Opening the reheating box in multifield inflation
  The robustness of multi-field inflation to the physics of reheating is
investigated. In order to carry out this study, reheating is described in
detail by means of a formalism which tracks the evolution of scalar fields and
perfect fluids in interaction (the inflatons and their decay products). This
framework is then used to establish the general equations of motion of the
background and perturbative quantities controlling the evolution of the system
during reheating. Next, these equations are solved exactly by means of a new
numerical code. Moreover, new analytical techniques, allowing us to interpret
and approximate these solutions, are developed. As an illustration of a
physical prediction that could be affected by the micro-physics of reheating,
the amplitude of non-adiabatic perturbations in double inflation is considered.
It is found that ignoring the fine-structure of reheating, as usually done in
the standard approach, can lead to differences as big as $\sim 50\%$, while our
semi-analytic estimates can reduce this error to $\sim 10\%$. We conclude that,
in multi-field inflation, tracking the perturbations through the details of the
reheating process is important and, to achieve good precision, requires the use
of numerical calculations.
