Dynamics of Non-minimally Coupled Scalar Fields in the Jordan Frame
  The presence of scalar fields with non-minimal gravitational interactions of
the form $\xi |\phi|^2 R$ may have important implications for the physics of
the early universe. While many studies solve the dynamics of non-minimally
coupled scalars in the Einstein frame, where gravity is simply described by the
Einstein-Hilbert action, we instead propose a procedure to solve the dynamics
directly in the original Jordan frame where the non-minimal couplings are
maintained explicitly. Our algorithm can be applied to scenarios that include
minimally coupled fields and an arbitrary number of non-minimally coupled
scalars, with the expansion of the universe sourced by all fields present. This
includes situations when the dynamics become fully inhomogeneous, fully
non-linear (due to e.g.~backreaction or mode rescattering effects), and/or when
the expansion of the universe is dominated by non-minimally coupled species. As
an example, we study geometric preheating with a non-minimally coupled scalar
spectator field when the inflaton oscillates following the end of inflation. In
the future, our technique may be used to shed light on aspects of the
equivalence of the Jordan and Einstein frames at the quantum level.
