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A multiverse was a grouping of parallel universes. The omniverse was comprised of every multiverse. Esterath told the Eighth Doctor that in all his past travels across dimensional planes he had never left his own multiversal realm. Having been allowed to experience the lives as his alternate selves, the Doctor realised that together they comprised one being. (COMIC: The Glorious Dead) It was also known as reality the empty spaces between the universes was known as the Void with the parallel universes arranged together with the Void acting as the space inbetween. (TV: Army of Ghosts)

When the multiverse was young, the Sentients, the Modulars, the Clockworks, the Recursives, the Binaries and many more species were all allies. (PROSE: Elementary, My Dear Sheila) The people of the Clockworks were responsible for maintaining the basic structure of the multiverse. (PROSE: The Blue Angel)

The Celestial Toymaker explained that he existed in the universe because all beings in the multiverse dreamed, and he shaped those dreams lest they become stale. (PROSE: Divided Loyalties)

Iris Wildthyme's TARDIS could travel through the multiverse (AUDIO: Wildthyme at Large) and the Axis was capable of allowing an individual to travel through the multiverse via portals that led to different timelines. (AUDIO: Reborn)

Anubis attempted to use the Circle of Transcendence to ascend to a higher dimension to join the rest of the Osirans, but the technology maintaining the Circle had been degraded too much, and the Doctor claimed that Anubis' ascension would collapse the Circle and destroy not just the Multiverse, but multiple other Multiverses as well. (COMIC: Sins of the Father)

On the final day of the Last Great Time War, Rassilon planned to destroy the whole of creation with the Ultimate Sanction. To escape the destruction of reality, the Time Lords would shed their corporeal bodies and become creatures of consciousness alone, ones that would escape the effects of time and of cause and effect. They were defeated by the Tenth Doctor before they would do so. (TV: The End of Time)

A Cyber-Leader at Stonehenge warned that with total event collapse, "all universes will be deleted", indicating that the phenomenon would erase the whole multiverse from existence. (TV: The Pandorica Opens)

The Fractures were described as anti-bodies for the multiverse, finding any threats and destroying them. This even included crossing through the Void. The Fractures attempted to hunt down Paul Foster after he used Cybermen technology to cross universes, but were defeated by the Twelfth Doctor and sent back into Void. (COMIC: The Fractures)

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Robert Lawrence Kuhn is the creator, writer and host of "Closer to Truth," a public television series and online resource that features the world's leading thinkers exploring humanity's deepest questions. Kuhn is co-editor, with John Leslie, of "The Mystery of Existence: Why Is There Anything at All?" (Wiley-Blackwell, 2013). This article is based on a "Closer to Truth" episode produced and directed by Peter Getzels and streamed at http://www.closertotruth.com. Kuhn contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Since childhood, I've obsessed about existence. What is existence? What's the extent of existence? What's the purpose of existence? Now, six decades on, having explored many things, I'm no surer (and feeling no smarter), but I continue my pursuit.

What's the largest, surest fact about existence that I can know with confidence? For me, it's the vastness of the cosmos. The universe is huge, but it is only with recent discoveries that we can realize how inconceivably immense the universe, or multiple universes, may actually be.

It's now one of humanity's ultimate questions and until relatively recently, we didn't know enough to even ask it. How many universes exist?

Related: Parallel Universes: Theories and Evidence

If we define "universe" as "all there is" or "all that exists," then obviously, by definition, there can be only one universe.

But if we define "universe" as "all we can ever see" (no matter how large our telescopes) or "space-time regions that expand together," then many universes may indeed exist. There is nothing in science more awesome, more majestic. To discern the nature of ultimate reality, one must begin with the challenge of multiple universes.

So what is a "multiverse"?

As physicist and Nobel laureate Steven Weinberg told me on "Closer to Truth" (the source of all following interviews), "The word 'universe,' I suppose, should properly mean the whole thing everything. But whenwe think of 'universe,' we sometimes use the word to mean just our Big Bang, the things we can see out to almost 14 billion light-years in all directions. And in this manner, it's reasonable to question: Is our universe unique? Are there multiple Big Bangs? Could there be multiple Big Bangs in different senses?" [What Triggered the Big Bang? It's Complicated (Op-Ed )]

"We started calling it a 'multiverse,'" meaning the entire ensemble of innumerable regions of disconnected space-time, said Andrei Linde, the Russian-American physicist now at Stanford. He developed the theory of "eternal chaotic inflation," which generates ever-increasing numbers of universes without end. Scientists created the neologism "multiverse," Linde continued, "because we found that what we had called 'the universe' can be divided into extremely large regions, which may have different laws of physics. And one part may be suitable for life, and other parts unsuitable."

Linde portrays "universes" as painted balloons or bubbles on canvas, "squeezing off" from one another via eternal chaotic inflation. Each of his bubbles is a separate universe, each with different laws of physics. The whole collection of universes, the multiverse, is incomprehensibly vast and growing ever more so.

Is such a multiverse merely speculation? Certainly it is not as widely accepted by scientists as quantum physics or the Standard Model of particle physics. But it is motivated by real science, and it does follow from the equations of cosmology that optimally explain the origin and structure of our universe. In fact, in some of Linde's mathematical models, cosmic inflation must be expanding eternally and chaotically.

