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By Root 1968 0

expansion, together with the production of particles—may happen over and over again at various far-flung locations throughout the cosmos. From a bird’s-eye view, the cosmos would appear riddled with innumerable widely separated regions, each being the aftermath of a portion of space transitioning out of the inflationary burst. Our realm, what we have always thought of as the universe, would then be but one of these numerous regions, floating within a vastly larger spatial expanse. If intelligent life exists in the other regions, those beings would just as surely have thought their universe to be the universe, too. And so inflationary cosmology steers us headlong into our second variation on the theme of parallel universes.

To grasp how this Inflationary Multiverse comes about, we need to engage two complications that my Cartman analogy glossed over.

First, the image of Cartman perched high on a mountaintop offered an analogy to an inflaton field harboring significant potential energy and negative pressure, poised to roll to lower values. But whereas Cartman is perched on a single mountaintop, the inflaton field has a value at each point in space. The theory posits that the inflaton field starts off with the same value at each location within an initial region. And so we’d achieve a more faithful rendering of the science if we imagine something a little odd: numerous Cartman clones perched on numerous, closely packed, identical mountaintops throughout a spatial expanse.

Second, we’ve so far barely touched on the quantum aspect of quantum field theory. The inflaton field, like everything else in our quantum universe, is subject to quantum uncertainty. This means that its value will undergo random quantum jitters, momentarily rising a little here and dropping a little there. In everyday situations, quantum jitters are too small to notice. But calculations show that the larger the energy an inflaton has, the greater the fluctuations it will experience from quantum uncertainty. And since the inflaton’s energy content during the inflationary burst was extremely high, the jitters in the early universe were big and dominant.8

We should thus not only picture a platoon of Cartmans perched high on identical mountaintops; we should also imagine that they are all subject to a random series of tremors—strong here, weak there, very strong way over there. With this setup, we can now determine what will happen. Different Cartman clones will stay perched on their mountaintops for different durations. In some locations, a strong tremor knocks most Cartmans down their slopes; in other locations, a mild tremor coaxes only a few to tumble down; in others still, some Cartmans may have started to roll down until a strong tremor knocked them back up. After a while, the terrain will be divided into a random assortment of domains—much as the United States is divided into states—in some of which no Cartmans are left on mountaintops, while in others many Cartmans remain securely perched.

The random nature of quantum jitters yields a similar conclusion for the inflaton field. The field begins high up on its potential energy slope at every point in a region of space. The quantum jitters then act like tremors. Because of this, as illustrated in Figure 3.2, the expanse of space rapidly divides into domains: in some, quantum jitters cause the field to topple down the slope, while in others it remains high.

So far, so good. But now stay with me closely; here’s where cosmology and Cartmans differ. A field that’s perched high up on its energy curve affects its environment far more significantly than a similarly perched Cartman does. From our familiar refrain—a field’s uniform energy and negative pressure generate repulsive gravity—we recognize that the region the field permeates expands at a fantastic rate. This means that the inflaton field’s evolution across space is driven by two opposing processes. Quantum jitters, by tending to knock the field off its perch, decrease the amount of space suffused with high field energy. Inflationary expansion, by