Emerging Newtonian potential in pure R2 gravity on a de Sitter background

Abstract

In [27] Alvarez-Gaume et al. established that pure mathcal{R} 2 theory propagates massless spin-2 graviton on a de Sitter (dS) background but not on a locally flat background. We build on this insight to derive a Newtonian limit for the theory. Unlike most previous works that linearized the metric around a locally flat background, we explicitly employ the dS background to start with. We directly solve the field equation of the action {left(2kappa right)}^{-1}int {d}^4xsqrt{-g}{mathcal{R}}^2 coupled with the stress-energy tensor of normal matter in the form Tμν = Mc2δ( overrightarrow{r} ) {delta}_{mu}^0{delta}_{nu}^0 . We obtain the following Schwarzschild-de Sitter metric d{s}^2=-left(1-frac{Lambda}{3}{r}^2-frac{kappa {c}^2}{48pi Lambda}frac{M}{r}right){c}^2d{t}^2+{left(1-frac{Lambda}{3}{r}^2-frac{kappa {c}^2}{48pi Lambda}frac{M}{r}right)}^{-1}d{r}^2+{r}^2d{varOmega}^2 which features a potential V(r)=-frac{kappa {c}^4}{96pi Lambda}frac{M}{r} with the correct Newtonian tail. The parameter Λ plays a dual role: (i) it sets the scalar curvature for the background dS metric, and (ii) it partakes in the Newtonian potential V(r). We reach two key findings. Firstly, the Newtonian limit only emerges owing to the de Sitter background. Most existing studies of the Newtonian limit in modified gravity chose to linearize the metric around a locally flat background. However, this is a false vacuum to start with for pure mathcal{R} 2 gravity. These studies unknowingly omitted the information about Λ of the de Sitter background, hence incapable of attaining a Newtonian behavior in pure mathcal{R} 2 gravity. Secondly, as Λ appears in V(r) in a singular manner, viz. V(r) ∝ Λ−1, the Newtonian limit for pure mathcal{R} 2 gravity cannot be obtained by any perturbative approach treating Λ as a small parameter.