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In cosmology, reheating is an increase in temperature theorized to have occurred in the early universe immediately after inflation. Inflation dramatically lowered the temperature by lowering the density of the universe (the heat energy, i.e., kinetic energy of randomly-moving particles, was diluted, spread over a much larger volume). With the end of inflation, the energy which drove inflation converted into additional particles with kinetic energy, which, with the existing particles, settled into thermodynamic equilibrium at a higher temperature.
The term preheating is used for an additional theory which posits much of this heating process was slightly earlier in time, due to conditions that made the process especially efficient.