Cosmological perturbation theory

Cosmological perturbation theory is a way of understanding the small variations or changes in the density of matter in the universe. It's based on the idea that although the universe is really big, there are ways that scientists can take that big universe and break it down into tiny pieces so they can study what's happening in the universe. Basically, if you look around the universe, you'll notice that in some places it's a bit denser (there's more matter in one place) than in other places. Cosmological perturbation theory helps us figure out how all this matter is arranged and how it changes over time.