We construct a massive spin-2 theory in 2+1 dimensions that is immune to the bulk-boundary unitarity conflict in anti-de Sitter space and hence amenable to holography. The theory is an extension of Topologically Massive Gravity, just like the recently found Minimal Massive Gravity (MMG), but it has two massive helicity modes instead of a single one. The theory admits all the solutions of TMG with a redefined topological parameter. We calculate the Shapiro time-delay and show that flat-space (local) causality is not violated. We show that there is an interesting relation between the theory we present here (which we call MMG2), MMG and the earlier New Massive Gravity (NMG): Namely, field equations of these theories are non-trivially related. We study the bulk excitations and boundary charges of the conformal field theory that could be dual to gravity. We also find the chiral gravity limit for which one of the massive modes becomes massless. The virtue of the model is that one does not have to go to the chiral limit to achieve unitarity in the bulk and on the boundary and the log-terms that appear in the chiral limit and cause instability do not exist in the generic theory.PACS numbers:
I. INTRODUCTIONGravity in 2+1 dimensions has a counterintuitive richness: On the one side, naive counting from the metric leads to the conclusion that once gauge invariance (diffeomorphism invariance) is taken into account, no local propagating degrees of freedom (gravitons or gravity waves) exist, as in the case of Einstein's gravity. On the other hand, modifications of the theory, such as with higher powers of curvature introduce non-trivial local dynamics along with, usually, massive gravitons. Therefore, while it is very hard to get non-linear, unitary, ghostfree massive gravity in 3+1 dimensions with 5 degrees of freedom, it is embarrassingly easy to get massive gravity with 2 degrees of freedom in one lower dimensions. By now there there are several 3D models : Topologically Massive Gravity (TMG),[1], New Massive Gravity (NMG) [2] and the recent Minimal Massive Gravity (MMG) [3,4]. TMG is a parity-violating theory with a single spin-2 degree of freedom, NMG has a massive spin-2 excitation with both helicities, while MMG has a single massive parity-violating spin-2 excitation (same as TMG) but free of the bulk-boundary unitarity conflict in anti-de Sitter(AdS)spacetime that inflicts NMG and TMG. Ultimately, of course, research in 3D gravity aims at understanding or building "quantum gravity" in the physically relevant spacetime. For this purpose, obtaining a unitary, nontrivial gravity theory that has a welldefined unitary conformal field theory on the boundary is an important step. For example, NMG which has 2 massive bulk excitations just like General Relativity in 4D (with a massless graviton) does not have a unitary * Electronic address: btekin@metu.edu.tr conformal field theory (CFT) on the boundary. [Elaborate extensions of NMG could not resolve the issue [5,6].] Hence MMG stands alone as a curious non-trivial case of a 3...