Redshift

Picture yourself standing on the roadside as an ambulance races toward you with its siren blaring. While it approaches, the sound is high and sharp. But the moment it passes and pulls away, the very same siren suddenly drops to a lower pitch.

Sound is a wave that spreads out like ripples on water. When the vehicle comes closer, the waves ahead of it get squeezed together. The wavelength gets shorter, and your ears hear a higher pitch. When it moves away, the waves behind it get stretched out. The wavelength gets longer, and the pitch drops.

Light is a wave too, so the same thing happens. Light from a star moving away from us arrives with a longer wavelength. In visible light, the long-wavelength end is red, so the light as a whole is pushed toward red.

This shift is called redshift. It is the light version of the siren dropping in pitch.

To notice that a siren has dropped in pitch, you need to know what it sounded like to begin with. You know the siren normally switches between two notes, so if it sounds lower than that, you can guess the ambulance is driving away.

Starlight works the same way. Spread starlight through a prism and you get a rainbow-like band called a spectrum. Scattered across it are dark stripes. They are marks left behind when elements like hydrogen or calcium swallow light of particular wavelengths. Scientists can find out exactly where these stripes should appear by testing in a lab.

But when you spread out the light from a distant galaxy, the whole set of stripes has slid toward red together. It is like both notes of the siren dropping at once. How far the stripes have moved tells you how fast the galaxy is moving away.

Astronomers also found that the farther away a galaxy is, the more its stripes have shifted. That means more distant galaxies are moving away faster.

Here is where the siren comparison breaks down. The reddening of distant galaxies is not caused by galaxies racing along a road. Instead, the space between the galaxy and us is itself stretching, and the light traveling through it gets stretched along with it.

In siren terms, it is not that the ambulance is speeding off. It is that the road between the ambulance and you keeps getting longer. That is why this kind of reddening has its own name: cosmological redshift. Its cause is different from Doppler redshift, which happens when a star really is moving away from us through space.

Not all starlight turns red, either. Light from a nearby object that is heading toward us, like the Andromeda Galaxy, gets pushed toward blue instead. This is called blueshift.

So by checking which way the stripes have shifted, you can tell whether an object is approaching or moving away.