A couple days ago I wrote about how atoms and molecules emit (or absorb) light at only certain colors, and how this produces line spectra. The pattern of bright or dark lines is unique to the type of atom or molecule, so we can use them to identify the atoms and molecules in a star. I mentioned in passing that there are other effects of the line spectra we can use to study stars and galaxies. One of these is an overall shift in the line pattern, known as a Doppler shift.
In our everyday lives, we’re familiar with the Doppler effect as it applies to sound. You might notice when a car or train passes you, its sound shifts downward as it passes. This is because the sound waves from an object are bunched together as it moves toward you, and stretched apart as it moves away from you. The bunched together waves sound higher, and the stretched waves sound lower, hence a shift in tone known as the Doppler shift.
For light a similar thing occurs. Waves of light coming from an object are compressed as an object moves toward us, making them look bluer (blue shifted). They’re stretched as the object moves away from us and look more red (red shifted). The Doppler effect shifts the entire spectrum to the blue or red, and that means the bright line or dark line spectra shifts to the blue or red, as seen in the figure above. By comparing the lines we see from a star or galaxy with the lines we observe in the lab, we can measure just how fast it is moving toward us or away from us.
Redshift and blueshift can also be caused by gravity. If you were to shine a beam of light into space, the light has to escape Earth’s gravity. As it does this it loses a little bit of energy, making it slightly more red than it was initially. If you were to shine a light down to Earth from space, the light would gain a bit of energy, making it slightly bluer as it reaches Earth. This means that we can use the Doppler shift of light to measure both relative motion and gravitational strength.
One of the things that’s often noted is that the light of more distant galaxies is redshifted more than the light of closer galaxies. This is known as Hubble’s law, and demonstrates that the more distant a galaxy the more redshifted its light tends to be. This relationship between distance and redshift is roughly linear, and it is typically explained by imagining galaxies drawn on an inflating balloon. As the balloon expands, galaxies near each other move slightly apart, while widely separated galaxies separate much more quickly.
This tends to reinforce the idea that Hubble redshift is due entirely to the relative motions of galaxies, that it is caused purely by the standard Doppler effect. In actuality it’s a little more complicated. There is a redshift due to the relative motion of a galaxy when the light leaves the source, but light can travel for billions of years before reaching us, and during that time the universe continues to expand. Since space itself is expanding, the wavelength of the traveling light also stretches. This means that while the light travels it continues to redshift due to cosmic expansion. But we can account for that in determining the distance to a galaxy.
So by looking at the color of lines, we can determine the motion of the universe.