The Third Way to 3D Gravity

Understand

Consistency of Einstein's gravitational field equation $G_{\mu\nu} \propto T_{\mu\nu}$ imposes a "conservation condition" on the $T$-tensor that is satisfied by (i) matter stress tensors, as a consequence of the matter equations of motion, and (ii) identically by certain other tensors, such as the metric tensor.

• However, there is a third way, overlooked until now because it implies a "non-geometrical" action: one {\it not} constructed from the metric and its derivatives alone.
• The new possibility is exemplified by the 3D "minimal massive gravity" model, which resolves the "bulk vs.
• boundary" unitarity problem of topologically massive gravity with anti-de Sitter asymptotics.