Modified gravity with f(R)=√R2-R02

In this work we propose a modified gravity with the action f(R)=root R-2-R-0(2) instead of the Einstein-Hilbert action to describe the late time acceleration of the Universe. We obtain the equation of the modified gravity both in the metric and Palatini formalisms. An asymptotic solution of gravity equations corresponding to a constant Ricci scalar causes a late time acceleration of the Universe. We do a conformal transformation in the action of the modified gravity and obtain the equivalent minimally coupled scalar-tensor gravity. The equivalent Brans-Dicke gravity of this model is also studied. To examine this model with observation, the perihelion precession of the Mercury is compared with our prediction and we put an upper constraint of R-0 < H-0(2). This range for R-0 is also in agreement with the cosmological acceleration at the present time. Finally we show that this action has instability for the small perturbations of the metric in vacuum solution in which adding a quadratic term of Ricci scalar can stabilize it.