Eternal chaotic inflation, which generates multiple universes, builds from the theory of cosmic inflation, originated by physicist Alan Guth at the Massachusetts Institute of Technology. He formulated cosmic inflation to solve several deep problems in the cosmology of our universe for example, why was the early universe extremely (and strangely) homogeneous, even though separated regions were causally disconnected? (Regions could not cause effects with others because the distances were too great and the elapsed time was too short, even though information was being exchanged at the speed of light, what theorists call the "horizon problem".)

On an episode of Closer To Truth, Guth described inflation this way: "Our universe began with a startlingly brief period of enormous, exponential expansion driven by a peculiar state of matter, called a 'false vacuum,' that actually creates a gravitational repulsion." (Guth's "startling brief period" is indeed startlingly brief from 10^-36 seconds after T0,the origin of the universe, to about 10^-32 seconds.)

According to quantum field theory, the word "vacuum" indicates a sector of space that has local-minimum energy, but not the lowest possible energy, and the word "false" is used to mean ultimately unstable, though the vacuum can remain stable for a very long time. The significance is that a "false vacuum" can "tunnel" into a lower energy state, releasing or "creating" enormous energy.

In this inflationary scenario, Guth said, the exponential expansion ends, because the false vacuum that's driving the repulsive gravity is unstable, and so it decays, much like radioactive elements decay.

The end of that initial inflation, after space expanded exponentially in this fleeting fraction of a second, becomes the traditional Big Bang, in which the vast energy that was locked up in the false vacuum is released and converted into the energy and matter of the early universe. This energy is what produces an unimaginably hot, uniform soup of plasmic particles, which is the starting point in traditional Big Bang theory.

But according to the model of cosmic inflation, while the decay of the false vacuum triggered the Big Bang in one part of expanding space, that decay did not happen in all of expanding space because not all of expanding space decays at the same time. Those varying rates of decay provide the special key that enables multiple universes to be generated without end.

Here's why: Sectors of false-vacuum space that do not decay go on expanding exponentially, stretching space even faster than the parts of false-vacuum space that do decay and because this false-vacuum, still-expanding space is "metastable," parts of it will eventually decay (over very long periods of time).

Each decay generates a single so-called "pocket universe," which is Guth's term for a connected region of space-time. Because the rate of expansions is much faster than the rate of decays, new regions are created with potentials for new decays, and hence potentials for new pocket universes.

That all works because the negative gravitational pressure of the false vacuum (in the sectors of space that do not decay) continues to create a repulsive gravitational field, which is the driving force behind inflation's exponential expansion of space. This creates more opportunities for local decays, which in turn generate pocket universes literally ad infinitum. Once again, it is because the rate of expansions of false-vacuum space, which stretches space, exceeds the rate of decays of false-vacuum space, which generates Big Bangs, that the process of generating multiple universes never stops.

Alex Vilenkin, a cosmologist at Tufts University in Massachusetts, explained that because "the space between these bubble or pocket universes is expanding very fast, room is being made for new bubbles to form, so there will be an unlimited number of pocket universes formed in the course of inflation."

If this picture is right, Guth said, "we see no end to it."

To get our bearings in understanding how a multiverse could be generated, let's begin with the theory of cosmic inflation, which explains the origin and structure of our universe. Then, because cosmic inflation does not end everywhere at the same time, this leads to the theory of eternal chaotic inflation, which generates multiple pocket universes continuously and without end.

It is easy to forget that each of these new pocket universes is vastly larger than our observable universe (see next section). But these new universes are not superstrange, since they all exist within the same space-time framework that we know in our universe though they erupt far beyond what we can see in our observable universe. What's more, once these new pocket universes are born, they are totally and forever disconnected from every other universe (including ours).

But how to commence cosmic inflation? Some physical material, however minuscule, is required. So from where does this primordial matter come?

Vilenkin said that the answer is quantum tunneling. He told me, "Quantum tunneling can create a universe 'from nothing' [because] in quantum mechanics things that are classically forbidden by energy barriers can happen by tunneling through energy barriers. So a universe of zero size that is, no universe at all can originate spontaneously by 'tunneling through' an energy barrier and then expanding by inflation." (Of course, the existence of the laws of quantum mechanics does not constitute "nothing" but that's a story for another time.)

A similar conundrum is that cosmic inflation seems to create energy "out of nothing," which would violate the law of conservation of energy (a physical "no-no"). In fact, the net energy of the universe is zero, because the positive energy of all the matter that is created is balanced by the negative energy of the gravitation. Guth calls it the "ultimate free lunch."

What's the evidence for these theories of cosmic inflation, eternal chaotic inflation and multiple universes? It's challenging, to say the least, considering that cosmic inflation began and ended within the tiniest fraction of the first second of our universe's existence. And, eternal chaotic inflation, by definition, cannot be seen, as other universes, arguably generated by eternal chaotic inflation, are disconnected permanently from our universe.

