wave moving away from an observer takes on an extra time lag, which makes it appear to vibrate slower. Conversely, a wave moving closer saves time due to its forward motion and seems to vibrate quicker. In the former case its frequency shifts downward, and in the latter case its frequency shifts upward.

Imagine a steady letter writer, traveling around the world, who mails a postcard to a close friend every single day. If she is traveling away from her friend, to lands increasingly remote, her postcards would likely take longer and longer to arrive. Thus, the frequency by which her friend would receive them would steadily drop. On the other hand, if she is making her way back home, her friend’s letter box would likely fill up at an increasing pace. Letters sent weeks before from faraway lands might arrive at the same time as those sent days before from nearer locales, leading to a glut of mail. Similarly, as the Doppler effect informs us, the direction of a signal’s sender affects its frequency upon arrival.

For sound waves the Doppler effect explains the high-pitched shrieking of a fire engine as it races toward a scene and the low-pitched moan as it speeds away. In the case of light waves, the Doppler effect is visual. Applied to the inward or outward motion of a source, it predicts a shift in luminous frequencies toward the higher or lower ends of the spectrum, respectively. In terms of colors, blue has a relatively high frequency and red a low frequency. Therefore, the increase in frequency for approaching sources (from green to blue, for example) is known as a “blueshift,” and the decrease of frequency for receding sources (from orange to red, for instance) is called a “redshift.”

Hubble pioneered the use of this effect to probe galactic motions. Pointing the Hooker Telescope (at that time the largest in the world) at various galaxies, he recorded shifts in the frequencies of their atomic spectral lines. He used this information to calculate the velocities (either incoming or outgoing) of each galaxy relative to Earth. Plotting these with respect to galactic distances, he discovered, to his amazement, an unmistakable pattern. With the notable

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