https://doi.org/10.1140/epjc/s10052-025-14969-2
Regular Article - Theoretical Physics
Inflationary models with Gauss–Bonnet coupling in light of ACT observations
1
Institute of Fundamental Physics and Quantum Technology, Department of Physics, School of Physical Science and Technology, Ningbo University, 818 Fenghua Rd, 315211, Ningbo, Zhejiang, China
2
School of Physical Science and Technology, Southwest University, 2 Tiansheng Rd, 400715, Chongqing, China
3
Faculty of Arts and Sciences, Beijing Normal University, 18 Jinfeng Rd, 519087, Zhuhai, China
Received:
16
August
2025
Accepted:
17
October
2025
Published online:
30
October
2025
Recent analyses combining Atacama Cosmology Telescope (ACT) data with other cosmological datasets report a higher scalar spectral index 
, creating tension with a wide range of inflationary models. Since a Gauss–Bonnet term with a coupling function ![$$\xi (\phi ) = 3\lambda /[4V(\phi )]$$](/articles/epjc/abs/2025/10/10052_2025_Article_14969/10052_2025_Article_14969_tex_eq2.png)
leaves nearly unchanged (up to a field rescaling) while reducing the tensor-to-scalar ratio r by a factor 
, so choosing sufficiently small effectively removes r as a limiting observable, making it easier for inflationary models to satisfy the latest observational constraints and alleviating this tension. Applying this mechanism to chaotic inflation, E-models, T-models, and hilltop inflation, we find that broad regions of parameter space become consistent with the latest ACT-based cosmic microwave background (CMB) constraints. These results demonstrate that Gauss–Bonnet couplings can help bring a broad class of inflationary models into agreement with current CMB measurements.
© The Author(s) 2025
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