Constant-roll inflation in modified $$f(R,\phi )$$ f ( R , ϕ ) gravity model using Palatini formalism

Abstract

Abstract In this work, we study a constant-roll inflationary model in the Palatini formalism using modified gravity. Here our action consists a non-minimal coupling of a scalar field $$\phi $$ ϕ with Ricci scalar R in a general form of $$f(R,\phi )$$ f ( R , ϕ ) . Using Palatini approach, we write its equivalent scalar-tensor form in the Einstein frame and then apply the constant-roll condition in the equation of motion for the inflaton field. Later the tensor-to-scalar ratio and the spectral index are calculated using the slow-roll parameters and the results obtained are matched with the Planck 2018 data. We found that the results agree nicely with the observations within the parameter regime under consideration.