  During (re)heating of the universe after inflation, the relativistic decay
products of the inflaton field $\phi$ must lose energy and additional particles
must be produced to attain a thermalised state at a temperature $T_{\reh}$. We
estimate the rate of energy loss via elastic and inelastic scattering
interactions. Elastic scattering is an inefficient energy loss mechanism so
inelastic processes, although higher order in the coupling $\alpha$, can be
faster because more energy is transfered. The timescale to produce a particle
number density of ${\cal O}(T_{\reh}^3)$ is the inelastic energy loss
timescale, $\sim(\alpha^3 n_\phi/T_{\reh}^2)^{-1}$.
