Physical Foundations of Cosmology

5.5 Preheating and reheating 243

V

φc φ

φ>>>φc

V∼lnφφc

Fig. 5.4.

for 1>|φ|φc(see Figure 5.4), we infer from (5.56) thatw→−1. This is easy
to understand. In the case of a convex potential, an oscillating scalar field spends
most of the time near the potential walls where its kinetic energy is negligible and
hence the main contribution to the equation of state comes from the potential term.

Problem 5.9Which general conditions must a potentialVsatisfy to provide a
stage of fast oscillating inflation? How long can such inflation last and why is it not
very helpful for solving the initial conditions problem?

5.5 Preheating and reheating

The theory of reheating is far from complete. Not only the details, but even the over-
all picture of inflaton decay depend crucially on the underlying particle physics
theory beyond the Standard Model. Because there are so many possible exten-
sions of the Standard Model, it does not make much sense to study the particulars
of the reheating processes in each concrete model. Fortunately we are interested
only in the final outcome of reheating, namely, in the possibility of obtaining a
thermal Friedmann universe. Therefore, to illustrate the physical processes which
could play a major role we consider only simple toy models. The relative impor-
tance of the different reheating mechanisms cannot be clarified without an under-
lying particle theory. However, we will show that all of them lead to the desired
result.