Inflation and Preheating in NO models

TL;DR

NO models feature a $V(\phi)$ without a minimum, so reheating via inflaton decay is ineffective and gravitational production risks large isocurvature perturbations and gravitino/moduli relics. The paper analyzes initial-condition and perturbation issues in NO models and demonstrates that instant preheating with a $\frac{g^{2}}{2}\phi^{2}\chi^{2}$ coupling provides a dramatically more efficient energy transfer than gravitational production (valid for $g^{2}\gtrsim 10^{-14}$). This mechanism can suppress isocurvature by keeping $\chi$ heavy during inflation and rapidly transfers energy to radiation through $\chi$-decays, addressing gravitino/moduli concerns. It further generalizes to fermions and other couplings, offering a robust path to viable NO-model cosmologies with practical reheating consequences.

Abstract

We study inflationary models in which the effective potential of the inflaton field does not have a minimum, but rather gradually decreases at large $φ$. In such models the inflaton field does not oscillate after inflation, and its effective mass becomes vanishingly small, so the standard theory of reheating based on the decay of the oscillating inflaton field does not apply. For a long time the only mechanism of reheating in such non-oscillatory (NO) models was based on gravitational particle production in an expanding universe. This mechanism is very inefficient. We will show that it may lead to cosmological problems associated with large isocurvature fluctuations and overproduction of dangerous relics such as gravitinos and moduli fields. We also note that the setting of initial conditions for the stage of reheating in these models should be reconsidered. All of these problems can be resolved in the context of the recently proposed scenario of instant preheating if there exists an interaction ${g^2} φ^2χ^2$ of the inflaton field $φ$ with another scalar field $χ$. We show that the mechanism of instant preheating in NO models is much more efficient than the usual mechanism of gravitational particle production even if the coupling constant $g^2$ is extremely small, $10^{-14} \ll g^2 \ll 1$.