Slow-roll Natural & Hilltop Inflation in Rastall Gravity

Abstract

This study provides a concise analysis of inflation under Rastall gravity by examining three types of potential such as the power law, natural, and hilltop potentials. Choosing a minimal interaction between matter and gravity, we derived the modified slow-roll parameters, the scalar spectral index $(n_s)$, the tensor spectral index $(n_T)$, and the tensor-to-scalar ratio $(r)$. For a general power-law potential as well as for Natural & Hilltop inflation, we calculated these quantities and subsequently plotted their trajectories in the $(n_s, r)$ plane. For the power-law potential, only the cases $n = 2/3$ and $n = 1$ satisfy the observational constraint of the Planck 2018 data. The natural potential analysis shows that the mass scale is crucial, with better compatibility achieved at $f = 5M_{p}$ compared to $f = 10M_{p}$. Lastly, the Hilltop potential results indicate that among the cases studied $m = 3/2, 2, 3$, and $4$, only $m = 3/2$ exhibits marginal consistency with observational bounds, while the other cases fail to produce acceptable $(n_s - r)$ trajectories.