Application of the Separate Universe Approach to Preheating

TL;DR

The paper examines whether preheating after inflation can generate significant metric perturbations on super-Hubble scales. It adopts the separate universe approach, linking curvature perturbations to spatial variations in the expansion history via $N_{H_r}$ and $\Delta N_{H_r}$ in a two-field setup. In a conformal two-field model, time-averaged analysis suggests little super-Hubble growth, while non-averaged numerics reveal small, chaotic, initial-condition-dependent fluctuations in $N$ and possible routes to converting isocurvature into curvature perturbations when conformal invariance is broken. The work argues that, under their framework, large-scale perturbations are suppressed unless specific mode-coupling and decay scenarios enable coherent growth, challenging previous expectations and clarifying the role of backreaction and resonance structure.

Abstract

The dynamics of preheating after inflation has not been clearly understood yet.In particular, the issue of the generation of metric perturbations during preheating on super-horizon scale is still unsettled. Large scale perturbations may leave an imprint on the cosmic microwave background, or may become seeds for generation of primordial black holes. Hence, in order to make a connection between the particle physics models and cosmological observations, understanding the evolution of super-Hubble scale perturbations during preheating is important. Here, we propose an alternative treatment to handle this issue based on the so-called separate universe approach, which suggests less efficient amplification of super-Hubble modes during preheating than was expected before. We also point out an important issue which may have been overlooked in previous treatments.