Loop Blow-up Inflation: An Overview

TL;DR

This paper reviews Loop Blow-up Inflation, updates its observational predictions with recent CMB and BAO data, and discusses how string loop corrections influence inflationary dynamics and cosmological parameters.

Contribution

It demonstrates that string loop corrections can generate a new slow-roll regime, altering inflationary predictions and maintaining consistency with current observational constraints.

Findings

Loop corrections invalidate the original non-perturbative picture.

A new slow-roll regime at larger field values is generated.

Predictions for tensor-to-scalar ratio and dark radiation are updated.

Abstract

This proceedings contribution provides an overview of Loop Blow-up Inflation and updates its observational predictions and their comparison with the latest CMB and BAO data from combined analyses of SPT, Planck, ACT, and BICEP/Keck, as well as ACT DR6 constraints on extra dark radiation. It is based on work originally published in arXiv:2403.04831, carried out in collaboration with L. Brunelli, M. Cicoli, A. Hebecker, and R. Kuespert, and presented at the 2025 Workshop on Quantum Gravity and Strings. We focus on string loop corrections to the K\"ahler potential, long regarded as a potential threat to blow-up inflation in the Large Volume Scenario. We argue that these corrections, previously assumed avoidable, are in fact generically present and qualitatively alter the original non-perturbative picture: they invalidate slow-roll near the minimum and instead generate a new slow-roll regime at larger field values, where the scalar potential transitions from an exponential to a power-law plateau. This leads to modified inflationary dynamics and distinct cosmological predictions, including an increased tensor-to-scalar ratio. We confront all three SM-location scenarios with the latest constraints on $n_s$, $r$, and $\Delta N_{\rm eff}$. The tighter bound on extra dark radiation requires an updated Giudice-Masiero coefficient in Scenario III, yielding revised predictions presented here for the first time. All scenarios remain consistent with recent observations, with the ACT+DESI combination yielding near-perfect agreement in $n_s$ for vanishing extra dark radiation at $0.03\sigma$ deviation. We also comment on subleading loop corrections, which improve robustness by reducing the field value required for slow roll. These results highlight that string loop effects, rather than being merely detrimental, can play a constructive role in realising viable inflation in string compactifications.