Toward an Effective Field Theory Approach to Reheating
  We investigate whether Effective Field Theory (EFT) approaches, which have
been useful in examining inflation and dark energy, can also be used to
establish a systematic approach to inflationary reheating. We consider two
methods. First, we extend Weinberg's background EFT to the end of inflation and
reheating. We establish when parametric resonance and decay of the inflaton
occurs, but also find intrinsic theoretical limitations, which make it
difficult to capture some reheating models. This motivates us to next consider
Cheung, et. al.'s EFT approach, which instead focuses on perturbations and the
symmetry breaking induced by the cosmological background. Adapting the latter
approach to reheating implies some new and important differences compared to
the EFT of Inflation. In particular, there are new hierarchical scales, and we
must account for inflaton oscillations during reheating, which lead to discrete
symmetry breaking. Guided by the fundamental symmetries, we construct the EFT
of reheating, and as an example of its usefulness we establish a new class of
reheating models and the corresponding predictions for gravity wave
observations. In this paper we primarily focus on the first stages of
preheating. We conclude by discussing challenges for the approach and future
directions. This paper builds on ideas first proposed in the note
arXiv:1507.06651.
