Modified gravity with $\ln R$ terms and cosmic acceleration

TL;DR

This paper explores modified gravity models with logarithmic and inverse curvature terms that grow at small curvature, showing they can explain cosmic acceleration without dark energy and remain stable with the inclusion of R^2 terms.

Contribution

It demonstrates that modified gravity with ln R and R^{-n}(ln R)^m terms can account for cosmic acceleration and stability, highlighting the importance of R^2 terms for viability.

Findings

Models with ln R terms can eliminate the need for dark energy.

R^2 terms are crucial for avoiding instabilities and ensuring a large scalar mass.

Modified gravity models can be consistent with current cosmic acceleration observations.

Abstract

The modified gravity with $\ln R$ or $R^{-n} (\ln R)^m$ terms which grow at small curvature is discussed. It is shown that such a model which has well-defined newtonian limit may eliminate the need for dark energy and may provide the current cosmic acceleration. It is demonstrated that $R^2$ terms are important not only for early time inflation but also to avoid the instabilities and the linear growth of the gravitational force. It is very interesting that the condition of no linear growth for gravitational force coincides with the one for scalar mass in the equivalent scalar-tensor theory to be very large. Thus, modified gravity with $R^2$ term seems to be viable classical theory.