several decades of refinement and elaboration, cosmologists now accept it as a strongly established theory. But one curious corollary of the inflationary cosmology is that the full universe created at the big bang is some 1023 times larger than our observable universe! [Guth1997, pg. 186].
- Flatness problem. Cosmologists have known for many years that the expansion of the universe since the big bang is related to the "omega" constant -- the ratio between the actual mass density of the universe to the critical density (the mass density at which the universe is perfectly "flat"). Back in the 1960s, it was recognized that this "omega" constant had to be exceedingly close to one -- if it was very slightly lower, the universe would have dispersed too rapidly for stars and galaxies to have formed, but if it were very slightly larger, the universe would have long ago recollapsed in a big brunch. Calculations had established, in fact, that omega must be 1.0 within one part in 1014 [Guth1997, pg. 25].
- Horizon problem. Cosmologists had also noted for many years that different regions of space, which have been causally disjoint since the big bang (since no physical force can travel faster than the speed of light), appear to be essentially indistinguishable in nature. For example, the intensity of the cosmic microwave background, first discovered by Penzias and Wilson in 1964, was known to be isotropic (uniform in all directions) to within one part in 100,000. But how can regions on opposite sides of the universe be in such close coordination, since no physical force, not even light rays, could have traversed the distance between them since the big bang?
More recently, Andrei Linde of Stanford University and Alex Vilenkin of Tufts University observed that one weakness in Guth's theory, namely how inflation started and stopped, could be reasonably explained by quantum fluctuations of a certain type. But one implication of this notion is that space is generated by a process of "chaotic eternal inflation" that has no beginning or end. As Linde describes it, "The whole process ... can be considered as an infinite chain reaction of creation and self-reproduction which has no end and which may have no beginning." [Susskind2005, pg. 81]. Thus what we have been calling the "universe" is just one "bubble" or "pocket universe" amid an enormous ensemble of pocket universes.
What's more, scientists have been led to the notion of a multiverse (also known as "landscape" in this context) from theoretical physics, in particular from "string theory," which is currently the leading candidate for a "theory of everything" that encompasses all known physical laws. For nearly 30 years, string theorists have been exploring the notion that all physical phenomena are, at the lowest level of reality, tiny vibrating strings and membranes -- roughly 10-34cm in size, or vastly smaller even than a proton. What's more, these strings or membranes live in a 10-dimensional space, not the 3-dimensional space that we are accustomed to, with other dimensions rolled up into tiny tubes. According to Brian Greene, author of a widely read semi-popular exposition of string theory, the principal advantage of string theory is that appears to neatly represent all known physical forces, including gravity (which had been excluded in more conventional theories) in one elegant package [Greene2003; Greene2011]. The original dream of string theory was that theorists could eventually derive a single, unique theory, from which all aspects of our current physical laws, including various constants such as the speed of light and the strength of gravitational attraction, could be deduced. Instead, recent research in the field has led to an enormous ensemble of possible universe designs, which by one reckoning number 10500.
In short, both cosmological and theoretical work is leading to a very large "multiverse," and so researchers are led to consider that the reason that our universe is so remarkably well-suited for life is merely an anthropic-principle-based selection effect -- if it were not so finely tuned for life, we wouldn't be here discussing the topic.
Assessments of the multiverse, pro and con
Needless to say, the various multiverse proposals have their detractors. With regards to the multiverse proposals suggested by string theory, Peter Woit writes that "any further progress toward understanding the most fundamental constituents of the universe will require physicists to abandon the now ossified ideology of supersymmetry and superstring theory that has dominated the last two decades" [Woit2006, pg. 264].
In a 2006 work, Lee Smolin wrote [Smolin2006, pg. 352]:
We physicists need to confront the crisis facing us. A scientific theory [the string theory/multiverse] that makes no predictions and therefore is not subject to experiment can never fail, but such a theory can never succeed either, as long as science stands for knowledge gained from rational argument borne out by evidence. There needs to be an honest evaluation of the wisdom of sticking to a research program that has failed after decades to find grounding in either experimental results or precise mathematical formulation. String theorists need to face the possibility that they will turn out to have been wrong and others right.
More recently, Smolin was even more explicit [Smolin2015]:
Cosmology is in crisis. Recent experiments have given us an increasingly precise narrative of the history of our universe, but attempts to interpret the data have led to a picture of a "preposterous universe" that eludes explanation in the terms familiar to scientists. ...
As a result, some cosmologists suggest that there is not one universe, but an infinite number, with a huge variety of properties: the multiverse. There are an infinite number of universes in the collection that are like our universe and an infinite number that are not. But the ratio of infinity to infinity is undefined, and can be made into anything the theorist wants. Thus the multiverse theory has difficulty making any firm predictions and threatens to take us out of the realm of science.
These other universes are unobservable and because chance dictates the random distribution of properties across universes, positing the existence of a multiverse does not let us deduce anything about our universe beyond what we already know. As attractive as the idea may seem, it is basically a sleight of hand, which converts an explanatory failure into an apparent explanatory success. The success is empty because anything that might be observed about our universe could be explained as something that must, by chance, happen somewhere in the multiverse.
