RESUMEN
A new class of non-Gaussian curvature fluctuations zeta(pr)(x) identical with deltaN(chi(i)) arises from the postinflation preheating behavior of a noninflaton field chi(i). Its billiardlike chaotic dynamics imprints regular log-spaced narrow spikes in the number of preheating e-folds N(chi(i)). We perform highly accurate lattice simulations of supersymmetry-inspired quartic inflaton and coupling potentials in a separate-universe approximation to compute N(chi(i)) as a function of the (nearly homogeneous) initial condition chi(i). The superhorizon modes of chi(i)(x) result in positive spiky excursions in zeta(pr) and hence negative gravitational potential fluctuations added to the usual sign-independent inflaton-induced perturbations, observably manifested in large cosmic structures and as (polarized) temperature cosmic microwave background cold spots.
RESUMEN
In this Letter, I point out that there is a curvature singularity problem appearing on the nonlinear level that generally plagues f(R) models that modify Einstein gravity in the infrared. It is caused by the fact that for the effective scalar degree of freedom, the curvature singularity is at a finite field value and energy level, and can be easily accessed by the field dynamics in the presence of matter. This problem is invisible in a linearized analysis, except for the telltale growing oscillatory modes it causes. In view of this, the viability of many f(R) models in the current literature will have to be reevaluated.