Knocking on Heaven's Door - Lisa Randall [169]
No one knew the universe was expanding when the twentieth century began. At the time that Edwin Hubble first peered into the sky, very little was known. Harlow Shapley had measured the size of the Milky Way to be 300,000 light-years across, but he was convinced that the Milky Way was all that the universe contained. In the 1920s, Hubble realized that some of the nebula that Shapley had thought were clouds of dust—which did indeed merit this uninspiring name—were in fact galaxies, millions of light-years away.
Once he identified galaxies, Hubble made his second stunning discovery—the universe’s expansion. In 1929, he observed that galaxies red-shifted, which is to say there was a Doppler effect in which light waves shifted to longer wavelengths for more distant objects. This red shift demonstrated that galaxies were receding, much as the high-pitched wail of a siren decreases in frequency as an ambulance speeds away. (See Figure 71.) The galaxies he had identified were not stationary with respect to our location, but were all expanding away from us. This was evidence that we live in an expanding universe, in which galaxies are growing farther apart.
[ FIGURE 71 ] The light from an object moving away from us is shifted to lower frequencies—or shifted toward the red end of the spectrum—whereas light from objects moving away is shifted to higher frequencies, or blue shifted. This is analogous to the noise from a siren that is lower pitched when an ambulance moves away and higher pitched when it approaches.
The universe’s expansion is different from the pictures we might first imagine since the universe doesn’t expand into some preexisting space. The universe is all there is. Nothing is present for it to expand into. The universe, as well as space itself, expands. Any two points within it grow farther apart as time progresses. Other galaxies move farther away from us, but our location is not special—they move farther from each other as well.
One way to picture this is to imagine the universe as the surface of a balloon. Suppose you had marked two points on the balloon’s surface. As the balloon blows up, the surface becomes stretched and those two points grow farther apart. (See Figure 72.) This is in fact what happens to any two points in the universe as it expands. The distance between any two points—or any two galaxies—increases.
[ FIGURE 72 ] The “ballooniverse” illustrates how all points move away from one another as the balloon (universe) expands.
Notice in our analogy that the points themselves don’t necessarily expand—just the space between them. This is in fact what happens in the expanding universe as well. Atoms, for example, are tightly bound together via electromagnetic forces. They don’t get any bigger. Neither do relatively dense strongly bound structures such as galaxies. The force driving the expansion acts on them too, but because other force contributions are at work, the galaxies don’t themselves grow with the overall expansion of the universe. They feel such strong attractive forces that they remain the same size while their relative distance from each other gets bigger.
Of course, this balloon analogy is not perfect. The universe has three spatial dimensions, not two. Furthermore, the universe is large and probably infinite in size, and not small and curved like the balloon’s surface. On top of that, the balloon exists in our universe and expands into existing space, unlike the universe, which permeates space and doesn’t expand into something else. But even with these caveats, the surface of a balloon illustrates quite nicely what it means for space to expand. Every point moves away from every other point at the same time.
A balloon analogy—this time referring to the interior—is also helpful for understanding how the universe cooled from its initial hot dense fireball existence. Imagine an extremely hot balloon that you allow to expand to a very big size. Though it might have been too hot to handle at first, the expanded