Hi, I'm stuggeling trying to understand the physics behind this. I just don't know where to start. Everything I read just goes into too much detail. And my lecturer didn't explain it well at all he basically just gave us this http://www.warwick.ac.uk/~phsep/px311/notescosmology/cosmology4-7.doc (page 6) without explaining what was going on. Can someone help me?