Yet lines of corroborating evidence (beyond the elegance of the equations) have convinced many cosmologists to such a degree that cosmic inflation and eternal chaotic inflation have become, in essence, the "standard model" of cosmology. As for cosmic inflation, it seems to solve, at the same time, several, separate enigmas in the origin and structure of the universe (including the horizon problem I mentioned earlier). Moreover, cosmic inflation makes interesting predictions, especially about the cosmic microwave background (CMB) radiation, a remnant of the Big Bang predictions that have been confirmed and reconfirmed by increasingly precise data from satellites.

As for eternal chaotic inflation, it seems to be the unavoidable consequences of the mathematics of cosmic inflation: Once cosmic inflation starts, it seems impossible to stop. However, while there have been tantalizing hints of possible corroborating data, none is convincing (yet?).

Cosmic inflation revolutionized cosmology, and if ultimately confirmed a demanding task it may come to be recognized as one of humanity's most fundamental realizations.

One great question of existence is simply, how big is it? By "it" I mean everything that exists. All there is. What are the physical dimensions of all-there-is?

A place to start is the size of the universe in which we find ourselves. According to one of Guth's models, our pocket universe may be at least 10^23 times larger than our observable universe (because, in order to work, inflation requires at least 100 doublings of the size of the universe, 2^100= roughly 10^30). This means that the pocket universe we call home would be 100 billion trillion times larger than everything we can see with our largest telescopes. (Models, no surprise, do jump around.)

Thus the vast expanse of our visible universe, Guth said, is but an insignificant speck within just our own inflating pocket universe. And this universe itself is only one pocket universe among an innumerable or even an infinite number of other pocket universes.

When I first realized how cosmic inflation majestically swells the size of the multiverse, how unutterably vast is the totality of the cosmos, it was unnerving. I can still feel the shock. It happened in the late 1990s when I was preparing for my first "Closer to Truth" interview with Andrei Linde.

In reading Linde's papers on eternal chaotic inflation (the parts I could understand), I kept coming across extremely large numbers that he was using to express the size of the universe but often I did not see any units of measurement associated with those numbers. I was perplexed: Size always needs units, I thought.

I did understand that many of the large numbers were relative to the size of our current universe. But how big is our current universe? Limited by the speed of light traveling since the Big Bang, we cannot see it all. I craved units with all the numbers so I could get a sense of the actual size of Linde's multiverse vision (in the different models).

Was Linde talking about centimeters or kilometers? Or for that matter, about nanometers (10^-9 meters) or kiloparsecs (3,262 light-years)? It seemed impossible to describe absolute size with just a number, no units. Why was Linde frustrating me by not using units?

Suddenly, it hit me. Units don't matter! This sounds counterintuitive: How could the difference between nanometers and kiloparsecs not matter? It gets worse. Compare the smallest and largest known sizes: the Planck length, which is ~10^-35 m (a proton is about 100 million trillion times larger than the Planck length), and the diameter of the visible universe, which is ~10^27 m. The difference between these smallest and largest potential units of measure is ~10^62 orders of magnitude but even this gigantic range shrinks to essentially zero when compared with Linde's numbers.

Here's what Linde told me: "If we're talking about the simplest models of inflation, the size of the universe is expected to be at least several orders of magnitude greater than what we see now. But it is very difficult to explain why would it be only several orders of magnitude greater than what we see now. In the past I used the estimate 10^10^12 (10 raised to the power of 10 raised to the power of 12), but if it is a self-reproducing, eternally inflating universe, then it is, most likely, simply infinite." (Linde stressed that "any estimates are very crude and model-dependent.")

Let's take Linde's noninfinite number, 10^10^12. As 10^12 is 1,000,000,000,000, or 1 trillion, the expression is 10^1,000,000,000,000. This number means one trillion orders of magnitude, one trillion consecutive multiplications by 10.

In comparison, even the vast expanse between the Planck length and the diameter of the entire visible universe 62 orders of magnitude becomes meaningless! It's not the difference between 62 and 1 trillion (which itself is obviously huge). It's the difference between 62 and 1 trillion orders of magnitude, the difference between multiplying by ten 62 times and multiplying by ten 1 trillion times. That is, this is a difference in sequential multiplications of 10 of 999,999,999,938.

This is why Linde's number for the size of an eternally chaotic inflating universe needs no units of measure. Units would add nothing to its meaning. The size of the cosmos is too vast.

Most scientists support cosmic inflation because it can account for the origin and structure of our cosmos and explain several profound problems. Sir Martin Rees, the U.K.'s Astronomer Royal, calls the multiverse "speculative science, not just metaphysics." He said he's confident there is far more to physical reality than the vast domain that we see through our telescopes, and he would be amazed, he said, "if the universe didn't extend thousands of times beyond what we can see."

But there are unanswered questions, and Rees raised two critical ones: "First, is our Big Bang the only one? And, second, if there are many BigBangs, are they all governed by the same laws of physics?"

The "fascinating option," said Rees, is that different physical laws govern the other universes "space may be different, gravity may be different, atoms may be different. This would mean that reality would consist of all these universes, governed by different laws, and only some tiny subset of them would be governed by laws that would allow complexity to evolve. Most universes would be sterile because, for example, gravity would be too strong to allow complex structures, or atoms would not be stable."

