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The diagram on this page shows a simplified picture of how the evolution would work. The top bar illustrates a region of false vacuum. The second bar shows the same region one half-size later. I have assumed for illustration that it has enlarged by a factor of 4, but the actual factor could have been much larger. The second bar shows a region of false vacuum that has decayed, producing a bubble universe, and two regions that remain false vacuum. Each of these two remaining regions of false vacuum are as large as the original region. The third bar shows the region after another half-life has elapsed, with two more bubble universes formed from the false vacuum regions on the second bar, and four regions of false vacuum, each as large as the original. The process would go on forever. The bubbles would collide so rarely that any observer would have no more than a negligible probability of seeing any sign of the other bubbles’ existence. Nonetheless, an understanding of the infinite tree of universes seems to be needed in order to make statistical predictions about the properties of our own universe, which is assumed to be a typical “branch” on the tree.

Bubble Universe Diagram

       In studying a scenario such as this, cosmologists generally assume that the laws of physics are the same throughout this multi-bubble universe. We don’t really have any way of knowing, but our goal is to understand the consequences of the laws of physics as we know them, and not to idly speculate about other mythical worlds. Nonetheless, there is a possibility that the other bubbles could be very different from our own. While empty space appears to be devoid of properties, to a modern particle physicist empty space, also called the vacuum, is an enormously complicated substance. Particle-antiparticle pairs are incessantly appearing and disappearing, and space itself is believed to break up into a poorly understood “quantum foam” when magnified enough so that distances as short as 10-33 centimeter become visible. Because of this complexity, physicists do not know whether only one kind of empty space is stable, or whether there are many kinds. Other kinds of space might not be three-dimensional, and they might alter the masses of elementary particles, or the forces that govern their behavior. If there are many kinds of space, the infinite tree of bubble universes would sample all the possibilities.

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