The Expansion of the Universe

Remember absorption spectra? We said we could use it identify what a star was made out of, or even a planet's atmosphere. We weren't lying, but that's only the beginning of it.

Let's go back to the Doppler Effect in our study of sound waves¹³. The siren of a police car moving towards us has a higher pitch. As the car moves away, the pitch drops. When a sound pitch "drops," the frequency decreases. That makes sense. The police car is moving away from us, so the sound waves have to travel a little more distance to reach our ears. Their wavelength increases.

The same thing happens with objects that give off light. In the 1920's, an astronomer named Edwin Hubble studied distant galaxies and analyzed their light. He came to the remarkable conclusion that their absorption spectrum was shifted to longer wavelengths, and that the exact shift depended on the exact distance to the galaxy. Longer wavelengths means redder colors. This phenomenon is called redshift.

If light from galaxies is redshifted, then they are moving away from us at velocities v proportional to their distance d from Earth. With math, Hubble wrote the Hubble law: v = H_od, where H_ois the Hubble constant. Its exact value varies from 15 to 30 kilometers per second per million light years.

If the universe is expanding, then there was a point in the distant past when it was a lot smaller. The Hubble Law hints at the fact that all galaxies originated from the same place. This is where the idea of the Big Bang comes from. The universe could've been created with a huge explosion– an explosion so powerful that we're still observing its (accelerating) effects.