Ozkan Mehmet, Pang Yi, Zorba Utku
Department of Physics, Istanbul Technical University, Maslak 34469, Istanbul, Turkey.
Mathematical Institute, University of Oxford, Woodstock Road, Oxford OX2 6GG, United Kingdom.
Phys Rev Lett. 2019 Jul 19;123(3):031303. doi: 10.1103/PhysRevLett.123.031303.
We obtain a new 3D gravity model from two copies of parity-odd Einstein-Cartan theories. Using Hamiltonian analysis, we demonstrate that the only local degrees of freedom are two massive spin-2 modes. Unitarity of the model in anti-de Sitter and Minkowski backgrounds can be satisfied for vast choices of the parameters without fine-tuning. The recent "exotic massive 3D gravity" model arises as a limiting case of the new model. We also show that there exist trajectories on the parameter space of the new model which cross the boundary between unitary and nonunitary regions. At the crossing point, one massive graviton decouples resulting in a unitary model with just one bulk degree of freedom but two positive central charges at odds with the usual expectation that the critical model has at least one vanishing central charge. Given the fact that a suitable nonrelativistic version of bigravity has been used as an effective theory for gapped spin-2 fractional quantum Hall states, our model may have interesting applications in condensed matter physics.
