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
The inflaton undergoes damped oscillations and decays into radiation which equilibrates rapidly at a temperature known as the reheat temperature TRH.
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 TRH. 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].