Preheating and gravitational waves in large-field hilltop inflation

TL;DR

This paper investigates preheating and gravitational wave production after inflation in a large-field hilltop model, finding that resonances are weak and gravitational waves remain below current detection limits.

Contribution

It provides a detailed analysis of preheating dynamics and gravitational wave generation in a large-field hilltop inflation model using linear perturbation theory and lattice simulations.

Findings

Resonance effects are weak, preventing exponential growth of fluctuations.

Generated gravitational waves are below current observational limits.

Fluctuation occupation numbers remain close to zero during preheating.

Abstract

The combined Planck, BICEP/Keck Array and BAO measurements of the scalar spectral index and the tensor-to-scalar ratio from the cosmic microwave background observations severely constrain or completely rule out several models of inflationary potentials. On the other hand, the data seems to favor concave potentials over convex ones. In this paper, we study preheating and gravitational waves after inflation in a large-field, regularized hilltop potential where inflation takes place in the concave plateau. The inflaton, $\phi$, is coupled to a subdominant scalar field, $\chi$, through a quartic coupling. After inflation ends, $\phi$ oscillates about the potential minimum and becomes inhomogeneous. The growth of the fluctuation modes, $\delta\phi_k$ and $\delta\chi_k$, in a homogeneous, oscillating background is analyzed in linear perturbation theory, revealing that small modes likely experience broad self-resonance or external parametric resonance. To determine if the resonances are sufficiently strong to cause unstable growth of the modes we perform a lattice simulation. The lattice simulations demonstrate that, although the initial inhomogeneities generate a stochastic gravitational wave background that remains below the present observational limit, the fluctuations do not grow exponentially, and the occupation numbers of $\delta\phi_k$ and $\delta\chi_k$ remain close to zero.