  General relativistic effects in the form of metric perturbations are usually
neglected in the preheating era that follows inflation. We argue that in
realistic multi-field models these effects are in fact crucial, and the fully
coupled system of metric and quantum field fluctuations needs to be considered.
Metric perturbations are resonantly amplified, breaking the scale-invariance of
the primordial spectrum, and in turn stimulate scalar field resonances via
gravitational rescattering. This non-gravitationally dominated nonlinear growth
of gravitational fluctuations may have significant effects on the Doppler peaks
in the cosmic background radiation, primordial black hole formation,
gravitational waves and nonthermal symmetry restoration.

High Energy Physics - Phenomenology;Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Theory