Abstract
We study the inflation scenarios, in the framework of superstring theory, where the inflaton is an axion producing the adiabatic curvature perturbations while there exists another light axion producing the isocurvature perturbations. We discuss how the non-trivial couplings among string axions can generically arise, and calculate the consequent cross-correlations between the adiabatic and isocurvature modes through concrete examples. Based on the Planck analysis on the generally correlated isocurvature perturbations, we show that there is a preference for the existence of the correlated isocurvature modes for the axion monodromy inflation while the natural inflation disfavors such isocurvature modes.
Highlights
The recent Cosmic Microwave Background (CMB) data is still consistent with the simple ΛCDM model with a nearly scale-invariant, adiabatic and Gaussian power spectrum which can well be represented by the single-field slow-roll inflation models [1,2,3]
The forthcoming cosmological data with even better precision could reveal the potential deviations from such pure adiabatic perturbations, and it would be worth exploring the possibilities for the nonadiabatic perturbations in existence of correlations among the adiabatic and non-adiabatic modes along with their indications for the early Universe dynamics in the fundamental physics models
The natural inflation and axion monodromy inflation with moduli-dependent sinusoidal corrections can generically appear in the scalar potential through the non-perturbative effects
Summary
The recent Cosmic Microwave Background (CMB) data is still consistent with the simple ΛCDM model with a nearly scale-invariant, adiabatic and Gaussian power spectrum which can well be represented by the single-field slow-roll inflation models [1,2,3]. The robust parameter estimation, without significantly affecting the bounds on the conventional (ΛCDM) cosmological parameters even with the inclusion of isocurvature modes, was not previously realized due to strong degeneracies among the parameters involving the isocurvature perturbations which the previous CMB data sets suffered from and the Planck data could greatly reduce [3] With such a precise cosmological parameter estimation including the correlated isocurvature perturbations at hand, it would be intriguing to explore the indications of the generally correlated isocurvature perturbations for the early Universe phenomena, and we in this paper aim to study the presumably ubiquitous light degrees of freedom in the early Universe through their isocurvature fluctuations. The mixing of the string axions arises from the non-perturbative effects in the sinusoidal form, and the consequent cross-correlations between the adiabatic and isocurvature modes are studied.
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