Phenomenology of Spillway Preheating: Equation of State and Gravitational Waves

TL;DR

This paper investigates how spillway preheating, involving perturbative decays, influences inflationary observables and gravitational wave production, revealing modifications to the scalar spectrum tilt and producing sharper, high-frequency gravitational wave spectra.

Contribution

It introduces the spillway preheating scenario as a way to enhance energy transfer and explores its effects on inflationary parameters and gravitational wave signatures.

Findings

Spillway preheating predicts a bluer scalar spectrum tilt.

It results in sharper-peaked high-frequency gravitational wave spectra.

The scenario modifies the evolution of the equation of state after inflation.

Abstract

In the canonical tachyonic resonance preheating scenario, only an order one fraction of energy density in the inflaton is transferred to radiation, due to backreaction effects. One possible way to improve the energy transfer efficiency is to allow for the perturbative decays of the resonantly produced daughter particles, which serve as the "spillway" to drain the direct decay products from inflaton and to reduce the backreaction. In this article, we study two observational consequences of spillway preheating. The first is on the inflationary observables: the scalar spectrum tilt $n_s$ and tensor-to-scalar ratio $r$. The spillway scenario modifies the evolution of the equation of state between the end of inflation and the thermal big bang. As a result, it affects the time elapsed from inflation to the Cosmic Microwave Background (CMB), as well as the fits of inflationary models and their corresponding prediction for $n_s$ and $r$. We map out the equation of state by systematically scanning the parameter space of the spillway scenario, and show that the most efficient spillway scenario predicts a bluer spectrum, compared to the tachyonic preheating scenario. Another consequence is the production of high-frequency gravitational waves (GWs). Comparing the simulation results with those of tachyonic preheating, we find that the existence of spillways leads to sharper-peaked GW spectra with a mildly damped amplitude.