The Universe - I'm Expanding!
In the 1920's, an astronomer named Edwin Hubble began looking at the spectral lines of faraway stars. He noticed that they did not match any of the known elements at the time. However, he also noticed that they were parallel to the spectral lines produced by hydrogen, just shifted toward the red slightly. He noticed that almost all of the stars he looked at were shifted toward the red by a small percent. He calculated that theses galaxies and stars he was looking at were moving away from Earth at a rate of 1 million meters per second.
As astronomers studied the universe more intensely, they noticed that they rarely saw anything but red shifts, indicating that the galaxies in the universe were moving farther and farther apart from one another. The universe is getting bigger.
The expanding universe can be compared to a loaf of raisin bread before, during, and after baking. When first mixed, all of the raisins are fairly close together. Before baking, the bread is allowed to rise. As it rises, the raisins (representing all the galaxies in the universe) spread out away from one another. Those on the edge spread out faster than those in the center.
This raisin example is similar to how galaxies move away from the center of the universe. Those that are closest to the center of the universe do not move as quickly or as far as the stars and galaxies that are farther from the center of the universe. Because of the Doppler Effect, scientists realize that the shift towards the red part of the spectrum indicates the light source is moving away. If stars or galaxies were moving towards Earth the shift would be toward the blue part of the spectrum.