5.6. Reheating after inflation
During inflation, all matter except the scalar field (usually called the inflaton) is redshifted to extremely low densities. Reheating is the process whereby the inflaton's energy density is converted back into conventional matter after inflation, re-entering the standard big bang theory.
Once the slow-roll conditions break down, the scalar field switches from being overdamped to being underdamped and begins to move rapidly on the Hubble timescale, oscillating at the bottom of the potential. As it does so, it decays into conventional matter. The details of reheating are an important area of research in inflationary cosmology at the moment for several reasons, but are not important for the generation and evolution of density perturbations which is the main focus of the remainder of this article. Consequently, I'll just note that recently there has been quite a dramatic change of view as to how reheating takes place. Traditional treatments (e.g. as given in Kolb & Turner ) added a phenomenological decay term; this was constrained to be very small and hence reheating was viewed as being very inefficient. This allowed substantial redshifting to take place after the end of inflation and before the Universe returned to thermal equilibrium; hence the reheat temperature would be lower, by several orders of magnitude, than suggested by the energy density at the end of inflation.
This picture is radically revised in work by Kofman, Linde & Starobinsky  (see also Ref. ), who suggest that the decay can undergo broad parametric resonance, with extremely efficient transfer of energy from the coherent oscillations of the inflaton field. This initial transfer has been dubbed preheating. With such an efficient start to the reheating process, it now appears possible that the reheating epoch may be very short indeed and hence that most of the energy density in the inflaton field at the end of inflation may be available for conversion into thermalized form.