Learn about the Doppler Effect

doppler shift
Astronomers use the Doppler effect to measure the frequency of light waves as an object is moving with respect to the observer. The frequency is shorter as it moves toward you, and the object shows a blueshift. If the object is moving away, it shows a redshift. This shows up in spectra of stellar light as a shift in the black lines (called absorption lines) as shown here). Carolyn Collins Petersen

What Is the Doppler Effect We Hear about in Astronomy? 

The Doppler effect is a shift in the frequency or wavelength of radiation emitted from an object as it moves with respect to an observer. It's named after Austrian physicist Christian Doppler who first proposed it in 1842. 

How does it work? If the source of radiation, say a star, is moving toward an astronomer on Earth (for example), then the wavelength of its radiation will appear shorter (higher frequency, and therefore higher energy).

On the other hand, if the object is moving away from the observer then the wavelength will appear longer (lower frequency, and lower energy). You have probably experienced a version of the effect when you heard a train whistle or a police siren as it moved past you, changing pitch as it passes by you and moves away.

The Doppler effect is behind such technologies as police radar, where the "radar gun" emits light of a known wavelength. Then, that radar "light" bounces off a moving car and travels back to the instrument. The resulting shift in wavelength is used to calculate the speed of the vehicle. (Note: it is actually a double shift as the moving car first acts as the observer and experiences a shift, then as a moving source sending the light back to the office, thereby shifting the wavelength a second time.)

Redshift

When an object is receding (i.e. moving away) from an observer, the peaks of the radiation that are emitted will be spaced farther apart than they would be if the source object were stationary.

The result is that the resulting wavelength of light appears longer.

By similar means, spectral lines (seen in measurements of light called "spectra") will appear shifted toward the red end of the optical spectrum. (The same effect applies to other bands of the electromagnetic spectrum, such as radio, x-ray or gamma-ray, but optical measurements are the most common and are the source of the term "redshift".)

The more quickly the source moves away from the observer, the greater the redshift. From an energy standpoint, longer wavelengths correspond to lower energy radiation.

Blueshift

Conversely, when a source of radiation is approaching an observer the wavelengths of light appear closer together, effectively shortening the wavelength of light. (Again, shorter wavelength means higher frequency, and therefore higher energy.) Spectroscopically, the emission lines would appear shifted toward the blue side of the optical spectrum, hence the name blueshift.

As with redshift, the effect is applicable to other bands of the electromagnetic spectrum, but the effect is most often times discussed when dealing with optical light, though in some fields of astronomy this is certainly not the case.

Expansion of the Universe

In the early 1900s it was believed that the Universe was static. In fact, this led Albert Einstein to add the cosmological constant to his famous field equation in order to "cancel out" the expansion (or contraction) that was predicted by his calculation. Specifically, it was believed that the edge of the Milky Way represented the boundary of the static Universe.

Then, Edwin Hubble found that the so-called "spiral nebulae" that had plagued astronomy for decades were not nebulae at all.

They were actually other galaxies. It was an amazing discovery, and told astronomers that the universe is much larger than they knew.

Hubble then proceeded to measure the Doppler shift, specifically finding the redshift of these galaxies. He found that that the farther away a galaxy is, the more quickly it recedes. This led to the now-famous Hubble's Law, which says that an object's distance is proportional to its speed of recession.

This revelation led Einstein to write that his addition of the cosmological constant to the field equation was the greatest blunder of his career. Interestingly, however, some researchers are now placing the constant back into general relativity.

As it turns out Hubble's Law is only true up to a point, since research over the last couple decades has found that distant galaxies are receding more quickly than predicted.

This imples that the expansion of the universe is accelerating. The reason for that is a mystery, and scientists have dubbed the driving force of this acceleration dark energy. They account for it in the Einstein field equation as a cosmological constant (though it is of a different form than Einstein's formulation).

Other Uses in Astronomy

Besides measuring the expansion of the universe, the Doppler effect can be used to model the motion of things much closer to home; namely the dynamics of the Milky Way Galaxy.

By measuring the distance to stars and their redshift or blueshift, astronomers are able to map the motion of our galaxy and get a picture of what our galaxy may look like to an observer from across the universe.

The Doppler effect also allows us to measure the pulsations of variable stars, as well as motions of particles traveling at incredible velocities inside relativistic jet streams emanating from supermassive black holes.

Edited and updated by Carolyn Collins Petersen.