One Universe: At Home in the Cosmos

Motion | Pages 10-11 | See Linked Version

of Cepheids in many other galaxies in painstaking detail. He also noted how bright the Cepheids appeared in the telescope. Putting those two quantities together let Hubble determine the distances to the stars and, by association, to their host galaxies. You would do much the same thing to estimate the distance to a house across a dark meadow by observing the lightbulb outside the front door. Perhaps you know roughly how bright a 100-watt bulb appears from half a mile away. If someone changed the light to a 60-watt bulb, you'd have to be much closer to the house for it to appear as bright to your eyes.

When Hubble combined his data, the results were stunning. Except for the few nearest galaxies, each galaxy's speed through space was directly related to its distance from Earth. If one galaxy was twice as far away as another, it moved away from Earth about twice as fast. Five times farther away meant five times faster, and so on. That's the telltale signature of an explosion. Colorful fragments of fireworks expand into the sky in the same way: Those near the center move slowly, while the flaming bits near the edge blast outward fastest of all. Hubble's work showed beyond doubt that the apparent repose of galaxies against the dark vault of space is an illusion. No longer did the cosmos seem a static place. In honor of this discovery, the orbiting Hubble Space Telescope carries Hubble's name. Appropriately, one of the telescope's primary missions is to nail down the rate at which galaxies are flying apart from one another.

Since Hubble's research, we have gradually realized that galaxies do not literally stream through space at warp speeds. Rather, they are carried along for the ride as space itself grows in all three dimensions. To imagine this, suppose that the galaxies are ladybugs sitting on the surface of a rapidly inflating balloon. Each bug sees its neighbors on all sides moving away from it, even though the bugs all think they are sitting still. No bug is at the "center" of the expansion because the expansion happens everywhere on the surface at once. The fabric of the balloon stretches with time, as does the very fabric of space in the cosmos.

This universal expansion gives us the strongest piece of evidence we have about our origins. If the cosmos is growing, it was smaller yesterday than it is now. One year ago it was smaller still. The farther back in time we go, the smaller the cosmos must have been. By following this line of reasoning to its logical conclusion, we know there must have been a time when the entire universe existed as a tiny point--and that (continued)

The Expanding Universe

Like ladybugs on the surface of a rapidly inflating balloon, galaxies in the expanding fabric of space in the cosmos move away from one another in all directions. The notion that the universe is expanding grew from the 1929 discovery by Edwin Hubble that the farther away a galaxy is, the faster it moves away from us.

some explosion triggered its rapid growth. As far as we can tell, that cataclysmic event occurred about 13 billion years ago. Its name is now part of our popular culture: the "Big Bang." This mind-boggling scenario raises many questions. However, of all the stories yet devised about the birth of the universe, the Big Bang is best supported by solid scientific evidence.

MOTIONThrough the Millennia

Hubble's work was a natural climax to thousands of years of curiosity about motion in its many guises. If he had to pick two words to summarize the state of the universe, Hubble might well have chosen those used to open this chapter: "Everything moves." The ancient Greeks understood this idea and captured it in an elegant sentence, translated centuries later into Latin: "Ignorato motu ignorator natura," or "One who knows not motion knows not nature." It's fair to say that to comprehend the universe at large we first must understand how things move in the world around us and in the close confines of our own solar system.

Observations of the Sun, Moon, and planets moving in the sky gave our ancestors their first clues about our place in the cosmos. They watched the Sun arc from east to (continued)