Reheating and Dark Radiation after Fibre Inflation

TL;DR

This paper investigates reheating processes and dark radiation production after fibre inflation, deriving spectral parameters, decay rates, and comparing predictions with observational data, highlighting the role of ultra-light axions and flux configurations.

Contribution

It provides a detailed analysis of reheating and dark radiation in fibre inflation, including spectral index and tensor-to-scalar ratio predictions with flux-dependent variations.

Findings

Reheating temperature around 10^{10} GeV with 52 efoldings.

Ultra-light axions contribute to dark radiation, affecting N_eff.

Predicted r of order 0.007 to 0.01 depending on parameters.

Abstract

We study perturbative reheating at the end of fibre inflation where the inflaton is a closed string modulus with a Starobinsky-like potential. We first derive the spectral index $n_s$ and the tensor-to-scalar ratio $r$ as a function of the number of efoldings and the parameter $R$ which controls slow-roll breaking corrections. We then compute the inflaton couplings and decay rates into ultra-light bulk axions and visible sector fields on D7-branes wrapping the inflaton divisor. This leads to a reheating temperature of order $10^{10}$ GeV which requires $52$ efoldings. Ultra-light axions contribute to dark radiation even if $\Delta N_{\rm eff}$ is almost negligible in the generic case where the visible sector D7-stack supports a non-zero gauge flux. If the parameter $R$ is chosen to be small enough, $n_s\simeq 0.965$ is then in perfect agreement with current observations while $r$ turns out to be of order $r\simeq 0.007$. If instead the flux on the inflaton divisor is turned off, $\Delta N_{\rm eff}\lesssim 0.6$ which, when used as a prior for Planck data, requires $n_s\simeq 0.99$. After $R$ is fixed to obtain such a value of $n_s$, primordial gravity waves are larger since $r\simeq 0.01$.