On Inflation and Axionic Dark Matter in a Scaled Gravity

Motivated by the modified gravity theories $F(R)\neq R$ and inflationary physics, we first propose and investigate an inflation model in a scaled gravity $F(R)=R\,+\beta R$, where $\beta$ is a dimensionless scaling parameter. The latter is also implemented in a particular potential $V(\phi )=M^{4}\left[ 1-\cos \left( \frac{\phi }{\mu}\right)^{\beta }\right]$ being considered to drive the inflation via a parameter coupling scenario. Using the slow-roll approximations, the gravity scale parameter $\beta$ is approached with respect to the range of the associated computed cosmological observables $n_{s}$ and $r$ according to the recent Planck and BICEP/Keck data. Then, we discuss the axionic dark matter in the suggested gravity model by considering the case where the inflaton is taken to be identified with an axion-like field $\phi =f_{a}\theta$ with the decay constant $f_{a}=\mu$. Referring to the known data, the underlying inflation scale $M$ is constrained to be much lower than the corresponding axion scale $M\ll f_{a}$.