Physics 207 Introduction
The image above denotes spectra of galaxies that are receding, or moving away from us. The Ha line is the alpha line for hydrogen gas. You can see that as the speed of the galaxy increases, the Ha line moves farther and farther into the red end of the light spectrum.
The redshift of the galaxies shows that they are moving away from us, and that farthest galaxies are moving the fastest. This means that the universe is presently expanding. This means that the universe was smaller in the past. If we run the clock all the way back to the beginning, the universe must have been incredibly small. In fact, we believe it began in an event we call the Big Bang. We will spend considerable time in this course considering these implications, and learning about the scientific evidence supporting them.