Similarly, Jim Baggott rather bluntly writes [Baggott2013]:
I reject the weak anthropic principle because it is simply empty of scientific content. It adds absolutely nothing to the debate. And yet it is used by some contemporary theorists to provide a rather facile logic, a veneer to deflect the fact that multiverse theories themselves are not scientific. Anthropic reasoning is the last refuge of theorists desperate to find a way to justify and defend their positions.
Here are the positions of some other leading scientists on the multiverse and the anthropic principle that is often invoked in conjunction with it:
The latest physics findings, the multiverse, cosmic coincidences and the anthropic principle are nicely summarized in two recent Quanta Magazine articles [Wolchover2013; Wolchover2014].
- Paul Davies: Davies, a leading physicist, notes that the multiverse represents an inconceivably flagrant violation of Occam's razor -- postulating an enormous ensemble of essentially unobservable universes, just to explain our own. What's more, if the multiverse exists, then not only would universes like ours exist, but also vastly more universes where advanced technological civilizations acquire the power to simulate universes like ours on computer. Thus our entire universe, including all "intelligent" residents, are merely avatars in some computer simulation. In that case, how can we possibly take the "laws of nature" seriously? [Davies2007, pg. 179-185].
- George F. R. Ellis: In a August 2011 feature article in Scientific American, Ellis addresses several multiverse proposals, and then concludes "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 a well-defined theory. ... The challenge I pose to the multiverse proponents is: can you prove that unseeable parallel universes are vital to explain the world we do see? And is the link essential and inescapable?" [Ellis2011].
- David Gross: As a leading string theorist, he invokes Winston Churchill in urging fellow researchers to "Never, ever, ever, ever, ever, ever, ever, ever give up" in seeking a single, compelling theory that eliminates the need for anthropic/multiverse arguments [Susskind2005, pg. 355].
- Stephen Hawking: In a 1999 lecture, Hawking declared, "I will describe what I see as the framework for quantum cosmology, on the basis of M theory [one formulation of string theory]. I shall adopt the no boundary proposal, and shall argue that the Anthropic Principle is essential, if one is to pick out a solution to represent our universe, from the whole zoo of solutions allowed by M theory." [Susskind2005, pg. 353].
- Andrei Linde: "Those who dislike anthropic principles are simply in denial. This principle is not a universal weapon, but a useful tool, which allows us to concentrate on the fundamental problems of physics by separating them from the purely environmental problems, which may have an anthropic solution. One may hate the Anthropic Principle or love it, but I bet that eventually everyone is going to use it." [Susskind2005, pg. 353].
- Juan Maldacena: Maldacena remarked, "I hope [the multiverse-anthropic principle] isn't true." However, when asked whether he saw any hope in the other direction, he answered, "No, I'm afraid I don't." [Susskind2005, pg. 350].
- Joseph Polchinski: Polchinski is one of the leading researchers in string theory, but he sees no alternative to the multiverse-anthropic view [Susskind2005, pg. 350].
- Paul Steinhardt: "I consider this approach to be extremely dangerous for two reasons. First, it relies on complex assumptions about physical conditions far beyond the range of conceivable observation so it is not scientifically verifiable. Secondly, I think it leads inevitably to a depressing end to science. What is the point of exploring further the randomly chosen physical properties in our tiny corner of the multiverse if most of the multiverse is so different. I think it is far too early to be so desperate. This is a dangerous idea that I am simply unwilling to contemplate." [Steinhardt2006].
- Leonard Susskind: "The fact that [the cosmological constant] is not absent is a cataclysm for physicists, and the only way that we know how to make any sense of it is through the reviled and despised Anthropic Principle." [Susskind2005, pg. 22].
- Gerard 't Hooft: 't Hooft, in response to a query by Susskind, wrote: "Nobody could really explain to me what it means that string theory has 10100 vacuum states. Before you say such a thing you must first give a rigorous definition on what string theory is, and we haven't got such a definition. Or was it 10500 vacua, or 1010000000000? As long as such 'details' are still up in the air, I feel extremely uncomfortable with the anthropic argument. ... However, some form of anthropic principle I cannot rule out." [Susskind2005, pg. 350].
- Max Tegmark: As mentioned above, Tegmark not only endorses the multiverse suggested by others, but also goes further to propose that the multiverse ultimately consists of all logically consistent mathematical structures, which actually exist, although only a minuscule fraction contain sentient observers. In other words, Tegmark definitely endorses both the multiverse and the anthropic principle [Tegmark2014].
- Steven Weinberg: Wienberg is very reluctant to accept these theories: "For what it is worth, I hope that [the multiverse-anthropic view] is not the case. As a theoretical physicist, I would like to see us able to make precise predictions, not vague statements that certain constants have to be in a range that is more or less favorable to life. I hope that string theory really will provide a basis for a final theory and that this theory will turn out to have enough predictive power to be able to prescribe values for all the constants of nature including the cosmological constant. We shall see." [Weinberg1993, pg. 229].