What? Many universes? As in more than one? Is this a trick question? Well, if we define "universe" as "all there is" or "all that exists," then sure, by definition there can be only one.
But if we define "universe" as "all we can ever see," no matter how large our telescopes, then many universes may indeed exist. Talk about expanding our horizons! There is nothing in science more awesome, more majestic. To discern what's ultimately real, it is here, with multiple universes, that one must start.
We begin with basics: How could multiple universes be generated? The person most responsible for conceiving how multiple universes might come about is theoretical physicist Alan Guth at the Massachusetts Institute of Technology, whose theory of "cosmic inflation" revolutionized cosmology. (If ultimately proven to be correct-a difficult task-cosmic inflation may come to be recognized as one of the most fundamental realizations that humanity has ever had). According to Guth, inflation proposes that our universe commenced with a startlingly brief period of enormous exponential expansion empowered by a gravitational repulsion that was generated by a particular state of matter that has a high energy density that cannot be rapidly lowered. Such a state is called a "false vacuum," where the word "vacuum" indicates a state of lowest possible energy density, and the word "false" is used to mean temporary. In this inflationary scenario, the exponential expansion ends because the stuff that's driving the repulsive gravity is unstable, so it decays, much like a radioactive element decays. The end of inflation is the traditional hot big bang, where the vast energy that had been locked in the false vacuum driving the exponential expansion is released and converted into the energy and matter of the early universe. This energy is what produces the hot, uniform soup of particles, which is exactly the assumed starting point of the traditional big bang theory.
Granting the one inflation-generated big bang, how could decaying inflation generate other big bangs? Decay means that in the time period of its "half-life," half of the stuff will disappear and half will remain, and in the subsequent half-life, another half will disappear (so that only one-quarter of the original stuff remains), and so on. Thus, it would seem that inflation would become progressively weaker and soon die out. But Guth has a catch.
The catch is that at the same time that this strange stuff generating repulsive gravity is decaying, it is undergoing exponential expansion. And this exponential expansion is faster, actually much faster, than the exponential decay. That is the key!
Note two counterintuitive corollary issues. First, there is no dilution of the driving force of the exponential expansion in the expanding space because the energy density of the expanding space remains the same (because the false vacuum cannot rapidly lower its energy density); this is why the energy density remains constant, and the total energy increases as the space expands. The negative pressure of the false vacuum, therefore, continues to create a repulsive gravitational field, which is the driving force behind the exponential expansion of inflation. Second, while it seems that energy is being created out of nothing, therefore violating the law of conservation of energy, the net energy is in fact zero because the positive energy of all the matter that is created is balanced by the negative energy of the gravitation. Guth calls the latter the "ultimate free lunch."
If this picture is right, Guth says, "we see no end to it." It appears that inflation is going to produce literally an infinite number of "pocket universes"-which is Guth's term for a connected region of space-time. Note that, on average, each one of Guth's "pocket universes" is vastly larger than our observable universe, hugely larger than all we can ever see (which is only our local part of one pocket universe). And new pocket universes form so rapidly that there may be infinite numbers of them.
Guth offers that it is "rather wild extrapolation to talk about these infinity of pocket universes," adding that "maybe it's all nonsense." But when the theory works as well as it does to describe the observed part of the universe, he continues, "I think it makes sense to at least explore the implications that the theory suggests for the part of the universe that we don't see."
If multiple universes do exist, the person who has shown them likely to be without number or end is physicist Andrei Linde, originally from Russia, now at Stanford University. In the early days of proposing inflation theory (the early 1980s), Linde showed how inflation could be expanding "chaotically and eternally." In some models, inflation must be expanding chaotically and eternally.
The entire ensemble of perhaps infinite regions of disconnected space-time, these innumerable pocket universes, has been affixed with a new term-"multiverse." Linde says that each of these extremely large regions within the multiverse may have different laws of physics. But since we live in one of these "universes" and because it is so large, we can only make measurements in our one universe-the others are too far away for us to ever receive any information-so all these laws seem unchanging and immutable.
Linde portrays "universes" as painted balloons on canvas. Each of his balloons is a separate universe, each with different laws of physics. The whole collection of universes, the multiverse, is incomprehensively vast. And growing ever more so.
Max Tegmark, a cosmologist at MIT, goes further still, seeing real possibilities for generating multiple universes through quantum parallel universes, where, with every tick of time (whether Planck time at 10-44 seconds or every observational instant), the universe branches into different realities. Tegmark says that "one of the most beautiful ideas in all of science is that the structure of the universe on a large scale actually originates from the microscopic quantum world" and that multiple universes by quantum branching would take this idea to its ultimate conclusion. Not yet satisfied, he conceives that multiple universes may also be generated by any coherent system of mathematics.
Multiple universes by quantum branching? By mathematics? Have we gone nuts? Or perhaps, our eyes are just starting to squint open.
