The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [22]
Might the universe’s spatial expanse be finite? Sure. Definitely possible. If space were finite yet large enough, there could still be some interesting patches way out there. But a smallish finite universe could easily fail to have adequate space to accommodate substantial numbers of distinct patches, let alone any that are duplicates of our own. A finite universe poses the most convincing way to upend the Quilted Multiverse.
But in the last few decades, physicists working to push the big bang theory back to time zero—in search of a deeper understanding of the origin and nature of Lemaître’s primeval atom—have developed an approach called inflationary cosmology. In the inflationary framework, the argument in support of an infinitely large cosmos, not only garners strong observational and theoretical support but, as we will see in the next chapter, becomes an almost inevitable conclusion.
What’s more, inflation brings to the fore another, even more exotic, variety of parallel worlds.
*It’s easier to envision curved space than curved time, and that’s why many popularizations of Einsteinian gravity focus solely on the former. However, for the gravity generated by familiar objects like the earth and sun, it is actually the curvature of time—not space—that exerts the dominant impact. For an illustration, think of two clocks, one on the ground, the other on top of the Empire State Building. Because the ground clock is closer to the earth’s center, it experiences slightly stronger gravity than the clock that’s high above Manhattan. General relativity shows that because of this, the rate at which time passes on each will be slightly different: the ground clock will run a tiny bit slow (billionths of a second per year) compared to the elevated clock. The temporal mismatch is an example of what we mean by time being curved or warped. General relativity then establishes that objects move toward regions where time elapses more slowly; in a sense, all objects “want” to age as slowly as possible. From an Einsteinian perspective, that explains why an object falls when you let go of it.
*Given our earlier discussion of how matter curves the region in which it is immersed, you might wonder how there can be no curvature even though there’s matter. The explanation is that a uniform presence of matter generally curves spacetime; in this particular case, there is zero space curvature but nonzero spacetime curvature.
*I will discuss black holes more fully in later chapters. Here we’ll stick to the familiar notion, by now well ingrained in popular culture, of a spatial region—think of it as a ball in space—whose gravitational pull is so strong that nothing crossing its edge can escape. The bigger the black hole’s mass, the larger its size, so when anything falls in, not only does the black hole’s mass increase but its size does too.
CHAPTER 3
Eternity and Infinity
The Inflationary Multiverse
A pioneering group of physicists in the mid-1900s realized that if you were to shut off the sun, remove the other stars from the Milky Way, and even sweep away the more distant galaxies, space would not be black. To the human eye it would appear black, but if you could see radiation in the microwave part of the spectrum, then every which way you turned you’d see a uniform glow. Its origin? The origin. Remarkably, these physicists discovered a pervasive sea of microwave radiation filling space that is a present-day relic of the universe’s creation. The story of this breakthrough recounts a phenomenal achievement of the big bang theory, but in time it also revealed one of the theory’s fundamental shortcomings and thus set the stage for the next major breakthrough in cosmology after the pioneering works of Friedmann and Lemaître: the inflationary theory.
Inflationary cosmology modifies the big bang theory by inserting an intense burst of enormously fast expansion during the universe’s earliest moments. This modification,