Modified gravity models for inflation: In conformity with observations

Xinyi Zhang, Che-Yu Chen, Yakefu Reyimuaji 2022 Physical Review D 105, 043514 (2022)  

We consider a modified gravity framework for inflation by adding to the Einstein-Hilbert action a direct f(ϕ)T term, where ϕ is identified as the inflaton and T is the trace of the energy-momentum tensor. The framework goes to Einstein gravity naturally when the inflaton decays out. We investigate inflation dynamics in this f(ϕ)T gravity (not to be confused with torsion-scalar coupled theories) on a general basis and then apply it to three well-motivated inflationary models. We find that the predictions for the spectral tilt and the tensor-to-scalar ratio are sensitive to this new f(ϕ)T term. This f(ϕ)T gravity brings chaotic and natural inflation into better agreement with data and allows a larger tensor-to-scalar ratio in the Starobinsky model.