Theoretical cosmologist George Ellis at the University of Cape Town does not like the term "multiverse." He prefers to talk about an "expanding universe" because to him, the "universe," by definition, is everything that exists. He stresses that the problem with other domains of space-time is that "because we cannot see them, we can't prove anything about them." He suggests a radical alternative that he finds "a rather nice option": Ellis posits that the vast universe picture may be wrong, that "maybe we are seeing the same patch of space-time over and over again."
Einstein's theory, he says, allows for this to happen because space-time is not only curved, it can also have a different connectivity structure. So maybe after several hundred million light-years, suddenly we return from the other side, just like Pac-Man did in those early computer games. In that case, there actually would be many fewer galaxies then we appear to see. We would be seeing many images, maybe hundreds of images, of the same galaxy. This is what Ellis calls a "small universe," which he finds "philosophically attractive." He says "it could be the case," but admits "it probably isn't."
At the University of Cambridge, Sir Martin Rees, the United Kingdom's Astronomer Royal, calls the multiverse "speculative science, not just metaphysics," and he compares the conceptual leap needed to comprehend it with humanity's intellectual leaps of the past-from the earth-centered Ptolemaic universe to the sun-centered Copernican universe; to the discovery that we are in a Milky Way galaxy with billions of suns; and then to the realization, since the 1920s, that our galaxy is one of untold billions of galaxies. Rees is confident that there's far more to physical reality then the vast domain that we can see through our telescopes, and he'd be amazed, he says, "if the universe didn't extend thousands of times beyond what we can see."
The "fascinating option," says Rees, is whether these other universes are governed by different physical laws-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. The most fascinating option Rees sees is the idea that many big bangs generate an immense variety of physical laws because, then, only science fiction can describe all that might happen.
Rees' two questions are profound: Was our big bang the only one? And if our big bang was not the only one, do the others have different laws?
I asked physicist Steven Weinberg at the University of Texas at Austin whether he has an aesthetic of multiple universes. "I suppose the word 'universe' should mean the whole thing, everything," he said. "But we tend to use 'universe' just to mean our big bang, the things we can see out to more than 10 billion light-years in all directions. And in that sense, it's a reasonable question to ask: Is this unique or are there other such domains? And there could be other domains in different senses. It could be as simple as the fact that the universe is bigger than we think; perhaps it's vastly bigger than 10 or so billion light-years across, and there are big bangs going off in different places.
"There's another possibility, which is also fairly simple to imagine: 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 other possibilities which are more recondite," he 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. Then, there are even more exotic possibilities: The philosopher Robert Nozick introduced the so-called 'principle of fecundity,' according to which everything imaginable exists someplace-not in our same space-time but entirely separate." (The philosopher David Lewis proposed a similar theory of "modal realism" in which all possible worlds are actual worlds, somewhere....)
Weinberg notes that the principle of fecundity avoids the question of why things are the way they are because whatever is possible does exist.
But to achieve such immensity and diversity, there has to be, at some deeper level, some rock-bottom, fundamental "universe-generating laws" to create all the multiple universes, each of which has different laws. Does that make sense?
Arizona State University physicist, cosmologist, and astrobiologist Paul Davies says "two cheers 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 says, 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."
This is simply not true, says Davies, 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 mechanics 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 claims that about 10 different basic assumptions of physical laws are required to get the multiverse theory to work. And he then says, "OK, where did those all come from? What about these meta-laws that generate all the universes in the first place? Where do those rules come from? Then what about the laws or rules which impose diverse local laws upon each individual universe? How does that work? What is the distribution mechanism?" Davies says that the only thing the multiverse 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."
How do I conclude? If multiple universes exist, our worldview changes. That's for sure. I like to categorize things, to discern the scope of what we're dealing with. So here are six potential mechanisms that could generate multiple universes, at least in theory:
1. Different regions of our ordinary space-time, so far away that even their light, traveling with enormous speed, will never have time enough to reach us.2. Different domains of ordinary space-time, "squeezed off" to become forever separate.3. Different dimensions of space and time, where, in some higher dimensions, entirely separate realities may be very close, but forever apart.4. Different universal histories through the strangeness of quantum mechanics, where at each observation (or at every fleeting moment of Planck time), all reality splits into many worlds.5. Whatever can be expressed by consistent mathematical systems and models exists in reality.6. The principle of fecundity: Whatever can be imagined does exist. Somewhere.
Multiple universes may be forever untestable, but when we consider their possibility, we come closer to truth.
Robert Lawrence Kuhn speaks with Andrei Linde, Alan Guth, Sir Martin Rees, Leonard Susskind, Max Tegmark, and Steven Weinberg in "How Many Universes Exist?" the 16th episode in the Closer to Truth: Cosmos, Consciousness, God TV series, which airs Thursdays on the PBS HD network and many other PBS stations. Every Friday, participants in the series will share their views on the previous day's episode.
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