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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...

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Super-horizon: lumpiness grows only by under-dense regions expanding faster (per general relativity). Within a speed-of-light horizon bubble, density is fairly even, so there is no (Newton-style) gravitation pull on uneven distribution. But different "bubbles" expand at different rates.

Sub-horizon: Now there are variations within each bubble, and so denser regions tend to grow denser.

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