If, indeed, many Big Bangs generate an immense variety of physical laws, then, Rees said, only science fiction can describe all that might happen.

"These inflationary bubble or pocket universes expand at speeds approaching the speed of light," Vilenkin noted. "So we cannot possibly travel to other universes. For practical purposes, each of these inflationary bubble universes is a separate, self-contained unit and they can in principle have different physical properties."

How could different laws in different universes be generated? Leonard Susskind, a physicist at Stanford University in California and one of the originators of string theory, gives one answer: Multiple universes (the innumerable pocket universes) are populated by possible laws of physics emerging from possible structures of string theory. This theory postulates that reality at its most fundamental level consists of miniscule, one-dimensional "strings," whose size is nearly the smallest possible Planck length, ~10^-35-m, or 100 million trillion times smaller than a proton. Their vibrations in multiple dimensions of space-time (10, 11 or 26 dimensions, depending on the specific string theory) give rise to all the laws of physics, the theory says. String theory seems almost impossible to test experimentally, but the elegance and the beauty of its explanatory powers convince its adherents.

"What string theory brings is something about the number of possibilities," Susskind said. "But its numbers are far, far larger than the number of atoms in the universe the number 10 to the 500 [10^500] gets bandied about. This does not mean 10^500different pocket universes, but 10^500 different types of them in a string theory 'landscape' each one being repeated over and over again [in different instances of the type]. So, on the one hand, string theory gives you the analog of the different number of ways of re-arranging a DNA molecule. What cosmic inflation theory gives you, on the other hand, is how do you bring these different [pocket] universes into existence?"

"That's why a multiverse of innumerable bubble or pocket universes can have a very wide variety of physical properties," Vilenkin said. "A property of inflation is that it will explore the whole landscape of these possibilities, or 'vacua,' because quantum mechanics allows tunneling through energy barriers to other minima. Moreover, quantum mechanics tells us that if a transition between two minima is not absolutely forbidden by some law, then it must inevitably happen. This means that all possible transitions between all possible states must happen."

It's a neat picture. String theory would provide the landscape of all permitted laws of physics, and cosmic inflation would provide the mechanism to generate actual universes to populate that landscape. This would mean that for each pocket universe, string theory would provide a particular set of physical laws. The smallest structures would determine the largest structures.

Max Tegmark, a cosmologist at MIT, goes further. He envisions four kinds of multiverses that may exist, labeling them "Levels":

All of these universes are immense beyond the imagination, but are any truly infinite in the literal meaning of the term, going on without end or limit?

As Linde told me recently, "This is subtle. Suppose a bubble of a new vacuum is created in an eternally expanding universe a standard picture in eternal chaotic inflation and the string theory landscape (Level II). Then, from the outside, each such bubble looks like a finite bubble that grows infinitely in time. But from the inside, it looks like an infinite open universe. Of course, 'looks like' means that someone is looking, but nobody can see an infinite universe."

Tegmark's Level I is accepted by almost all cosmologists (i.e., space in our universe extends far beyond that which we can see with our best telescopes); his Level II has become the "standard model" of cosmology (i.e., the cosmic inflation of Guth leads to the eternal chaotic inflation of Linde, generating disconnected pocket universes continuously and forever); his Level III is speculative and controversial (i.e., quantum branching); and his Level IV, while idiosyncratic, seeks deep truths of existence (i.e., reality is mathematics).

All this shows how far and how fast our knowledge of the cosmos has expanded: Generating multiple universes by eternal chaotic inflation, a theory developed in the last four decades, is now the standard model of cosmology.

I asked Steven Weinberg, a founder of the Standard Model of particle physics now at the University of Texas at Austin, about other kinds of multiple universes. "There's another possibility, which is fairly simple to imagine," he said. "Our Big Bang is one episode and may be followed [and/or may have been preceded] by a series of other bangs, and our universe will make a transition into a different kind of expanding universe so that we are just living through a particular age.

"There are still other possibilities, which are more recondite," Weinberg continued. "Quantum mechanics can be applied to the whole shebang. Because the fundamental quanta in quantum mechanics is not the individual particle or billiard ball but is something called the 'wave function,' which describes all possibilities, it may be that the universe, the comprehensive universe, the whole thing, is some kind of quantum mechanical superposition of different possibilities." (This is Tegmark's Level III.)

"Then, there are even more exotic possibilities," Weinberg added. "The philosopher Robert Nozick introduced the so-called 'principle of fecundity,' according to which everything imaginable exists some place not in our same space-time but entirely separate." Weinberg noted that the principle of fecundity undermines (or trivializes) the question of why things are the way they are (i.e., why "this way" rather than "that way"), because whatever is possible must and does exist (somewhere). (The philosopher David Lewis proposed a similar theory of "modal realism" in which all possible worlds, astonishingly, are actual worlds.)

But to achieve such immensity and diversity, wouldn't there still have to be, at a deeper level, some rock-bottom, fundamental "universe-generating laws" to create all the multiple universes in the first place, each of which has its own different laws? Where is bedrock reality?

