The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [32]
Figure 3.1 The energy contained in an inflaton field (vertical axis) for given values of the field (horizontal axis).
Two things, both critical. While the inflaton is on the plateau, it fills space with a large potential energy and negative pressure, driving a burst of inflationary expansion. But, just as Cartman releases his potential energy by rolling down the slope, so the inflaton releases its potential energy by its value, throughout space, rolling to lower numbers. And as its value decreases, the energy and negative pressure it harbors dissipate, bringing an end to the period of blistering expansion. Just as important, the energy released by the inflaton field isn’t lost—instead, like a cooling vat of steam condensing into water droplets, the inflaton’s energy condenses into a uniform bath of particles that fill space. This two-step process—brief but rapid expansion, followed by energy conversion to particles—results in a huge, uniform spatial expanse that’s filled with the raw material of familiar structures like stars and galaxies.
Precise details depend on factors that neither theory nor observation has as yet determined (the initial value of the inflaton field, the exact shape of the potential energy slope, and so on)5 but in typical versions the mathematical calculations show that the inflaton’s energy would roll down the slope in a tiny fraction of a second, on the order of 10–35 seconds. And yet, during that brief span, space would expand by a colossal factor, perhaps 1030 if not more. These numbers are so extreme that they defy analogy. They imply that a region of space the size of a pea would be stretched larger than the observable universe in a time interval so short that the blink of an eye would overestimate it by a factor larger than a million billion billion billion.
However difficult it is to envision such a scale, what’s essential is that the region of space that spawned the observable universe was so small that it would easily have come to a uniform temperature before it was stretched into our grand cosmic expanse by the rapid burst. The inflationary expansion, and billions of years of subsequent cosmological evolution, resulted in this temperature cooling substantially, but the uniformity set in place early on dictates a uniform result today. This resolves the mystery of how the universe’s uniform conditions came to be. In inflation, a uniform temperature across space is inevitable.6
Eternal Inflation
During the nearly three decades since its discovery, inflation has become a fixture of cosmological investigation. But to have an accurate picture of the research panorama, you should be aware that inflation is a cosmological framework, but it is not a specific theory. Researchers have shown that there are many ways to skin an inflationary cat, differing in details such as the number of inflaton fields supplying the negative pressure, the particular potential energy curves to which each field is subject, and so on. Fortunately, the sundry realizations of inflation have some implications in common, so we can draw conclusions even in the absence of a definitive version.
Among these, one first fully realized by Alexander Vilenkin of Tufts University and developed further by others, including most notably Linde, is of great importance.7 In fact, it’s the very reason I’ve spent the first half of this chapter explaining the inflationary framework.
In many versions of the inflationary theory, the burst of spatial expansion is not a onetime event. Instead, the process by which our region of the universe formed—rapid stretching of space, followed by a transition to a more ordinary, slower