Modulated Reheating and Large Non-Gaussianity in String Cosmology

TL;DR

This paper presents the first explicit string-theory realization of the modulation mechanism for generating primordial curvature perturbations within the LARGE Volume Scenario (LVS) of type IIB flux compactifications. By engineering two light Kähler moduli—a fibre inflaton and a poly-instanton–stabilised blow-up modulator—the authors realize reheating as a modulated process, converting inflationary isocurvature fluctuations into adiabatic perturbations. The model yields a local-type non-Gaussianity that can reach f_NL around 20 under mild tuning, with characteristic small trispectrum parameters; the analysis connects UV-consistent moduli stabilisation to observable cosmological signatures and outlines the required volume and coupling hierarchies. The work demonstrates that string compactifications can naturally accommodate modulated reheating and make Planck-scale predictions for non-Gaussianity, providing concrete benchmarks for upcoming observations.

Abstract

A generic feature of the known string inflationary models is that the same physics that makes the inflaton lighter than the Hubble scale during inflation often also makes other scalars this light. These scalars can acquire isocurvature fluctuations during inflation, and given that their VEVs determine the mass spectrum and the coupling constants of the effective low-energy field theory, these fluctuations give rise to couplings and masses that are modulated from one Hubble patch to another. These seem just what is required to obtain primordial adiabatic fluctuations through conversion into density perturbations through the `modulation mechanism', wherein reheating takes place with different efficiency in different regions of our Universe. Fluctuations generated in this way can generically produce non-gaussianity larger than obtained in single-field slow-roll inflation; potentially observable in the near future. We provide here the first explicit example of the modulation mechanism at work in string cosmology, within the framework of LARGE Volume Type-IIB string flux compactifications. The inflationary dynamics involves two light Kaehler moduli: a fibre divisor plays the role of the inflaton whose decay rate to visible sector degrees of freedom is modulated by the primordial fluctuations of a blow-up mode (which is made light by the use of poly-instanton corrections). We find the challenges of embedding the mechanism into a concrete UV completion constrains the properties of the non-gaussianity that is found, since for generic values of the underlying parameters, the model predicts a local bi-spectrum with fNL of order `a few'. However, a moderate tuning of the parameters gives also rise to explicit examples with fNL O(20) potentially observable by the Planck satellite.

Figures (1)

• Figure 1: The potentials for the inflaton (left) and the modulating field (right). Inflation occurs in the flat region of the inflaton potential while the modulating field gets large quantum oscillations around its classical value which is almost frozen during inflation at a value which is not necessarily close to its minimum.