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In the chapter on sound you learned about the Doppler shift for sound waves. A sound source moving away from you sounds as if it has lower frequency than if it is at rest, and when moving toward you sounds as if it had higher frequency than at rest. There is a vaguely similar behavior for light, but the physics is TOTALLY different. If you are looking at a double star a few hundred light years away, the light from the star moving toward earth is blue-shifted while the light from the star moving away is red shifted. Using Einstein's Special Theory of Relativity, you can accurately calculate the speeds of the stars relative to earth. However, when we look at another galaxy, many millions of light years away, we always see a red shift and this is due to a totally different fact. The universe is only 13.8 billion years old, and the space the universe fills is expanding explosively. So when we look at a galaxy 7 billion light years away, when that light left on its journey, the universe was extremely small compared to its present size. As the light made its way to us, space expanded drastically. The effect is to "stretch" the light waves... and the light always appears to have a much higher frequency and longer wavelength than it actually originally had. This can only be described accurately with Einstein's Theory of Gravitation, aka General Relativity.
We can directly observe the entire universe at the moment it first became transparent to visible light. It was only 380,000 years old, and the entire universe was at 3000 K. When we see it today nearly 14 billion years later, the frequency shift is 1/1000 --- the light has a temperature of only 3 K, so the dominant frequency is in the microwave, not the visible region. This is the "light" you see from the dark night sky itself. It looks black to our eyes, but not to a microwave telescope!
