Inflation with an extra light scalar field after Planck

TL;DR

This paper uses Bayesian inference to analyze how an extra light scalar field during inflation affects model viability, finding that most single-field models remain robust, with specific exceptions like quartic large-field inflation.

Contribution

It introduces a numerical pipeline to evaluate inflation models with an additional light scalar field and assesses their viability using Bayesian methods.

Findings

Single-field models are generally robust with added light scalar fields.

Most models do not improve significantly with extra fields unless fine-tuned.

Quartic large-field inflation remains viable with an extra scalar in specific scenarios.

Abstract

Bayesian inference techniques are used to investigate situations where an additional light scalar field is present during inflation and reheating. This includes (but is not limited to) curvaton-type models. We design a numerical pipeline where $\simeq 200$ inflaton setups $\times\, 10$ reheating scenarios $= 2000$ models are implemented and we present the results for a few prototypical potentials. We find that single-field models are remarkably robust under the introduction of light scalar degrees of freedom. Models that are ruled out at the single-field level are not improved in general, because good values of the spectral index and the tensor-to-scalar ratio can only be obtained for very fine-tuned values of the extra field parameters and/or when large non-Gaussianities are produced. The only exception is quartic large-field inflation, so that the best models after Planck are of two kinds: plateau potentials, regardless of whether an extra field is added or not, and quartic large-field inflation with an extra light scalar field, in some specific reheating scenarios. Using Bayesian complexity, we also find that more parameters are constrained for the models we study than for their single-field versions. This is because the added parameters not only contribute to the reheating kinematics but also to the cosmological perturbations themselves, to which the added field contributes. The interplay between these two effects lead to a suppression of degeneracies that is responsible for having more constrained parameters.