Constraining Quintessential Inflation with ACT: A Gauss-Bonnet Gateway

TL;DR

This paper explores how Einstein--Gauss--Bonnet gravity modifications can reconcile quintessential inflation models with recent ACT observations, by adjusting inflationary predictions and reheating scenarios.

Contribution

It demonstrates that specific Gauss--Bonnet couplings can bring quintessential inflation predictions into alignment with current cosmological data.

Findings

Exponential and sech-type couplings shift $r$ and $n_s$ into the ACT 1$\sigma$ region.

Tanh-type coupling remains disfavored, showing sensitivity to coupling structure.

Viable reheating temperatures are achievable without a potential minimum, consistent with BBN bounds.

Abstract

Recent results from the Atacama Cosmology Telescope (ACT), indicating a higher and more tightly constrained scalar spectral index, $n_s = 0.9743 \pm 0.0034$, place several inflationary models under tension, with quintessential inflation pushed close to or beyond the $2\sigma$ boundary in the $r$--$n_s$ plane. In this work, we revisit quintessential inflation within the framework of Einstein--Gauss--Bonnet (EGB) gravity, where a scalar field non-minimally coupled to the Gauss--Bonnet invariant modifies the inflationary dynamics. We consider three representative coupling functions -- exponential, hyperbolic secant, and hyperbolic tangent -- and show that the exponential and sech-type couplings can shift the predicted values of $r$ and $n_s$ into the $1\sigma$ region allowed by ACT, thereby restoring consistency with observations. In contrast, the tanh-type coupling remains disfavored, underscoring the sensitivity of inflationary observables to the coupling structure. We further investigate the reheating phase using a model-independent parametrization and demonstrate that viable thermal histories can be realized even in the absence of a potential minimum, with reheating temperatures consistent with Big Bang nucleosynthesis bounds. Overall, our analysis shows that EGB corrections provide a viable and robust extension that reconciles quintessential inflation with current precision cosmological data, and we identify the corresponding allowed parameter space.