Evidence for Big Bang Cosmology

At the beginning of this series, we talked about the Big Bang, which was the beginning of our universe, and what happened in the early epochs thereafter. But what we didn't talk about was how we know all that stuff! What is the evidence that supports this model? There is a lot of it, and we weren't ready to understand it then, but now we are. Let's talk about recession velocities, the cosmic microwave background radiation, the ratio of hydrogen to helium in the universe, and all the other empirical evidence that supports Big Bang cosmology.