Nonlinear Inflaton Fragmentation after Preheating

TL;DR

The paper addresses how nonlinear fragmentation unfolds during preheating after chaotic inflation, focusing on a short intermediate stage that bridges linear preheating and turbulence. It uses fully nonlinear lattice simulations of a two-field system with $V=\frac{1}{2} m^2 \phi^2$ and $\frac{1}{2} g^2 \phi^2 \chi^2$ to analyze both momentum- and configuration-space dynamics, revealing a bubble-like fragmentation phase where inflaton inhomogeneities grow at the peaks of the resonantly amplified field $\chi$, expand, and collide to drive phase mixing. The study finds strong non-gaussianity, rapid chaotic dynamics, and a subsequent return to turbulence and thermalization, with gravitational waves emerging from bubble collisions and potentially leaving observable imprints depending on the scale of inflation. This work links chaotic inflation preheating to tachyonic and hybrid preheating pictures, highlighting observable consequences in gravitational-wave spectra and offering insights into early-universe baryogenesis pathways via out-of-equilibrium bubbles.

Abstract

We consider the nonlinear dynamics of inflaton fragmentation during and after preheating in the simplest model of chaotic inflation. While the earlier regime of parametric resonant particle production and the later turbulent regime of interacting fields evolving towards equilibrium are well identified and understood, the short intermediate stage of violent nonlinear dynamics remains less explored. Lattice simulations of fully nonlinear preheating dynamics show specific features of this intermediate stage: occupation numbers of the scalar particles are peaked, scalar fields become significantly non-gaussian and the field dynamics become chaotic and irreversible. Visualization of the field dynamics in configuration space reveals that nonlinear interactions generate non-gaussian inflaton inhomogeneities with very fast growing amplitudes. The peaks of the inflaton inhomogeneities coincide with the peaks of the scalar field(s) produced by parametric resonance. When the inflaton peaks reach their maxima, they stop growing and begin to expand. The subsequent dynamics is determined by expansion and superposition of the scalar waves originating from the peaks. Multiple wave superposition results in phase mixing and turbulent wave dynamics. Thus, the short intermediate stage is defined by the formation, expansion and collision of bubble-like field inhomogeneities associated with the peaks of the original gaussian field. This process is qualitatively similar to the bubble-like inflaton fragmentation that occurs during tachyonic preheating after hybrid or new inflation.

Figures (6)

• Figure 1: Evolution of spectra in the combination $k^3 \omega_k n_k$ of the $\phi$ and $\chi$ fields during and immediately after preheating. Bluer plots show later spectra. Horizontal axis $k$ is in units of $m$
• Figure 2: Evolution of comoving number density of $\phi$ (red, lower plot) and $\chi$ (blue, upper plot) in units of $m^3$. Time is in units of $1/m$
• Figure 3: Evolution of the ratio $\langle f^2\rangle^2/\langle f^4\rangle$, where $f$ represents the $\phi$ field (red, solid) or the $\chi$ field (blue, dashed) and angle brackets represent a spatial average, is a measure of gaussianity. This ratio is one for a random gaussian field. Time is in units $1/m$
• Figure 4: Values of the $\phi$ and $\chi$ fields in a two dimensional slice through the lattice. The horizontal axes are spatial axes and the vertical axis is field value.
• Figure 5: Energy density and its components. The horizontal axes represent the same two dimensional slice through the lattice as in figure (\ref{['3dslices']}).