I'm just beginning physics and teaching myself classical physics from an intro physics textbook and in the 2nd chapter it described reference frames. This got me thinking. My reference frame is the earth, as that is what I use to describe position, displacement, ect. But what is the reference frame for the universe? I'm not even sure if this is a valid question because we don't know if our universe is moving or not; however we know it's expanding, so mustn't this motion of expansion be in some sort of reference frame?