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.. h pressure and gravity pull inward on the bubble wall, a bubble that is started from rest will start to contract. With nothing to halt the contraction, it will collapse to a black hole. While a black hole is interesting, it would certainly be viewed as a disappointment by our would-be universe creator. Suppose, however, that the bubble was not started from rest, but instead was given an outward push. If the initial outward velocity is large enough, then the bubble will follow the sequence shown in the figure on the facing page.

As the bubble grows, the indentation will become deeper, as shown at the top. The indentation will continue to deepen, developing a neck, or wormhole, as shown in the middle. Once the wormhole develops, a dramatic change takes place — the bubble has turned inside out. Now the region of false vacuum can grow larger and larger without encroaching on the original space. It creates new space as it expands, resembling an inflating balloon.

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The climax of the evolution is shown at the bottom: The region of false vacuum, with a region of true vacuum attached, disconnects from the parent space, forming a new, completely isolated closed universe. It will then continue to enlarge, going through the usual evolution of an inflationary universe. A new universe has been created, and the parent universe is unharmed — universe creation is not a doomsday machine. From the point of view of the parent universe, the umbilical cord of the child universe is indistinguishable from a black hole. The umbilical connection in the child universe would similarly look like a black hole.

If we assume that the false vacuum driving the inflation has a mass density of about 10[sup 80] grams per cubic centimeter, then the time that it takes the child universe to disconnect is roughly 10[sup -37] second. After this time there will be no contact between the parent universe and child. A false-vacuum-bubble universe creator would watch helplessly as his new universe slipped inexorably through the wormhole and severed all contact. Once the new universe has separated, the black hole that remains in the parent universe would evaporate. It would disappear in roughly 10[sup -23] second, releasing the energy equivalent of a 500-kiloton nuclear explosion.

While the parent universe would be in no danger of annihilation, the safety of the experimenters would require precautions similar to those used in hydrogen bomb tests. The story of inflationary universe creation sounds complete at this point, but there is an important and surprising twist: It is not clear whether it is possible, even in principle, to attain the expansion velocity needed for the false vacuum bubble to evolve into a new universe. One hope for evading this problem involves a peculiar consequence of quantum theory, the process of quantum tunneling. A quantum system can make a discontinuous transition from one configuration to another, even when the system would not normally have enough energy to exist in the intermediate configurations. This quantum leap is the origin of a frequently used metaphor (and a successful American television series), and it also plays a crucial role in the decay of the false vacuum.

But calculations show that each time a false vacuum bubble is set into expansion, the probability that it will tunnel to become a new universe is extraordinarily small. To write out the number in decimal notation would require a decimal point, followed by approximately 10[sup 13] zeros, followed by a one. So even if some super-advanced civilization develops the capacity to produce and manipulate regions of false vacuum, they would still require a fantastic amount of patience to produce a new universe. It may be premature, however, for us to bemoan the difficulties that our super-advanced descendants will face. Calculations show that the probability becomes higher as the mass density of the false vacuum increases. If there exists a false-vacuum state associated with the unification of gravity and the other three forces of nature, then the mass density would be about 10[sup 93] grams per cubic centimeter.

For this density, the answer to our probability calculation would be approximately one — a new universe would be created with just about every attempt. While advanced civilizations can conceivably create multiple universes, inflation itself can have the same effect. Most versions of the inflationary universe theory imply that the false vacuum does not decay all at once, but instead decays a fragment at a time. Each fragment produces a universe, while the bulk of the false vacuum continues eternally to double and redouble in size. Each doubling in size might occur in as little as 10[sup -37] second.

Because the time needed for the development of a super-advanced civilization is measured in billions of years or more, there appears to be no chance that laboratory production of universes could compete with the natural process of eternal inflation. On the other hand, a child universe created in a laboratory by a super-advanced civilization would set into motion its own progression of eternal inflation. Could the super-advanced civilization find a way to enhance its efficiency? We may have to wait a few billion years to find out. THE FALSE VACUUM The false vacuum is a peculiar form of matter predicted to exist by modern theories of elementary particles. If inflationary cosmology is correct, it was the driving force behind the Big Bang.

The false vacuum has an extraordinarily high density, perhaps 10[sup 80] grams per cubic centimeter, and also a pressure that is extraordinarily large, but negative — it acts like a suction. The huge negative pressure turns gravity on its head, producing a repulsive gravitational force that can launch a region of false vacuum into explosive expansion. Why does the false vacuum have such a peculiar name? While the word vacuum is often defined as a state devoid of matter, this definition is not precise enough for physicists, since it is not clear exactly what matter means. The physicist defines vacuum as the state with the lowest possible density of energy. The false vacuum is not really a vacuum, but its energy density can be lowered only by a very slow process, called the decay of the false vacuum. So, while the false vacuum is waiting to decay, it behaves as if its energy density cannot be lowered — as if it were a temporary vacuum.



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