33. Jeans length. In a self-gravitating fluid, pressure stabilizes perturbations on scales below the Jeans length, while larger-scale perturbations are unstable to gravitational collapse. Can be thought of as the scale where the sound crossing time equals the gravitational collapse time:
34. evolution of the early Universe. Gives us the connection between time since the big bang, temperature, and energy scale from ~1 second to 100,000 years after the big bang (before, the normalization varies; after, the exponents change):
36. the fluid equation. Essentially the continuity equation for an expanding Universe; takes into account that evolution of density depends on the pressure (in a Newtonian sense, one can think of this as coming from PdV work):
37. Newton's law of gravitation. The gravitational force between two bodies depends on the inverse square of their separation and the product of their masses:
38. Coulomb's law of electrostatics. The electrostatic force between two charged bodies depends on the inverse square of their separation and the product of their charges. Intriguingly similar to Newton's law of gravitation (Eq. 37):
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