Non-standard primordial fluctuations and nongaussianity in string inflation

TL;DR

The work addresses how string-theoretic multi-field dynamics can generate non-standard primordial fluctuations by embedding a curvaton mechanism in a LARGE Volume Scenario Type IIB compactification. It develops a concrete four-moduli setup with a light curvaton and a separate inflaton, derives the kinetic terms and scalar potential including loop and non-perturbative corrections, and demonstrates how isocurvature fluctuations from the curvaton can be converted into adiabatic perturbations after inflation. The main result is that the model typically yields a local-type non-Gaussianity with $f_{NL} \sim O(10)$ (and related $ au_{NL}$, $g_{NL}$) while satisfying CMB amplitude constraints and BBN bounds, with the magnitude of $f_{NL}$ tunable via the internal volume ${\cal V}$. This indicates Planck-like observations could robustly favor such string cosmologies, highlighting the relevance of multi-field dynamics in the early universe and offering a path to connect high-energy UV physics to observable cosmological signatures.

Abstract

Inflationary scenarios in string theory often involve a large number of light scalar fields, whose presence can enrich the post-inflationary evolution of primordial fluctuations generated during the inflationary epoch. We provide a simple example of such post-inflationary processing within an explicit string-inflationary construction, using a Kahler modulus as the inflaton within the framework of LARGE Volume Type-IIB string flux compactifications. We argue that inflationary models within this broad category often have a selection of scalars that are light enough to be cosmologically relevant, whose contributions to the primordial fluctuation spectrum can compete with those generated in the standard way by the inflaton. These models consequently often predict nongaussianity at a level, f_NL ~ O(10), potentially observable by the Planck satellite, with a bi-spectrum maximized by triangles with squeezed shape in a string realisation of the curvaton scenario. We argue that the observation of such a signal would robustly prefer string cosmologies such as these that predict a multi-field dynamics during the very early universe.