Super-renormalizable quantum gravity

In this paper we study perturbatively an extension of the Stelle higher derivative gravity involving an infinite number of derivative terms. We know that the usual quadratic action is renormalizable but suffers from the unitarity problem because of the presence of a ghost (state of negative norm) in the theory. In this paper, we reconsider the theory first introduced by Tomboulis in 1997, but we expand and extensively study it at both the classical and quantum level. This theory is ghost-free, since the introduction of (in general) two entire functions in the model with the property does not introduce new poles in the propagator. The local high derivative theory is recovered expanding the entire functions to the lowest order in the mass scale of the theory. Any truncation of the entire functions gives rise to the unitarity violation, but if we keep all the infinite series, we do not fall into these troubles. The theory is renormalizable at one loop and finite from two loops on. Since only one-loop Feynman diagrams are divergent, then the theory is super-renormalizable. We analyze the fractal properties of the theory at high energy showing a reduction of the spacetime dimension at short scales. Black hole spherical symmetric solutions are also studied omitting the high curvature corrections in the equation of motions. The solutions are regular and the classical singularity is replaced by a ``de Sitter-like core'' in $r=0$. Black holes may show a ``multihorizon'' structure depending on the value of the mass. We conclude the paper with a generalization of the Tomboulis theory to a multidimensional spacetime.

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