A Cosmic Microscope for the Preheating Era
  Light fields with spatially varying backgrounds can modulate cosmic
preheating, and imprint the nonlinear effects of preheating dynamics at tiny
scales on large scale fluctuations. This provides us a unique probe into the
preheating era which we dub the "cosmic microscope.'' We identify a distinctive
effect of preheating on scalar perturbations that turns the Gaussian primordial
fluctuations of a light scalar field into square waves, like a diode. The
effect manifests itself as local non-Gaussianity. We present a model,
"modulated partial preheating," where this nonlinear effect is consistent with
current observations and can be reached by near future cosmic probes.
