I've always had difficulty visualizing how sub-horizon scales work versus super-horizon scales.
Inflation causes perturbations due to quantum fluctuations. These are under-densities and over-densities on vastly different scales. Why is there such a variation of fluctuations? Why aren't all perturbations uniform in the same horizon?
Pressure and gravity effects these fluctuations depending on which scale they are in. So, for instance, super-horizon fluctuations are "frozen out" by the horizon. What does that mean, freezing out?
EDIT: I guess it is just a tenet of inflationary theory that densities are not uniform/not the same. Ok then...