Axion dark matter and Planck favor non-minimal couplings to gravity

Sarah Folkerts,Cristiano Germani,Javier Redondo

doi:10.1016/j.physletb.2013.12.026

Sarah Folkerts, Cristiano Germani + Show 1 more

Open Access

https://doi.org/10.1016/j.physletb.2013.12.026

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Abstract

Constraints on inflationary scenarios and isocurvature perturbations have excluded the simplest and most generic models of dark matter based on QCD axions. Considering non-minimal kinetic couplings of scalar fields to gravity substantially changes this picture. The axion can account for the observed dark matter density avoiding the overproduction of isocurvature fluctuations. Finally, we show that assuming the same non-minimal kinetic coupling to the axion (dark matter) and to the standard model Higgs boson (inflaton) provides a minimal picture of early time cosmology.

Highlights

The results of Planck are striking for cosmology [1]
The absence of an indirect observation of gravitational waves by Planck puts a tight, model-independent bound on the Hubble scale during inflation
The second mechanism involves instead a modification of how the QCD axion interacts to gravity: The amplitude of isocurvature perturbations is proportional to the axion fluctuations normalized to the axion expectation-value today

Summary

Introduction

The results of Planck are striking for cosmology [1]. The absence of non-Gaussianities generically point to inflationary models where the anisotropies of the Cosmic Microwave Background (CMB) are effectively generated by a single scalar field during inflation [2]. Large-field scenarios (chaotic slow-roll inflation) [3] require typical inflationary scales H I ∼ O(1013 GeV) and are disfavored with respect to models with a non-minimal kinetic coupling to gravity [4]. This is because the ratio of tensor to scalar perturbations depends on “how fast” the inflaton moves in its potential. There, a consistent single-field scenario, with sufficiently low Hubble scale H I so to fulfill the isocurvature constraints of Planck, is obtained by a non-minimal derivative coupling of a hidden axion to curvatures. During this transition topological defects are produced, which in the most generic cases over-close the Universe and are excluded [33,34]

Non-minimal couplings during inflation

Axion dark matter

Small scale inflation

Suppressing isocurvature perturbations

Inflation and dark matter from the standard model

Conclusions