3.8. 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. This may happen slowly due to decays of individual inflaton particles, or may begin explosively due to a coherent decay of the many-particle inflaton state. This latter possibility is known as preheating [12, 13] and may convert the bulk of the inflaton's energy density. Preheating will however necessarily end before all the inflaton energy density is converted.
In the single field paradigm presently under discussion, it does not really matter how reheating takes place, in the sense that the details of the mechanism will not affect the predictions for large-scale structure. I therefore won't discuss it further here, except to note that the situation can be considerably more complicated in multi-field theories. Then the details of reheating can be important for determining the magnitude of density perturbations in the Universe, and one also must consider possibilities such as that one or more of the fields need not decay at all, but may instead survive up to the present to act as dark matter or dark energy.