Not every cosmologist is a full convert to the multiverse. As cosmologist George Ellis told me, "I don't like the word 'multiverse.' I like the idea 'universe.'" He said he prefers to talk about "one universe with many different expanding domains," because to him, the "universe," by definition, is every physical thing that exists. Moreover, he stresses the basic problem of other domains of space-time. "Because we cannot see them," he said, "we can't prove anything about them.

"Now there is an alternative picture, which is actually rather nice," he said (with a twinkle). "Maybe this multiverse picture is wrong. Maybe we are seeing the same patch of space-time over and over again. Einstein's theory [of general relativity] allows this to happen because space-time not only is curved, but also it can have a different connectivity structure. So maybe we can go for several hundred million light-years [in one direction] and then suddenly we return from that side [to where we started from], just like Pac-Man did in those early computer games.

"In that case," Ellis concluded, "there actually would be many fewer galaxies than we appear to see. We would be seeing many images, maybe hundreds of images, of the same galaxy." This is what Ellis calls his "small universe" theory, which he finds "philosophically attractive." He said, "It could be the case," but admitted, "it probably isn't."

Physicist Paul Davies, director of the Beyond Center for Fundamental Concepts in Science at Arizona State University, said he gives "two cheers [not the full three] for the multiverse," because "although there are good reasons for supposing that what we see may not be all that exists, the hypothesis falls far short of being a complete theory of existence." A multiverse, Davies said, is often presented as solving the mysteries of existence by assuming that if there are an infinite number of universes, then "everything is out there somewhere, so that's the end of the story.

"That is simply not true," Davies said, because to get a multiverse, you need a universe-generating mechanism, "something that's going to make all those Big Bangs go bang. You're going to need some laws of physics. All theories of the multiverse assume quantum physics to provide the element of spontaneity, to make the bangs happen. They assume pre-existing space and time. They assume the normal notion of causality, a whole host of pre-existing conditions." Davies said there are about "10 different basic assumptions" of physical laws that are required "to get the multiverse theory to work."

Davies then made his deep point. "OK, where did those laws all come from? What about those meta-laws that generate all the universes in the first place? Where did they come from? Then what about the laws or meta-laws that impose diverse local laws upon each individual universe? How do they work? What is the distribution mechanism?" Davies argues that the only thing the multiverse theory does is shift the problem of existence up from the level of one universe to the level of multiple universes. "But you haven't explained it," Davies asserted.

Davies dismissed the idea that "any universe you like is out there somewhere. I think such an idea is just ridiculous and it explains nothing. Having all possible universes is not an explanation, because by invoking everything, you explain nothing."

Davies' critique of the multiverse goes deeper. To explain the universe, he rejects "outside explanations," he said.

"I suppose, for me, the main problem [with a multiverse] is that what we're trying to do is explain why the universe is as it is by appealing to something outside of it," Davies told me. "In this case, an infinite number of multiple universes outside of our universe is used as the explanation for our universe."

Then Davies makes his damning comparison. "To me, multiverse explanations are no better than traditional religion, which appeals to an unseen, unexplained God a God that is outside of the universe to explain the universe. In fact, I think both explanations multiverse and God are pretty much equivalent." To Davies, this equivalence is not a compliment.

Davies said he appreciates all the motivations and mathematics that drive inflation theory, along with the multiple universes that seem the compulsory consequences. But still, he said, he feels that an infinite number of universes does not make sense. Something's amiss.

What's my take? Long out of childhood, but still feeling childlike in the presence of a multiverse, I try to assess the possibilities. I like to categorize things, to discern scope and breadth. Here are seven possible mechanisms that could generate multiple universes.

What's more, these seven mechanisms for generating multiple universes are not mutually exclusive. Several, or even all of them, could be true and they could nest in various ways, one within others, others within one.

In a multiverse, one cannot avoid infinity, and infinity does strange things. There are two types of possible infinities in a multiverse: Type I: A single universe may be infinite in size (e.g., in our universe, if space and galaxies would continue forever without end or closure), or Type II: All the separate universes in a multiverse can be infinite in number (irrespective of whether any or all of the universes are infinite in size themselves).

The consequences of either infinity become bizarre. First of all, even Tegmark's Level I multiverse, assuming it's infinite, must contain everything that's physically possible. This means, for example, that every "Star Wars" scenario really exists out there, including those that didn't make it into the films and even all those the writers didn't think of!

Similarly, as long as there is sufficient space for unending random shufflings of particles (and a universe of infinite size certainly has sufficient space), there would have to be a sector of space out there identical to our sector of space, with persons identical to you and to me. Tegmark estimates that our closest identical copy is 10^10^28 m away.

I'm not so impressed even by this bizarre proposition. There would also have to be a sector of space identical to our sector of space except for, say, one hair on the head of one person, which is skewed 1 nanometer to the right. And another sector of space in which all else is the same except for that same hair, which is now skewed 2 nanometers to the left. Then all the hairs on all the people, skewed this way and that way. And then all the things in whole sectors of space, arranged in every possible combination and permutation. There would be innumerable minute differences and innumerable large differences, with every one a separate sector of space all enabled because the one infinite universe with infinite sectors of space goes on forever. Obviously, on this vision, randomized particles in the overwhelming majority of vast sectors of space yield nothing much at all.

