Preheating: The Decay of the Inflaton Condensate

Around the end of inflation, \(\epsilon _H=1\), the homogeneous inflaton begins to oscillate about the minimum of its potential. The inflaton condensate must decay into other forms of matter and radiation, eventually giving the particle content of the Standard Model and perhaps dark matter. These more familiar forms of matter and radiation must eventually reach thermal equilibrium at temperatures greater than \(1\,\mathrm {MeV}\) in order to recover the successful big-bang nucleosynthesis scenario. The transition of the universe from the supercooled state at the end of inflation to the hot, thermal, radiation dominated state required for big-bang nucleosynthesis is called reheating. The subject of this section is the early transfer of energy from the inflaton condensate to the fields it is coupled to. We begin with the perturbative theory of reheating—historically, the process was first treated this way. We then show the importance of non-perturbative effects arising from the coherent nature of the inflaton condensate. They include parametric resonances and tachyonic instabilities, all of which lead to exponential growth in the occupation numbers of the fields the inflaton decays to (i.e., the decay products). These kinds of rapid decay are called preheating, with the decay products in a highly non-thermal state. Finally, we discuss the implications from coupling these decay products to additional matter fields for the energy transfer from the inflaton condensate.