Observational Constraints on Warm Natural Inflation

TL;DR

This paper investigates warm natural inflation with a cosine potential, focusing on how temperature-dependent dissipation affects the need for large decay constants, and compares model predictions with CMB data.

Contribution

It analyzes the impact of temperature-dependent dissipation rates on the viability of sub-Planckian decay constants in warm natural inflation.

Findings

Weak dissipation allows decay constants down to 0.3 M_pl

Moderate dissipation requires decay constants above 0.8 M_pl

Models are constrained by CMB data in the r-n_s plane

Abstract

Warm natural inflation is studied for the case of the original cosine potential. The radiation bath during inflation induces a dissipation (friction) rate in the equation of motion for the inflaton field, which can potentially reduce the field excursion needed for an observationally viable period of inflation. We examine if the dissipation thus provides a mechanism to avoid the large decay constant $f \gtrsim M_{\mathrm{pl}}$ of cold cosine natural inflation. Whereas temperature independent dissipation has previously been shown to alleviate the need for a trans-Planckian decay constant $f$, we illustrate here the difficulties of accommodating a significantly sub-Planckian decay constant ($f<10^{-1}M_{\mathrm{pl}}$) in the case of the following temperature dependent dissipation rates, $\Gamma \propto T^c$, with $c=\{1,3\}$. Such dissipation rates represent physically well-motivated constructions in the literature. For each model, we map its location in the $r$-$n_s$ plane and compare with Cosmic Microwave Background data. For $c=1 \, (c=3)$, we find that agreement with CMB data requires that dissipation be in the weak (moderate) regime and that the minimum allowed value of the decay constant in the potential is $f_{\rm min} = 0.3 \, (0.8)\,M_{\mathrm{pl}}$ respectively.