To be clear, a truly infinite universe means that anything that is not impossible (no matter how obscure) will happen, must happen and must happen, weirdly, an infinite number of times. An infinite universe goes on forever, not only generating uncountable variations, but also requiring each of the uncountable variations to occur an infinite number of times. That's the strange nature of a true infinity.

I agree with Davies: Something is amiss.

If multiple universes are real, and especially if a true infinite number of universes really exist, then our worldview changes. Everything changes. Whatever you believe even about God (that God exists? that God doesn't exist?) nothing remains the same. If only the material world exists, then the material world becomes inconceivably larger. If an infinite God exists, then God's infinity becomes expressed by science and enriched with new meaning.

But would the real existence of a multiverse undermine arguments for the real existence of God (by undercutting the modern "argument from design" based on the "fine-tuning" of our universe)? How would God, if there is a God, relate to a multiverse? If one believes in God, or wonders whether to believe in God, this issue cries out to be addressed. Why would God, if there is a God, create multiple universes? Why would God create infinite multiple universes? Could a multiverse elucidate what God, if there is a God, would be like? And if God does not exist, then what? Does a multiverse have meaning? (All this I shall consider in a future essay.)

If, from this essay I seem rational, coolheaded and self-assured about multiple universes, then I have been unintentionally deceptive. I am intimidated by the ineffable endlessness of an overarching, overwhelming multiverse. I shrink before the terrifying vision of the 17th century philosopher Blaise Pascal: "The eternal silence of these infinite spaces frightens me."

Read more from Kuhn on hisSpace.com Expert Voices landing page. Follow all of the Expert Voices issues and debates and become part of the discussion on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com.

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Confronting the Multiverse: What 'Infinite Universes ...

Editor's note: In the August issue of Scientific American, cosmologist George Ellis describes why he's skeptical about the concept of parallel universes. Here, multiverse proponents Alexander Vilenkin and Max Tegmark offer counterpoints, explaining why the multiverse would account for so many features of our universeand how it might be tested.

Welcome to the MultiverseBy Alexander Vilenkin

The universe as we know it originated in a great explosion that we call the big bang. For nearly a century cosmologists have been studying the aftermath of this explosion: how the universe expanded and cooled down, and how galaxies were gradually pulled together by gravity. The nature of the bang itself has come into focus only relatively recently. It is the subject of the theory of inflation, which was developed in the early 1980s by Alan Guth, Andrei Linde and others, and has led to a radically new global view of the universe.

Inflation is a period of super-fast, accelerated expansion in early cosmic history. It is so fast that in a fraction of a second a tiny subatomic speck of space is blown to dimensions much greater than the entire currently observable region. At the end of inflation, the energy that drove the expansion ignites a hot fireball of particles and radiation. This is what we call the big bang.

The end of inflation is triggered by quantum, probabilistic processes and does not occur everywhere at once. In our cosmic neighborhood, inflation ended 13.7 billion years ago, but it still continues in remote parts of the universe, and other normal regions like ours are constantly being formed. The new regions appear as tiny, microscopic bubbles and immediately start to grow. The bubbles keep growing without bound; in the meantime they are driven apart by the inflationary expansion, making room for more bubbles to form. This never-ending process is called eternal inflation. We live in one of the bubbles and can observe only a small part of it. No matter how fast we travel, we cannot catch up with the expanding boundaries of our bubble, so for all practical purposes we live in a self-contained bubble universe.

The theory of inflation explained some otherwise mysterious features of the big bang, which simply had to be postulated before. It also made a number of testable predictions, which were then spectacularly confirmed by observations. By now inflation has become the leading cosmological paradigm.

Another key aspect of the new worldview derives from string theory, which is at present our best candidate for the fundamental theory of nature. String theory admits an immense number of solutions describing bubble universes with diverse physical properties. The quantities we call constants of nature, such as the masses of elementary particles, Newtons gravitational constant, and so on, take different values in different bubble types. Now combine this with the theory of inflation. Each bubble type has a certain probability to form in the inflating space. So inevitably, an unlimited number of bubbles of all possible types will be formed in the course of eternal inflation.

This picture of the universe, or multiverse, as it is called, explains the long-standing mystery of why the constants of nature appear to be fine-tuned for the emergence of life. The reason is that intelligent observers exist only in those rare bubbles in which, by pure chance, the constants happen to be just right for life to evolve. The rest of the multiverse remains barren, but no one is there to complain about that.

Some of my physicist colleagues find the multiverse theory alarming. Any theory in physics stands or falls depending on whether its predictions agree with the data. But how can we verify the existence of other bubble universes? Paul Steinhardt and George Ellis have argued, for example, that the multiverse theory is unscientific, because it cannot be tested, even in principle.

Surprisingly, observational tests of the multiverse picture may in fact be possible. Anthony Aguirre, Matt Johnson, Matt Kleban and others have pointed out that a collision of our expanding bubble with another bubble in the multiverse would produce an imprint in the cosmic background radiationa round spot of higher or lower radiation intensity. A detection of such a spot with the predicted intensity profile would provide direct evidence for the existence of other bubble universes. The search is now on, but unfortunately there is no guarantee that a bubble collision has occurred within our cosmic horizon.

