The Curvaton as a Pseudo-Nambu-Goldstone Boson

TL;DR

This work investigates the viability of a pseudo-Nambu-Goldstone boson as the curvaton responsible for cosmological curvature perturbations, offering a general PNGB framework in which the curvaton potential vanishes in the exact symmetry limit. It derives the key relations between the curvaton parameters, the curvature perturbation ζ, and the COBE normalization, and identifies stringent bounds from inflation, decay, and non-Gaussianity. The authors then analyze three explicit PNGB realizations—Wilson line in a 5D theory, Little Higgs mechanisms, and string axions—each yielding distinct parameter constraints (e.g., extra-dimensional scales, symmetry-breaking scales, and v/M_P) and, in some cases, requiring a suppressed inflation scale or additional entropy production to align with observations. Overall, PNGB curvatons offer a viable alternative to inflaton-generated perturbations, albeit with model-dependent requirements and potential moduli-related challenges in string-inspired scenarios.

Abstract

The field responsible for the cosmological curvature perturbations generated during a stage of primordial inflation might be the ``curvaton'', a field different from the inflaton field. To keep the effective mass of the curvaton small enough compared to the Hubble rate during inflation one may not invoke supersymmetry since the latter is broken by the vacuum energy density. In this paper we propose the idea that the curvaton is a pseudo Nambu-Goldstone boson (PNGB) so that its potential and mass vanish in the limit of unbroken symmetry. We give a general framework within which PNGB curvaton candidates should be explored. Then we explore various possibilities, including the case where the curvaton can be identified with the extra-component of a gauge field in a compactified five-dimensional theory (a Wilson line), where it comes from a Little-Higgs mechanism, and where it is a string axion so that supersymmetry is essential.