Gravitational Energy in Quadratic Curvature Gravities
We define the notion of energy, and compute its values, for gravitational systems involving terms quadratic in curvature. While our construction parallels that of ordinary Einstein gravity, there are significant differences both conceptually and concretely. In particular, for D=4, all purely quadratic models admit vacua of arbitrary constant curvature. Their energies, including that of conformal (Weyl) gravity, necessarily vanish in asymptotically flat spaces. Instead, they are proportional to that of the Abbott-Deser (AD) energy expression in constant curvature backgrounds and therefore also proportional to the mass parameter in the corresponding Schwarzschild-(Anti) de Sitter geometries. Combined Einstein-quadratic curvature systems reflect the above results: Absent a cosmological constant term, the only vacuum is flat space, with the usual (ADM) energy and no explicit contributions from the quadratic parts. If there is a Lambda term, then the vacuum is also unique with that Lambda value, and the energy is just the sum of the separate contributions from Einstein and quadratic parts to the AD expression . Finally, we discuss the effects on energy definition of both higher curvature terms and higher dimension.