String-inspired Gauss-Bonnet Gravity Inflation and ACT

TL;DR

This paper systematically tests a string-inspired Gauss-Bonnet gravity inflation model against Planck 2018 and ACT data, confirming its viability and exploring various coupling functions through Bayesian analysis.

Contribution

First comprehensive observational comparison of the ghost-free $f(R, ext{G})$ model with multiple coupling functions using Bayesian MCMC methods.

Findings

All models reproduce the observed scalar spectral tilt $n_s \\approx 0.97$ at 60 e-folds.

Preference for models depends on Hubble parametrization, not coupling function.

The parameter \\mu \\approx 0.1 remains stable across models.

Abstract

In this article we present a systematic observational verification of the ghost-free string-inspired $f(R,\mathcal{G})$ model, where the Gauss-Bonnet invariant is non-minimally coupled to an auxiliary scalar field $\chi$ through the coupling function $h(\chi)$. Previous studies confirmed the theoretical viability of this framework using phenomenological parameter choices. In this work, for the first time, a systematic comparison with observational data from Planck 2018 and the Atacama Comsology Telescope is carried out via a Bayesian MCMC analysis using the Cobaya code. We explore an extended set of sixteen models constructed from four types of the Hubble parameter combined with power-law, exponential, hybrid, and inverse logarithmic coupling functions $h(\chi)$. The hybrid coupling $h(\chi) = \gamma e^{b_1\chi}\chi^{b_2}$, introduced in this context, allows for interpolation between the power-law and exponential forms, providing additional flexibility in controlling the Gauss-Bonnet contribution at different stages of inflation. All sixteen models reproduce the red spectral tilt of scalar perturbations consistent with CMB observations, yielding $n_s \approx 0.97$ at $N = 60$ e-folds. We find that the preference for the dataset is systematically determined by the choice of Hubble parametrization rather than by the coupling function. The parameter $\mu\approx0.1$ remains stable in all configurations, suggesting its fundamental role within the ghost-free formalism.