Strong Einstein-Hilbert Gravity Inflation and ACT Phenomenology

TL;DR

This work analyzes single-field inflation within a rescaled gravity framework in which the Einstein-Hilbert term is effectively strengthened during inflation via a factor $α∈(0,1)$. By deriving the slow-roll relations under $S ∝ αR$ and applying them to four canonical models (D-Brane $p=2$, D-Brane $p=4$, Einstein-frame plateau, and exponential T-model), the authors assess compatibility with ACT measurements of the spectral index $n_s$ and the updated Planck/BICEP bounds on the tensor-to-scalar ratio $r$. They find that the rescaled gravity can render some otherwise disfavored models viable: the D-Brane $p=2$ and Einstein-frame plateau models can fully satisfy the observational constraints, while the D-Brane $p=4$ and exponential T-model may be marginally compatible depending on parameter choices. Overall, the study highlights that primordial stronger-gravity effects during inflation can broaden the viable model space under current cosmological data, offering a new angle on inflationary phenomenology.

Abstract

In this work we study rescaled effective single scalar field theories, and we confront these with the ACT constraint on the spectral index of the scalar primordial perturbations and the updated BICEP/Planck constraint on the tensor-to-scalar ratio. Rescaled scalar theories of gravity may be the result of an effective $f(R,φ)$ gravity at strong curvature regimes, which may result on a rescaling of the Einstein-Hilbert term of the form $\sim αR$. It turns out that canonical scalar field theories with stronger gravity compared to standard Einstein-Hilbert gravity can be compatible with the ACT and updated Planck/BICEP constraints, with stronger gravity meaning that the rescaling parameter $α$ takes values smaller than unity.

Figures (12)

• Figure 1: Contour plot for the spectral index of primordial scalar curvature perturbations $n_s$ (left plot) and the tensor-to-scalar ratio $r$ (right plot) for $\alpha = [0, 1]$, $m = [10^{-6}, 10^{0.3}]$ and $N = 60$ for the D-Brane Model ($p=2$).
• Figure 2: Contour plot for the spectral index of primordial scalar curvature perturbations $n_s$ (left plot) and the tensor-to-scalar ratio $r$ (right plot) for $\alpha = [0, 1]$, $m = [10^{-6}, 10^{0.3}]$ and $N = 52$ for the D-Brane Model ($p=2$). Here we are at the borderline of the constraint for $n_s$.
• Figure 3: Marginalized curves of the Planck 2018 data and the rescaled D-Brane gravity model $p=2$, confronted with the ACT data, the Planck 2018 data, and the updated Planck/BICEP constraints on the tensor-to-scalar ratio. We choose $\alpha=0.8$ and $m=0.1$ and $N$ in the range $N=[50,60]$.
• Figure 4: Contour plot for the spectral index of primordial scalar curvature perturbations $n_s$ (left plot) and the tensor-to-scalar ratio $r$ (right plot) for $\alpha = [0, 1]$, $m = [10^{-6}, 10^{0.3}]$ and $N = 60$ for the D-Brane Model (p=4).
• Figure 5: Contour plot for the spectral index of primordial scalar curvature perturbations $n_s$ (left plot) and the tensor-to-scalar ratio $r$ (right plot) for $\alpha = [0, 1]$, $m = [10^{-6}, 10^{0.3}]$ and $N = 58$ for the D-Brane Model (p=4). Here we are at the borderline of the constraint for $n_s$.