**5.9. Reheating and Preheating**

Clearly, one of the great strengths of inflation is its ability to
redshift away all unwanted relics, such as topological
defects. However, inflation is not discerning, and in doing so any
trace of radiation or dust-like matter is similarly redshifted away to
nothing. Thus, at the end of inflation the universe contains nothing
but the inflationary scalar field condensate. How then does that
matter of which we are made arise? How does the hot big bang phase of
the universe commence? How is the universe *reheated*?

For a decade or so reheating was thought to be a relatively well-understood phenomenon. However, over the last 5-10 years, it has been realized that the story may be quite complicated. We will not have time to go into details here. Rather, we will sketch the standard picture and mention briefly the new developments.

Inflation ends when the slow-roll conditions are violated and, in most
models, the field begins to fall towards the minimum of its
potential. Initially, all energy density is in the inflaton, but this
is now damped by two possible terms. First, the expansion of the
universe naturally damps the energy density. More importantly, the
inflaton may decay into other particles, such as radiation or massive
particles, both fermionic and bosonic
[188,
189].
To take account of this one introduces a phenomenological decay term
_{} into the
scalar field equation. If we focus on fermions only, then a rough
expression for how the energy density evolves is

(225) |

The inflaton undergoes damped oscillations and decays into radiation
which equilibrates rapidly at a temperature known as the *reheat
temperature* *T*_{RH}.

In the case of bosons however, this ignores the fact that the inflaton
oscillations may give rise to parametric resonance. This is signified
by an extremely rapid decay, yielding a distribution of products that
is far from equilibrium, and only much later settles down to an
equilibrium distribution at energy *T*_{RH}. Such a rapid decay
due to parametric resonance is known as *preheating*
[190,
191].

One interesting outcome of preheating is that one can produce particles with energy far above the ultimate reheat temperature. Thus, one must beware of producing objects, such as topological relics, that inflation had rid us of. However, preheating can provide some useful effects, such as interesting new ways to generate dark matter candidates and topological relics and to generate the baryon asymmetry [192, 193, 194, 195, 196, 197].