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 era which we dub "cosmic microscope.'' We identify distinctive effect scalar perturbations that turns Gaussian primordial fluctuations light field square waves, like diode. The manifests itself as local non-Gaussianity. present model, "modulated partial preheating," where this is consistent current observations be reached by near future probes.