 The SM’s account of the evolution of the matter and radiation in the universe reflects the dynamical effect of expansion. Consider a cube of spacetime in the early universe, filled with matter and radiation. The dynamical effects of the universe’s expansion are locally the same as slowly stretching the cube. For some stages of evolution the contents of the cube interact sufficiently quickly that they reach and stay in local thermal equilibrium as the cube changes volume. (Because of isotropy, equal amounts of matter and radiation enter and leave the cube from neighboring cubes.) But when the interactions are too slow compared to the rate of expansion, the cube changes volume too rapidly for equilibrium to be maintained. As a result, particle species “freeze out” and decouple, and entropy increases. Without a series of departures from equilibrium, cosmology would be boring—the system would remain in equilibrium with a state determined solely by the temperature, without a trace of things past. The rate of expansion of the cube varies with cosmic time. Because radiation, matter, and a cosmological constant term (or dark energy) dilute with expansion at different rates, an expanding universe naturally falls into separate epochs, characterized by different expansion rates.