There is also another approach that one can follow. The idea is to use our theoretical model of the multiverse to predict the constants of nature that we can expect to measure in our local region. If the constants vary from one bubble universe to another, their local values cannot be predicted with certainty, but we can still make statistical predictions. We can derive from the theory what values of the constants are most likely to be measured by a typical observer in the multiverse. Assuming that we are typicalthe assumption that I called the principle of mediocritywe can then predict the likely values of the constants in our bubble.

This strategy has been applied to the energy density of the vacuum, also known as dark energy. Steven Weinberg has noted that in regions where dark energy is large, it causes the universe to expand very fast, preventing mater from clumping into galaxies and stars. Observers are not likely to evolve in such regions. Calculations showed that most galaxies (and therefore most observers) are in regions where the dark energy is about the same as the density of matter at the epoch of galaxy formation. The prediction is therefore that a similar value should be observed in our part of the universe.

For the most part, physicists did not take these ideas seriously, but much to their surprise, dark energy of roughly the expected magnitude was detected in astronomical observations in the late 1990s. This could be our first evidence that there is indeed a huge multiverse out there. It has changed many minds.

The multiverse theory is still in its infancy, and some conceptual problems remain to be resolved. But, as Leonard Susskind wrote, I would bet that at the turn of the 22nd century philosophers and physicists will look nostalgically at the present and recall a golden age in which the narrow provincial 20th century concept of the universe gave way to a bigger better [multiverse] ... of mind-boggling proportions.

The Multiverse Strikes BackBy Max Tegmark

Do you really live in a multiverse, or is this notion beyond the pale of science?

Inspired by an interesting critique of multiverses in the August issue of Scientific American, penned by relativity pioneer George F. R. Ellis, let my give you my two cents' worth.

Multiverse ideas have traditionally received short shrift from the establishment: Giordano Bruno with his infinite-space multiverse got burned at the stake in 1600 and Hugh Everett with his quantum multiverse got burned on the physics job market in 1957. I've even felt some of the heat first-hand, with senior colleagues suggesting that my multiverse-related publications were nuts and would ruin my career. There's been a sea-change in recent years, however. Parallel universes are now all the rage, cropping up in books, movies and even jokes: "You passed your exam in many parallel universesbut not this one."

This airing of ideas certainly hasn't led to a consensus among scientists, but it's made the multiverse debate much more nuanced and, in my opinion, more interesting, with scientists moving beyond shouting sound bites past each other and genuinely trying to understand opposing points of view. George Ellis's new article is a great example of this, and I highly recommend reading it if you haven't already.

By our universe, I mean the spherical region of space from which light has had time to reach us during the 13.7 billion years since our big bang. When talking about parallel universes, I find it useful to distinguish between four different levels: Level I (other such regions far away in space where the apparent laws of physics are the same, but where history played out differently because things started out differently), Level II (regions of space where even the apparent laws of physics are different), Level III (parallel worlds elsewhere in the so-called Hilbert space where quantum reality plays out), and Level IV (totally disconnected realities governed by different mathematical equations).

In his critique, George classifies many of the arguments in favor of these multiverse levels and argues that they all have problems. Here's my summary of his main anti-multiverse arguments:

1) Inflation may be wrong (or not eternal)

2) Quantum mechanics may be wrong (or not unitary)

3) String theory may be wrong (or lack multiple solutions)

4) Multiverses may be unfalsifiable

5) Some claimed multiverse evidence is dubious

6) Fine-tuning arguments may assume too much

7) It's a slippery slope to even bigger multiverses

(George didn't actually mention (2) in the article, but I'm adding it here because I think he would have if the editor had allowed him more than six pages.)

What's my take on this critique? Interestingly, I agree with all of these seven statementsand nonetheless, I'll still happily bet my life savings on the existence of a multiverse!

Let's start with the first four. Inflation naturally produces the Level I multiverse, and if you add in string theory with a landscape of possible solutions, you get Level II, too. Quantum mechanics in its mathematically simplest ("unitary") form gives you Level III. So if these theories are ruled out, then key evidence for these multiverses collapses.

Remember: Parallel universes are not a theorythey are predictions of certain theories.

To me, the key point is that if theories are scientific, then it's legitimate science to work out and discuss all their consequences even if they involve unobservable entities. For a theory to be falsifiable, we need not be able to observe and test all its predictions, merely at least one of them. My answer to (4) is therefore that what's scientifically testable are our mathematical theories, not necessarily their implications, and that this is quite OK. For example, because Einstein's theory of general relativity has successfully predicted many things that we can observe, we also take seriously its predictions for things we cannot observe, e.g., what happens inside black holes.

Likewise, if we're impressed by the successful predictions of inflation or quantum mechanics so far, then we need to take seriously also their other predictions, including the Level I and Level III multiverse. George even mentions the possibility that eternal inflation may one day be ruled outto me, this is simply an argument that eternal inflation is a scientific theory.

