The Doppler Effect, the Expanding Universe, and the Search for Exoplanets

The words "Doppler Effect" get tossed around a lot by all sorts of people—highway patrol officers, TV weathermen, handsome physics education websites—but perhaps the people who toss it around with the most weight are astrophysicists and cosmologists. And no, this is not only because they sit around in their cubicles all day making WEEEE-OOOOOOOO noises with toy spaceships. Though that is a big part of it.

Astrophysicists and cosmologists study the universe and the really, really big things floating around within it. And even though in space, no one can hear you scream, everyone can see you move—and that's how these space scientists use the Doppler Effect. They look not at sound waves, but at light waves, which experience the same kind of frequency shifting (admittedly without the race car noises).

Recall in terrestrial applications of the Doppler Effect, sounds that are moving away from you sound lower, a decrease in frequency; sounds that are moving closer sound higher, an increase in frequency. The same thing happens with light—objects that are moving away appear redder (a decrease in frequency); objects that are moving closer appear bluer (an increase in frequency). ¹¹ This phenomena was described for the first time in a seminal scientific treatise by Dr. Theophrastus Seuss entitled One Shift, Two Shift, Redshift, Blueshift.

As scientists began noticing that light from stars seemed to be significantly redshifted—and increasingly redshifted as the stars got further away from earth—they realized that the entire universe must, in fact, be expanding.¹² If the universe is expanding, it's a logical jump to assume at some point in time the universe was just that: a point. This, in a nutshell, is the Big Bang theory, which postulates that all matter in the entire universe once existed in a tiny point and expanded outward to fill the vast gulf of space we now inhabit. (Not to be confused with "The Big Bang Theory," a TV sitcom which postulates that one joke should expand to fill endless seasons of television.)

While the idea of an ever-expanding universe may seem isolating if not a bit Lovecraftian, take heart—the Doppler Effect is also paramount in the quest to discover if humanity is alone in the galaxy or if there really are Wookiees out there waiting for us to find them. In the last few decades, humanity has discovered a number of extrasolar planets, or "exoplanets:" planets that orbit stars other than our own Sun. There are hundreds of billions of these planets in the Milky Way alone, with as many as 20% of those called habitable, meaning they are similar to Earth in size and distance from their star.¹³

There are many ways to discover these planets, but one way is by using the Doppler Effect. As a planet orbits a star, it pulls on the star with its own force of gravity—remember Newton's laws?—and this moves the star a tiny bit as the planet flies around. This star movement is enough to shift the light coming from the star, which tips off astronomers that the star has a planet circling it. If this seems like it might be hard to discern, well, four hundred and fifty-six exoplanets disagree.

The universe may be expanding, but the Doppler Effect has also shown it's a bit more crowded than we thought. There's hope for finding those Wookiees yet, and there will be a substantial reward for the one who does. But we want them alive—no disintegrations!