  Recently it has become clear that the resonant amplification of quantum field
fluctuations at preheating must be accompanied by resonant amplification of
scalar metric perturbations, since the two are united by Einstein's equations.
Furthermore, this "metric preheating" enhances particle production and leads to
gravitational rescattering effects even at linear order. In multi-field models
with strong preheating (q \gg 1), metric perturbations are driven nonlinear,
with the strongest amplification typically on super-Hubble scales (k \to 0).
This amplification is causal, being due to the super- Hubble coherence of the
inflaton condensate, and is accompanied by resonant growth of entropy
perturbations. The amplification invalidates the use of the linearized Einstein
field equations, irrespective of the amount of fine-tuning of the initial
conditions. This has serious implications at all scales - from the large-angle
cosmic microwave background (CMB) anisotropies to primordial black holes. We
investigate the (q,k) parameter space in a two-field model, and introduce the
time to nonlinearity, t_{nl}, as the timescale for the breakdown of the
linearized Einstein equations. Backreaction effects are expected to shut down
the linear resonances, but cannot remove the existing amplification, which
threatens the viability of strong preheating when confronted with the CMB. We
discuss ways to escape the above conclusions, including secondary phases of
inflation and preheating solely to fermions. Finally we rank known classes of
inflation from strongest (chaotic and strongly coupled hybrid inflation) to
weakest (hidden sector, warm inflation) in terms of the distortion of the
primordial spectrum due to these resonances in preheating.