String theory certainly hasn't come as far as inflation and quantum mechanics in terms of establishing itself as a testable scientific theory. However, I suspect that we'll be stuck with a Level II multiverse even if string theory turns out to be a red herring. It's quite common for mathematical equations to have multiple solutions, and as long as the fundamental equations describing our reality do, then eternal inflation generically creates huge regions of space that physically realize each of these solutions. For example, the equations governing water molecules, which have nothing to do with string theory, permit the three solutions corresponding to steam, liquid water and ice, and if space itself can similarly exist in different phases, inflation will tend to realize them all.

George lists a number of observations purportedly supporting multiverse theories that are dubious at best, like evidence that certain constants of nature aren't really constant, evidence in the cosmic microwave background radiation of collisions with other universes or strangely connected space, etc. I totally share his skepticism to these claims. In all these cases, however, the controversies have been about the analysis of the data, much like in the cold fusion debacle. To me, the very fact that scientists are making these measurements and arguing about data details is further evidence that this is within the pale of science: this is precisely what separates a scientific controversy from a nonscientific one!

Our universe appears surprisingly fine-tuned for life in the sense that if you tweaked many of our constants of nature by just a tiny amount, life as we know it would be impossible. Why? If there's a Level II multiverse where these "constants" take all possible values, it's not surprising that we find ourselves in one of the rare universes that are inhabitable, just like it's not surprising that we find ourselves living on Earth rather than Mercury or Neptune. George objects to the fact that you need to assume a multiverse theory to draw this conclusion, but that's how we test any scientific theory: we assume that it's true, work out the consequences, and discard the theory if the predictions fail to match the observations. Some of the fine-tuning appears extreme enough to be quite embarrassingfor example, we need to tune the dark energy to about 123 decimal places to make habitable galaxies. To me, an unexplained coincidence can be a tell-tale sign of a gap in our scientific understanding. Dismissing it by saying "We just got luckynow stop looking for an explanation!" is not only unsatisfactory, but is also tantamount to ignoring a potentially crucial clue.

George argues that if we take seriously that anything that could happen does happen, we're led down a slippery slope toward even larger multiverses, like the Level IV one. Since this is my favorite multiverse level, and I'm one of the very few proponents of it, this is a slope that I'm happy to slide down!

George also mentions that multiverses may fall foul of Occam's razor by introducing unnecessary complications. As a theoretical physicist, I judge the elegance and simplicity of a theory not by its ontology, but by the elegance and simplicity of its mathematical equationsand it's quite striking to me that the mathematically simplest theories tend to give us multiverses. It's proven remarkably hard to write down a theory which produces exactly the universe we see and nothing more.

Finally, there's an anti-multiverse argument which I commend George for avoiding, but which is in my opinion the most persuasive one of all for most people: the parallel universes just seems too weird to be true.

Having looked at anti-multiverse arguments, let's now analyze the pro-multiverse case a bit more closely. I'm going to argue that all the controversial issues melt away if we accept the External Reality Hypothesis: there exists an external physical reality completely independent of us humans. Suppose that this hypothesis is correct. Then most multiverse critique rests on some combination of the following three dubious assumptions:

1) Omnivision assumption: physical reality must be such that at least one observer can in principle observe all of it.

2) Pedagogical reality assumption: physical reality must be such that all reasonably informed human observers feel they intuitively understand it.

3) No-copy assumption: no physical process can copy observers or create subjectively indistinguishable observers.

(1) and (2) appear to be motivated by little more than human hubris. The omnivision assumption effectively redefines the word "exists'' to be synonymous with what is observable to us humans, akin to an ostrich with its head in the sand. Those who insist on the pedagogical reality assumption will typically have rejected comfortingly familiar childhood notions like Santa Claus, local realism, the Tooth Fairy, and creationismbut have they really worked hard enough to free themselves from comfortingly familiar notions that are more deeply rooted? In my personal opinion, our job as scientists is to try to figure out how the world works, not to tell it how to work based on our philosophical preconceptions.

If the omnivision assumption is false, then there are unobservable things that exist and we live in a multiverse.

If the pedagogical reality assumption is false, then the objection that multiverses are too weird makes no logical sense.

If the no-copy assumption is false, then there's no fundamental reason why there can't be copies of you elsewhere in the external realityindeed, both eternal inflation and unitary quantum mechanics provide mechanisms for creating them.

We humans have a well-documented tendency toward hubris, arrogantly imagining ourselves at center stage, with everything revolving around us. We've gradually learned that it's instead we who are revolving around the sun, which is itself revolving around one galaxy among countless others. Thanks to breakthroughs in physics, we may be gaining still deeper insights into the very nature of reality.

The price we have to pay is becoming more humblewhich will probably do us goodbut in return we may find ourselves inhabiting a reality grander than our ancestors dreamed of in their wildest dreams.

Additional Reading:

Many Worlds in One: The Search for Other Universes. Alex Vilenkin. Hill and Wang, 2006.

The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. Leonard Susskind. Back Bay Books, 2006.

The Hidden Reality: Parallel Universes and the Hidden Laws of the Cosmos. Brian Greene. Knopf, 2011.

See the original post:

The Case for Parallel Universes - Scientific American