If the big bang theory is correct, then astronomers should be able to detect the expansion of the universe. Edwin Hubble, the namesake of the Hubble Space Telescope, was one of the first scientists to observe and measure this expansion. In 1929, he was studying the spectra, or rainbows, of distant galaxies by allowing the light from these objects to pass through a prism on his telescope. He noticed that light coming from almost every galaxy was shifted to the red end of the spectrum. To explain the observation, he turned to the Doppler effect, a phenomenon that most people associate with sound. For example, as an ambulance approaches us on the street, the pitch of the siren seems to increase; as it passes, the pitch decreases. This happens because the ambulance is either catching up to the sound waves it is creating (increased pitch) or moving away from them (decreased pitch).
Hubble reasoned that light waves created by galaxies were behaving similarly. If a distant galaxy were rushing toward our galaxy, he argued, it would move closer to the light waves it was producing, which would decrease the distance between wave crests and shift its color to the blue end of the spectrum. If a distant galaxy were rushing away from our galaxy, it would move away from the light waves it was creating, which would increase the distance between wave crests and shift its color to the red end of the spectrum. After he consistently observed redshifts, Hubble developed what we call Hubble's law: Galaxies are moving away from us at a velocity proportional to their distance from Earth.
Today, the redshifts of distant celestial objects stand as strong evidence that the universe is expanding. But anything that expands must eventually stop, right? Won't the universe, just like a ball thrown into the sky, reach some maximum point of expansion, stop and then start falling back to where it started? As we'll see next, that's one of three possible scenarios.