“If space is truly infinite,” observes cosmologist Dan Hooper, head of the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, in At the Edge of Time, “the implications are staggering. Within an infinite expanse of space, it would be hard to see any reason why there would not be an infinite number of galaxies, stars, and planets, and even an infinite number of intelligent or conscious beings, scattered throughout this limitless volume. That is the thing about infinity: it takes things that are otherwise very unlikely and makes them all inevitable.”
Ensemble of Universes
The universe we see around us is a tiny sliver of a much larger multiverse. The multiverse theory says that what we have all along been calling “the universe” is in fact nothing of the kind. Rather, it is but an infinitesimal fragment of a much larger and more elaborate system—an ensemble of “universes,” or of distinct cosmic regions according to Davies, Paul. in The Goldilocks Enigma: Why Is the Universe Just Right for Life?
Epoch of Inflation Creates the Multiverse
Eternal inflation is one mechanism for generating a multiplicity, or as Davies describes it, “an ensemble of universes, known collectively as a multiverse. Individual universes within the multiverse could be very different from one another. Only a small fraction, such as the Universe harboring Earth, might be fit for life.”
A Small piece of the space that emerged from inflation went on to form our universe
“During the epoch of inflation,’ writes Davies, “space expanded at an absolutely staggering rate, tearing space and everything in it apart. No two objects—even elementary particles—remained close enough to one another for long enough to interact. Two objects separated by the width of an atom at the beginning of inflation were trillions of miles apart from one another by the time it was over—only a minuscule fraction of a second later. Inflation took regions of space that had once been neighbors, including the small piece of the space that emerged from inflation went on to form our universe, and forever disconnected them from each other. So utterly complete was this act of sequestration, that these regions became more than merely distant. Inflation left them in entirely different universes.”
Our Observable Universe –Tip of the Iceberg
But there is good reason, writes Hooper, to think that everything we can see in the Cosmos represents only the smallest tip of the cosmic iceberg. During inflation, countless pieces of space were stretched into newly formed universes, populating a greater multiverse of disconnected worlds.
Starkly Different Alien Physics
And despite the fact that we have no way to observe them, says Hooper, not all universes within the greater multiverse will be similar to our own, as the Earth’s flora and fauna varies from place to place. In some, the laws of nature could be subtly different, or starkly different, dictated by a diversity of physical laws. Patches of space separated by inflation could evolve in such a way that they come to support the existence of different forms of matter and forces.
The nature of infinity is such that with an infinite amount of space, there are an infinite number of universes — collections of atoms and other particles located at specific places at specific times oriented in almost exactly the same way that they are in our Earth world. Within an infinite space, suggests Hooper there are inevitably an infinite number of universes that are indistinguishable from our own.
Across the multiverse, life is very rare indeed
“These worlds contain a star that is nearly identical to the Sun, which is orbited by a planet that is nearly identical to the Earth, which contains upon it people who are nearly identical to you and me,” Hooper writes. “If space as we know it extends forever, this conclusion is inevitable. All things and all events that are possible, no matter how unlikely, will exist and will occur within this greater collection of space.”
But according to Hooper there’s a catch. “Throughout the overwhelming majority of the multiverse, dark energy is so abundant as to make life impossible—no living observers will ever find themselves in such a place. In fact, any universe with more than about ten times as much dark energy as there is in ours will almost certainly be devoid of life. Across the multiverse, life is very rare indeed.”
“Multiverse theory hovers between science and fantasy”
A frequently voiced criticism of the multiverse is that it isn’t science because it can’t be tested by experiment or observation, says Davies, who concludes with a note of optimism about the current state of multiverse theory:. “Because of the theory of eternal inflation we cannot directly observe the other universes for two reasons: because they are unbelievably far away, and because they are receding from us much faster than light. It can be validly objected that a theory that rests on entities that are in principle unobservable cannot be described as scientific. It is conceivable, however, that indirect evidence could be found to support the theory. Sometimes in science one can have confidence.”
“There are no new theories,” Hooper told the Daily Galaxy, “that have recently strengthened (or weakened) the case for a multiverse.”
But, says Nobel-Prize laureate physicist Steven Weinberg, it’s not a requirement of a successful physical theory that everything it describes be observable, nor that all possible predictions of a theory be verifiable. Weinberg helped lay the foundation of the Standard Model, the theory that classifies all known elementary particles in the universe, making it one of the most important breakthroughs in physics in the 20th century
The Big Bang Universe
In an interview with Quanta magazine, Weinberg proposed that our Big Bang universe is just part of a larger multiverse, which, he suggests, could explain why some constants of nature, particularly dark energy, have values that seem to be very favorable to the appearance of life. It’s only in big bangs where the dark energy takes a value favorable to the appearance of life that there are observers around to ask the question.
In 1987, years before the discovery of dark energy, Weinberg argued in a controversial paper that “our universe’s vacuum energy density could be explained if we take into account the fact that we are living in it. All one needs for this to be the case is for there to exist an incredibly large number of universes. The idea of the modern multiverse had been born.”
The Last Word -Dan Hooper and Yasunori Nomura
When asked about about the possibility of ever acquiring evidence of the existence of another universe, Yasunori Nomura, Director of the Berkeley Center for Theoretical Physics, replied in an email to The Daily Galaxy: “This picture of many universes—the multiverse—is not a random idea but a specific scenario suggested by theories of fundamental physics, such as string theory. As such, we could test predictions of the scenario, even without going to another universe. For example, our universe may collide with another universe, whose trace can, in principle, be seen in the sky, in particular as a characteristic pattern in the so-called cosmic microwave background radiation.
“Furthermore,” Nomura explains in his email, “according to this many universes scenario, our own universe must be ‘negatively curved,’ meaning it must have a certain geometric property. While the theory does not predict how much the curvature is, this prediction might be confirmed in future observation. Perhaps more importantly, if a future observation found that the curvature of our universe is positive, rather than negative, then the version of the multiverse many physicists are currently talking about would be observationally excluded.”
When we sked Nomura if it is probable that an adjacent universe would operate under different laws of physics, he replied: “According to what we currently know, we expect that other universes have very different properties than ours. For example, the nature of elementary particles and the law governing their behaviors take different forms. For some universes, even the number of spacetime dimension can be different. We certainly expect that universes ‘adjacent to’ ours look very different from ours. This, however, does not mean that there is no physical law governing the multiverse. For example, all the universes are expected to obey the principles of quantum mechanics. It is simply that some of the properties of our universe which we have conventionally regarded as fundamental are not as fundamental as we thought. The nature of elementary particles—even the existence of the electron, photon, and so on—is among them.”
“In some pockets of space, far beyond the limits of our observations,” wrote Dan Hooper in an email to The Daily Galaxy, referring to the theory of eternal inflation and the inflationary multiverse: “the laws of physics could be very different from those we find in our local universe. Different forms of matter could exist, which experience different kinds of forces. In this sense, what we call ‘the laws of physics’, instead of being a universal fact of nature, could be an environmental fact, which varies from place to place, or from time to time.”
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Paul Davies, The Goldilocks Enigma: Why Is the Universe Just Right for Life?, Yasunori Nomura and Dan Hooper, At the Edge of Time, both Kindle editions, and Scientific American.